Underwater endoscopic submucosal dissection in the gastrointestinal tract: technical review and dual-approach endoscopic submucosal dissection

Mitsuru Nagata

doi:10.5946/ce.2025.330

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Review Underwater endoscopic submucosal dissection in the gastrointestinal tract: technical review and dual-approach endoscopic submucosal dissection: Mitsuru Nagata; DOI: https://doi.org/10.5946/ce.2025.330
Published online: March 17, 2026

Department of Endoscopy, Shonan Fujisawa Tokushukai Hospital, Kanagawa, Japan

Correspondence: Mitsuru Nagata Department of Endoscopy, Shonan Fujisawa Tokushukai Hospital, 1-5-1 Tsujidokandai, Fujisawa, Kanagawa 251-0041, Japan E-mail: mitsuru10jp@yahoo.co.jp

• Received: September 7, 2025 • Revised: November 7, 2025 • Accepted: November 8, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Abstract
INTRODUCTION
BASICS OF UESD
COUNTERMEASURES FOR BLEEDING DURING UESD
COUNTERMEASURES FOR BUBBLES DURING UESD
UESD ADVERSE EVENTS
ORGAN-SPECIFIC UESD
DISCUSSION
CONCLUSIONS
NOTES
REFERENCES

See letter "Underwater endoscopic submucosal dissection in the stomach: where are we now?".

Abstract

Endoscopic submucosal dissection (ESD) has become an established technique for en bloc resection of superficial gastrointestinal neoplasms. However, conventional ESD (CESD) under gas insufflation remains technically demanding, particularly in gravity-side lesions or in cases with submucosal fibrosis. Underwater ESD (UESD) was developed to overcome these challenges. Complete submergence mitigates gravity-related difficulties, enhances visualization through the natural zoom effect, and facilitates submucosal dissection via buoyancy and water pressure. Retrospective data further suggest that UESD may reduce the risk of post-ESD coagulation syndrome, which is likely attributable to the heat sink effect. UESD can also be integrated with device-assisted traction and pocket creation methods. However, limitations, such as visual field impairment caused by bleeding or bubble formation, remain concerns, and their optimal management requires further investigation. Nevertheless, UESD allows procedural flexibility by switching between UESD and CESD within the same procedure, herein referred to as dual-approach ESD. Despite its routine use in clinical practice, dual-approach ESD has not been clearly distinguished from UESD in most clinical studies, complicating comparisons with CESD. To advance both clinical practice and research, future investigations should clearly differentiate UESD from dual-approach ESD for a more accurate evaluation. Furthermore, appropriate procedural selection requires careful consideration of multiple factors.
Keywords: Dual-approach endoscopic submucosal dissection; Endoscopic submucosal dissection; Gastrointestinal endoscopy; Gastrointestinal neoplasms; Underwater endoscopic submucosal dissection

INTRODUCTION

Endoscopic submucosal dissection (ESD) is a minimally invasive technique for en bloc resection of superficial gastrointestinal neoplasms, enabling accurate histopathological assessment and reducing recurrence rates.¹ Conventional ESD (CESD), performed under gas insufflation, remains technically demanding, particularly for lesions located on the gravity side. In such situations, insufficient tension in the dissection plane and visual field impairment due to partial submergence can hinder safe and efficient dissection. In addition, submucosal fibrosis and poor endoscopic maneuverability often contribute to procedural difficulties.²^,³ Moreover, post-ESD coagulation syndrome (PECS) remains a common adverse event of colorectal CESD that has not been adequately addressed.⁴^,⁵

Underwater ESD (UESD),⁶ also referred to as saline immersion therapeutic endoscopy,⁷ is performed during saline immersion and was developed to overcome these challenges. Complete submergence mitigates gravity-related difficulties,⁸ enhances visualization through the natural zoom effect, and facilitates submucosal dissection through buoyancy and water pressure even in fibrotic areas.⁹^-¹¹ Retrospective data suggest that UESD may reduce the risk of PECS, which is likely attributable to the heat sink effect.¹² UESD can also be combined with device-assisted traction methods¹³ or the pocket creation method (PCM).¹⁴ However, its disadvantages include visual field impairment due to bleeding and bubble formation. Although novel solutions have recently been reported,¹⁵^-²⁵ their safety and efficacy remain unclear.

Nevertheless, UESD provides a notable advantage, flexibility, allowing operators to switch between UESD and CESD during the same procedure according to procedural demands, herein referred to as dual-approach ESD, which is routinely used in clinical practice.²⁶^,²⁷ Despite its practical value, dual-approach ESD has not yet been clearly distinguished from UESD in the literature.

A recent meta-analysis of seven studies, most of which were retrospective and some available only as abstracts, reported faster procedures for UESD than for CESD in the colorectum.²⁸ However, one of these abstracts,²⁹ later published as a prospective trial, reported that 5.7% of UESD procedures required the use of dual-approach ESD,³⁰ while other studies did not clarify whether the dual approach was applied. Thus, the UESD group in this meta-analysis may have included patients who were treated with dual-approach ESD. Therefore, interpreting the favorable outcomes of UESD as evidence of its superiority over CESD entails two risks: one resulting from unclear procedural definitions and the other arising from an ill-defined comparator. To advance both clinical practice and research, it is important to explicitly differentiate between CESD, UESD, and dual-approach ESD.³¹

Accordingly, this review first focuses on the technical considerations and devices for UESD, including methods to address its limitations. The second section discusses organ-specific strategies that utilize the distinctive advantages of UESD while emphasizing the importance of distinguishing it from dual-approach ESD.

BASICS OF UESD

Definition of UESD

The terms UESD⁶ and saline immersion therapeutic endoscopy⁷ have been used interchangeably to describe ESD performed in saline. However, for clarity and consistency, the present review adopted “UESD” as the standard term. Currently, the stage at which immersion should be initiated during UESD is not clearly defined. However, immersion is most commonly introduced during submucosal dissection,³⁰^,³²^,³³ as most of its benefits are achieved at this stage.⁸ In this article, UESD is defined as a technique in which submucosal dissection is performed underwater, whereas marking, submucosal injection, mucosal incision, and hemostasis of the vessels at the resection site can be performed either under gas insufflation or underwater. Complete UESD is defined as a technique in which all the abovementioned procedures are performed underwater.

Definition of dual-approach ESD

UESD can be switched to the CESD at any moment without the need for special devices. Dual-approach ESD is a strategy in which the operator switches between CESD and UESD according to procedural demands.³¹ Dual-approach ESD can be further categorized as planned or unplanned. Planned dual-approach ESD is a proactive strategy in which both techniques are deliberately combined to maximize their respective advantages. In contrast, unplanned dual-approach ESD refers to reactive switching when continuation of the initial method becomes difficult, serving as a rescue approach.

Rationale for using saline instead of water in UESD

Saline is generally used instead of water to create underwater conditions in UESD for the following reasons⁷^,⁸^,³⁴: (1) water is hypotonic and easily penetrates the submucosa, which lowers electrical conductivity and interferes with effective electrocautery, whereas saline provides sufficient conductivity; (2) the hypotonic nature of water may cause tissue and specimen friability, whereas isotonic saline is safer; (3) water can cause water intoxication, particularly in duodenal procedures; and (4) saline provides greater buoyancy than water.

Classification of lesion position relative to gravity

Liquids accumulate on the gravity side, whereas gases accumulate on the opposite side (Fig. 1). Therefore, a good field of vision can be obtained on the gravitational side in UESD and on the opposite side in CESD. Additionally, buoyancy acts in the direction opposite to gravity, and buoyancy and gravity can be employed as traction for lesions on the gravity and opposite sides of gravity, respectively (Fig. 2). Therefore, it is essential to consider the relationship between lesions and gravity when determining the indications for UESD. In this article, the lumen was divided into three equal parts according to the direction of gravity: the gravity side, the intermediate side, and the opposite side of gravity. The latter two are collectively classified as the non-gravity side.

Advantages of UESD

1) Improved visualization on the gravity side

In CESD, gravity-side lesions tend to be partially submerged, creating a gas–liquid interface that impairs visualization (Fig. 2A). In contrast, UESD eliminates the interface and enables a clear visual field (Fig. 2B). Conversely, on the opposite side of gravity, a gas–liquid interface is problematic in UESD (Fig. 2C) but is avoided in CESD (Fig. 2D).⁸^,³⁰

2) Buoyancy traction

When the lesion is on the gravitational side, the buoyancy acts in a direction that opens the mucosal flaps and provides natural traction on the dissection plane (Fig. 2B). However, buoyancy impedes the opening of the mucosal flap on the opposite side of gravity (Fig. 2C), whereas gravity offers natural traction (Fig. 2D).⁸^,³⁰

3) Water pressure traction

The water jet pumped from the endoscope opens the incised mucosal edges without splashing, making it easier to approach the submucosa. Moreover, traction can be added to the dissection plane (Fig. 2B).⁹

4) Magnification and anti-fog effect

Underwater conditions provide a magnified visual field without halation, making it easier to recognize the submucosa, even with severe fibrosis, while preventing lens fogging due to fat scattering.⁸

5) Heat sink effect

An ex vivo study suggested the existence of a heat sink effect in underwater endoscopic resection.³⁵ This effect may be achieved with UESD and further enhanced when full submersion is maintained (complete UESD), thereby potentially reducing the incidence of PECS.¹²

6) Low-pressure endoscopy

The usefulness of low-pressure endoscopy with gel immersion has been reported.³⁶ Saline immersion may also provide the same effect, because saline is less diffusive than gases and can distend the gastrointestinal tract locally. This may reduce abdominal distension, improve endoscope maneuverability, support access to narrow spaces, and facilitate access to areas that are difficult to reach under gaseous conditions. However, these effects are based on clinical experience and should be validated in future studies.

Devices for UESD

1) Endoscopes with a water jet function

The water jet function is essential for water pressure traction and maintenance of the visual field. Additionally, a wide angulation range and a large accessory channel are preferable.

2) Pumps

A pump with an adjustable water volume, such as the OFP-2 (Olympus), is recommended. When used with endoscopes from the same manufacturer, this pump enables the waterjet switch to be placed on the endoscope button rather than on the foot pedal, which is advantageous in underwater techniques.¹⁷^,²⁴

3) Hoods

UESD is effective for treating lesions in narrow spaces or with thin fibrotic submucosa, and tapered hoods can further enhance these advantages. Although tapered hoods provide lower visibility than straight hoods owing to their narrow openings and longer tips, underwater magnification mitigates these limitations. Tapered hoods, such as the short small-caliber tip, transparent hood (short ST hood; Fujifilm Medical, Fig. 3A), and the ST hood (Fujifilm Medical; Fig. 3B),³⁷ are widely used under both gas and underwater conditions. The calibrated small-caliber-tip transparent hood (CAST hood; Top, Fig. 3C), originally developed for small intestinal strictures in Crohn’s disease,³⁸ has also been adopted for UESD. Its conical shape facilitates submucosal access even with severe fibrosis; however, visibility is poor under gas insufflation, particularly in wide lumens, owing to its extremely narrow tip. Therefore, this hood is recommended for underwater conditions.³⁹ If a straight hood is used under immersion, a longer protrusion type is recommended because the underwater magnification effect may obscure the hood tip on the monitor if the protrusion is short.

4) Electrosurgical units

VIO300D (Erbe) or VIO3 (Erbe) is commonly used for UESD because it provides stable cutting and coagulation functions, even under underwater conditions. In this review, the names of the cutting and coagulation modes of these electrosurgical units (ESUs) are used.

5) Electrosurgical knives

As saline is highly conductive and easily dissipates currents, a high current density is required for an electrosurgical knife. For needle-type knives, the DualKnifeJ (0.65-mm tip diameter, Olympus) can be used with the same ESU settings as under gas insufflation, whereas the FlushKnife ball-tip type (0.9-mm tip diameter, Fujifilm Medical) requires a higher ESU output.⁸ The bipolar Jet-B Knife (Zeon Medical) can also be used under the same ESU settings.³⁴ Although the HookKnifeJ (Olympus) cuts poorly in the coagulation mode (e.g., Swift coagulation), it is effective in the cut mode (e.g., Endocut I). Precoagulation of vessels with the Soft coagulation, followed by cutting in the cut mode, helps avoid bleeding.⁴⁰ An insulated-tip knife (Olympus) is ineffective in saline, but becomes usable by modulating the electrical flow with diluted saline (1:2 saline:distilled water).⁴¹ Among scissor-type knives, the SBKnifeJr2 (SB-Kawasumi) can be used in saline,⁴² while the ClutchCutter (Fujifilm Medical) is better used with the dedicated gel Viscoclear (Otsuka Pharmaceutical Factory).⁴³

6) Hemostatic forceps

Bipolar hemostatic forceps, such as the Tighturn (Zeon Medical) and HemoStat-Y (Pentax) forceps, can be used even under underwater conditions at the same ESU settings as under gas insufflation conditions.⁸^,³⁴ In contrast, for monopolar hemostatic forceps such as the Coagrasper (Olympus), a higher ESU setting is required in underwater conditions to achieve sufficient coagulation compared to gas insufflation conditions.

7) Continuous liquid-suction catheter

The Endosweeper (Yamashina Seiki), which is attached to the endoscope and connected to a suction connector, enables continuous suction independent of the built-in suction channel of the endoscope, thereby reducing excessive saline accumulation and cloudiness.⁴⁴

8) Gel

While saline clouds the field when mixed with blood, gel maintains clear visibility by preventing mixing owing to its viscosity.⁴⁵ Recently, gel-immersion ESD (GI-ESD)²¹ and the combined use of gels and UESD have been reported.⁴⁶ Viscoclear (Otsuka Pharmaceutical Factory) is commonly used for GI-ESD because it is isotonic and viscous, prevents tissue penetration, and its low electrical conductivity allows monopolar devices to cut and coagulate.

9) Traction devices

In UESD, the lumen is usually narrowed owing to deaeration; therefore, devices that can maintain traction strength, even in a confined lumen, are desirable. The selectivity of the traction direction is also important. In the colon, devices that do not require reinsertion of the endoscope are preferable.⁴⁷

COUNTERMEASURES FOR BLEEDING DURING UESD

Bleeding without visual field loss

If mild bleeding is observed, similar to a beacon, identifying the bleeding point is easy. It is possible to achieve hemostasis under underwater conditions while maintaining visibility by supplying a waterjet as needed. To maintain visibility, a flow system using two pumps was proposed. In this system, an assistant produces a continuous low flow, and endoscopists apply a high flow in the event of bleeding.²²

Bleeding with visual field loss (red-out)

Severe bleeding causes the monitor to turn red, requiring gas insufflation to visualize the bleeding point, followed by hemostasis. However, this approach delays timely hemostasis and may require saline replacement once the visual field becomes cloudy. In contrast, underwater or under-gas compression hemostasis using a hood combined with irrigation restores visibility and buys time for intervention, preventing the saline from becoming cloudy.²⁴^,⁴⁸ However, the compression force should be carefully controlled to avoid mechanical perforation, particularly in the duodenum. The choice between underwater and under-gas compression depends on the relationship between the lesion and gravity, as liquid gathers on the gravity side after gas insufflation (Fig. 4).

1) Bleeding point on the gravity side

Even with gas insufflation, the bleeding point may be covered with blood, which makes identification difficult, particularly when excess saline accumulates. Therefore, the underwater compression hemostasis method is effective (Figs. 4A–C, 5).²⁴ Gel immersion hemostasis may also be beneficial.²¹^,⁴⁵^,⁴⁶ In the colorectum, although time-consuming, changing the patient’s posture to move the blood away from the bleeding point, combined with gas insufflation, can help identify the bleeding point.

2) Bleeding point on the non-gravity side

When the gas is insufflated, the liquid moves to the gravity side, allowing identification of the bleeding point, and under-gas compression hemostasis is effective (Fig. 4D–F).⁴⁸ However, excess saline can prevent displacement from the bleeding point, even with gas insufflation; therefore, appropriate suction is advisable. Underwater compression hemostasis can also be used to eliminate the need for degassing after hemostasis.

Flush-assisted presealing/precoagulation and flow-assisted coagulation techniques

A recent report demonstrated that flushing from the waterjet channel of the J-type knife (ClearCut Knife J-type, 1.5 mm; Finemedix) around the knife tip enables “presealing” of vessels without the sparking that causes undesired cutting.⁴⁹ Another report showed that flushing generated by the EIP2 (Erbe) also provides “precoagulation” of vessels even at a high radiofrequency output.⁵⁰ A similar precoagulation technique can be achieved using the system for the gas-free immersion dissection technique.⁵¹ These techniques allow for prophylactic vessel coagulation under saline immersion before incision. Furthermore, the knife-coagulated cut,⁵² widely applied in CESD, can also be used underwater. The appropriate application of these techniques requires further investigation.

COUNTERMEASURES FOR BUBBLES DURING UESD

During mucosal incision and submucosal dissection in UESD, bubbles form around the tip of the electrosurgical knife, impairing visualization, particularly when trapped inside the hood. These bubbles must be removed using suction or water jets. Because the coagulation mode (e.g., Swift coagulation) generates more bubbles than the cut mode (e.g., Endocut I), the cut mode is preferable in areas with few vessels. However, the cut mode is less feasible when vessels are abundant because insufficient coagulation increases the risk of bleeding. Novel solutions have recently been reported, as described below.

Special devices for bubble removal

1) Hoods with wide holes

As the currently commercially available hoods are designed for use under gas conditions, small side holes are created to drain the liquid by capillary action. However, under underwater conditions, wider holes are required to remove bubbles efficiently (Figs. 6, 7).¹⁵^,¹⁶ Although bubbles larger than the holes are difficult to remove, they can be efficiently cleared using the continuous low water pressure dissection technique¹⁷ described in “Flow-assisted dissection for bubble removal” below.

2) Automatic irrigation pump

The EIP2 (Erbe) is an endoscopic water irrigation pump that automatically irrigates water from the tip of the endoscope in response to the activation of the VIO3 (Erbe), thereby promoting bubble removal.¹⁹

3) Paired-pump immersion system

This system uses two flushing pumps. The one used by the operator is connected to the endoscope’s water supply channel as usual, whereas the other, operated by the assistant with the minimum output, is connected to an accessory channel via a BioShield irrigator (STERIS), allowing a wider water stream from the accessory channel, aligned with the ESD device, to remove bubbles.²⁵

Flow-assisted dissection for bubble removal

A recent report showed that activating an electrosurgical knife under continuous saline flow produces different effects, which may depend on the flow strength. Strong flows impair cutting, whereas weak flows preserve cutting while minimizing bubbles.¹⁷^,⁵³ A combination of weak flow and specific hood structures can spontaneously clear bubbles within the hood. Such techniques are classified as flow-assisted dissection. Hood design, such as volume and side holes, may influence bubble removal efficiency, although the underlying mechanism remains unclear. Flow-assisted dissection can be classified into three types based on the flow source.

1) Flow from solution infusion for lens clearance

Gas-free saline immersion dissection technique: The combination of saline flow generated by firmly pressing the air/water valve and a CAST hood (Top) enhances bubble clearance, but CO₂ insufflation must be prevented from mixing with saline. Two approaches have been reported: closing the CO₂ hole in the bottle cap with a plug¹⁸ or using a pressure infusion bag.²³

2) Flow from the endoscope’s water jet

Continuous low water pressure dissection technique: Even when a hood with wide holes¹⁵ is used, bubbles may still accumulate within the hood when the tissue obstructs the bubble outlet, large bubbles form, or the lesion is located opposite to gravity, necessitating repeated waterjet activation. However, this technique enables spontaneous removal of bubbles entering the hood without repeatedly turning the water jet on and off, thereby streamlining the procedure (Fig. 8).¹⁷

3) Flow from the endoscope’s accessory channel

Continuous irrigation method: This method utilizes the flow delivered through the accessory channel of the endoscope connected to a pump via a BioShield irrigator (STERIS) with an extension tube. This flow, combined with a CAST hood (Top), can remove bubbles.²⁰

UESD ADVERSE EVENTS

Common adverse events

A previous meta-analysis found no significant differences between CESD and UESD in the incidence of perforation, delayed bleeding, and PECS.²⁸ However, this meta-analysis may have included dual-approach cases; therefore, the findings should be carefully interpreted.³¹ In the upper gastrointestinal tract, the differences in the incidence of adverse events between CESD and UESD are unclear because of a lack of comparative studies. However, large amounts of saline may cause reflux or absorption in the small intestine, leading to aspiration pneumonia or electrolyte abnormalities. Although a retrospective study of the duodenum showed a similar incidence of aspiration pneumonia between CESD and UESD (1.2% vs. 2.3%) and no incidence of electrolyte abnormalities,⁵⁴ these issues should be evaluated in a large number of cases in the future. Although concerns have been raised regarding recurrence due to dissemination after perforation when UESD was first developed, no such reports were found in the literature search for this review.

Explosion

One case report described a minor explosion during UESD.⁵⁵ In this study, an audible explosion occurred when submucosal dissection was performed near the trapped bubbles; however, no perforation occurred. Bubbles generated by electrocautery may contain vapor⁵⁶ and possibly hydrogen,⁵⁷ which becomes explosive only when mixed with oxygen and triggered by a spark.⁵⁸ As hydrogen–oxygen mixtures can be explosive, an endoscopic gas insufflation system without oxygen might be safer. Therefore, further investigations, including gas analyses, are warranted.

ORGAN-SPECIFIC UESD

This chapter provides an overview of UESD for each organ. UESD has been reported mainly in the colorectum, where several randomized controlled trials (RCTs) have been conducted. Most reports on the laryngopharynx, esophagus, stomach, and duodenum are case reports. This chapter reviews UESD of the colorectum, duodenum, stomach, esophagus, and laryngopharynx.

Colorectal UESD

Colorectal ESD is more challenging than esophageal or gastric ESD because endoscope maneuverability is demanding, and the colorectal lumen is angulated, making the procedure time-consuming. Moreover, PECS is a common adverse event, and the muscle layer is thin and easily perforated.

Recently, a meta-analysis demonstrated that UESD was faster but had comparable rates of en bloc resection, R0 resection, and adverse events to CESD.²⁸ However, this meta-analysis may not represent a pure comparison between UESD and CESD. Instead, it may compare “UESD plus dual-approach ESD” with CESD. In other words, the comparative framework itself may have been distorted; thus, the findings should be interpreted in light of this limitation. Such distortions stem from the lack of a clear definition of UESD (i.e., misclassification bias), and similar issues have been repeatedly reported in previous epidemiological and clinical studies. In the present analysis, various strategies, including the CESD-dominant, mixed, and UESD-dominant dual approaches, as well as pure UESD, may have been treated as a single category. Future research should clearly distinguish between these approaches when making comparisons with CESD.³¹

Although two RCTs compared CESD and UESD with a focus on procedure-related outcomes, their results regarding procedural efficacy were inconsistent (Table 1).³⁰^,³² Both were single-center studies performed by one experienced endoscopist. One RCT³² comparing CESD (28 cases) with UESD (28 cases) reported a significantly shorter procedure time (75.7 vs. 49.5 minutes) and faster dissection speed (15.2 vs. 21.9 mm²/min) with UESD, while rates of R0 resection and adverse events were comparable. In contrast, another RCT³⁰ comparing CESD (69 cases) with UESD (70 cases) found no significant differences between CESD and UESD in terms of dissection speed (17.4 vs. 19.9 mm²/min), procedure time (55.5 vs. 48.3 minutes), R0 resection rates, or adverse events. To the best of our knowledge, no other studies have clearly reported the proportion of UESD cases requiring dual-approach ESD, which was 5.7% in this study. Two factors are suggested as possible reasons why this RCT did not demonstrate a significantly faster dissection speed in the UESD group. First, the bubbles generated during UESD impaired the visual field, particularly when they accumulated within the hood. Once trapped, they must be removed to restore visibility, a step that is occasionally difficult and time-consuming. Second, in the UESD group, the interval from the initiation of hemostasis to the resumption of ESD tended to be longer, along with a higher frequency of hemostasis. However, multiple regression analysis revealed that a suitable positional relationship between the lesion and the direction of gravity (non-gravity side for CESD and gravity side for UESD) was independently and positively associated with dissection speed. This result suggests that CESD and UESD may be complementary in the colorectum, depending on the lesion-gravity relationship, and having the lesion on the gravity side may enhance the effectiveness of UESD.

Submucosal fibrosis is a major challenge in colorectal ESD,³ as it hinders the identification of the dissection plane and deployment of the mucosal flap. UESD can be advantageous in such cases, offering traction from the water pressure and a magnified view without halation. A retrospective study compared UESD (n=54) and CESD (n=79) for 133 fibrotic lesions.¹¹ Severe fibrosis was observed in 72.2% of the cases. The median procedure time was significantly shorter with UESD (43.5 vs. 72 minutes), and multivariate analysis identified UESD as an independent factor associated with a reduced procedure time. R0 resection and adverse event rates, including PECS and perforation, were comparable. Although the study was limited by a shift from CESD to UESD during the study period, which likely favored UESD through the learning curve, the findings suggest that UESD reduces the procedure time for fibrotic lesions without increasing the risk of adverse events.

UESD can also be combined with other methods. PCM with a saline pool in the submucosal pocket is termed saline-pocket ESD (SP-ESD). A single-center RCT⁵⁹ comparing CESD (n=45) with SP-ESD (n=46) by one endoscopist showed a significantly faster median dissection speed in the SP-ESD group (16.3 vs. 20.1 mm²/min). However, this trial compared SP-ESD with CESD, but not PCM, which demonstrated faster dissection than CESD in a meta-analysis.⁶⁰ Thus, it remains unclear whether the faster speed is derived from PCM, saline immersion, or both, and further studies comparing PCM and SP-ESD are needed. Moreover, UESD with PCM has also been reported to be distinct from SP-ESD,¹⁴^,⁶¹ and differences in the location of the saline immersion—pocket only versus pocket and lumen—require clarification.

The effectiveness of the combined use of device-assisted traction and UESD has been reported, especially in difficult cases, such as lesions on the gravity side with difficulty in changing the patient’s posture,⁶² diverticulum-associated lesions,¹³ peri-appendiceal lesions,²⁶ and lesions with submucosal fibrosis⁶³ or poor endoscope maneuverability.⁶⁴ When selecting a traction device for colorectal UESD, a device that does not require endoscope reinsertion and has a traction direction-selectable function with sufficient traction force even in a collapsed lumen is desirable.⁶⁵ Although the clip-with-line pulley method requires endoscope reinsertion, it can be an effective option for rectal lesions.⁶²

Although PECS is a common adverse event of colorectal ESD, there is currently no established method for preventing PECS.⁴^,⁵ An ex vivo study suggested that a heat sink effect exists,³⁵ which may prevent PECS by reducing damage to the muscle layer. Propensity score matching analysis revealed that the incidence of PECS in the UESD group was significantly lower than that in the CESD group (0% vs. 11.1%).¹² Although it remains unclear whether a dual-approach ESD was used in this study, complete UESD is likely to maximize the heat sink effect. Further RCTs focusing on PECS are required to evaluate this issue.

In summary, UESD appears useful for gravity-side and fibrotic lesions but is less effective for those on the opposite side of gravity or with frequent visual field impairment due to bleeding or bubbles. Although dual-approach ESD may be effective in clinical practice, it should be considered separately when comparing CESD to UESD. Given the discrepancies between the two single-center RCTs, a multicenter RCT with defined protocols and evaluation of the maintenance of immersion during the procedure is warranted.

Duodenal UESD

Duodenal ESD is highly challenging owing to the fragile muscle layer, limited endoscope maneuverability, and difficulty in accessing the submucosa; therefore, it is performed only in select high-volume centers. A meta-analysis showed that duodenal ESD achieved acceptable outcomes in terms of en bloc resection (98.1%) and R0 resection (86.3%) rates but was associated with a high incidence of intraoperative perforation (8.5%) and delayed perforation (2.0%).⁶⁶ Therefore, safer and more effective methods for duodenal ESD must be developed.

A retrospective study evaluated duodenal ESD performed by a single expert (n=246) by dividing the period into early, mid, and late phases.¹⁰ UESD was not performed in the early phase, but was applied in all late-phase cases. The electrosurgical knives used were the DualKnife (without injection function) and the DualKnifeJ (with injection function); the latter was applied in 39% of early-phase cases and all late-phase cases, comprising 78% overall. The mean lesion size was 29.4 mm, and the overall R0 resection rate was 84.1%. The median procedure time was significantly longer in the early phase than in the other phases (60 vs. 40 minutes). Perforation rates were lowest in the late phase (8.6%); however, the difference among the groups was not significant. Multivariate analysis showed that UESD reduced intraprocedural perforations, and both UESD and DualKnifeJ use were independently and negatively correlated with procedure time. Despite the limitations of its retrospective design and single-operator setting, this study indicated that UESD may be a safe and effective option for duodenal lesions.

However, concerns remain regarding adverse events due to reflux and influx of large amounts of saline during duodenal UESD. A retrospective study comparing 43 UESD and 83 CESD cases reported a significantly higher incidence of intraoperative fecal incontinence with UESD (28% vs. 0%).⁵⁴ Multivariate analysis identified a saline infusion rate of ≥17 mL/min as a significant risk factor for fecal incontinence. Although an overtube was not used, no significant differences were observed in the incidence of oral regurgitation (14% vs. 4.8%) or aspiration pneumonia (2.3% vs. 1.2%). No cardiac or renal failure, or postoperative electrolyte abnormalities were observed. In the UESD group, the median lesion diameter was 20 mm, and the median resection time was 55 minutes. Although no serious adverse events owing to reflux or saline influx occurred, caution is warranted when dealing with large lesions.

Under gas insufflation, the PCM and device-assisted traction methods have been reported to be effective for duodenal ESD. In a retrospective study, PCM demonstrated lower perforation rates, faster dissection speeds, and higher en bloc resection rates than CESD.⁶⁷ Moreover, a propensity score-matched study showed that patients treated with clip-and-line-assisted ESD had a higher R0 resection rate than those treated with CESD.⁶⁸ Although these methods may be ineffective in certain situations, such as lesions on the gravity side or in an angulated and narrow lumen, combining these methods with UESD may be effective in such situations. Indeed, one case report demonstrated the feasibility of UESD combined with PCM and traction techniques for duodenal ESD.⁶⁹

Although UESD, PCM, device-assisted traction, and their combinations are useful, there are areas in the duodenum in which ESD is challenging when using resection techniques alone. For example, the duodenal bulb just behind the pyloric ring is challenging to achieve a stable endoscopic approach. However, UESD with a thin therapeutic endoscope (EG-840TP; Fujifilm Medical) may be a useful option in this area. Although its diameter is only 7.9 mm, this endoscope features a 3.2-mm accessory channel, waterjet function, and a wide downward angulation of up to 160°, allowing a stable approach even in the duodenal bulb just behind the pyloric ring.⁷⁰

Bubbles can cause visual field impairment, especially when they are trapped inside the hood. However, a hood equipped with wide holes serving as bubble outlets may offer a potential solution,¹⁵ as demonstrated in a report of complete resection of a subcircumferential duodenal tumor using this device (Figs. 6, 7).¹⁶ The continuous low water pressure dissection technique can further enhance bubble removal using this hood.¹⁷ Although this device is not yet commercially available, continued refinement of hood design and dissection techniques may ultimately enable bubble-related visual impairments to be overcome.

Although bleeding-induced visual field impairment can hinder duodenal UESD, gel immersion may help address this issue. A retrospective study comparing 49 GI-ESD procedures with 51 CESD procedures demonstrated that muscle layer exposure was significantly lower with GI-ESD than with CESD (1.9% vs. 16.7%).⁴³ No perforation was observed in the GI-ESD group, and the R0 resection rates were similar (90.2% vs. 92.6%). Although Viscoclear (Otsuka Pharmaceutical Factory) was used as the gel and entailed a higher cost (approximately 15 United States dollars per 200 mL bag at current exchange rates) than saline, it may enhance procedural safety and potentially reduce overall medical costs.

In conclusion, UESD appears to be a promising approach for resecting duodenal lesions. The combination of UESD with a PCM or device-assisted traction may be an option in difficult situations. However, caution is warranted regarding adverse events associated with large-volume saline infusions, such as electrolyte abnormalities and aspiration pneumonia.

Gastric UESD

Technical challenges of gastric ESD include sites that hinder endoscope access and gravity-side locations where the mucosal flap opening is limited and fluid-related visual impairment is more likely to occur. Furthermore, submucosal fibrosis due to ulcer scars may occasionally hinder dissection. Multibending endoscopes⁷¹ and direction-selectable traction devices⁷²^-⁷⁷ have been reported to address these problems. UESD may also offer potential solutions; however, gastric ESD is more prone to severe bleeding than ESD in other organs, making it difficult to maintain saline immersion. Therefore, the application of UESD in the stomach has not yet become common practice and remains largely limited to case reports.

Lesions in the greater curvature of the gastric fundus are difficult to approach using an endoscope and lie on the gravity side. The muscle layer may also face vertical to the knife tip, increasing the risk of perforation; thus, ESD in this area is highly challenging. Nevertheless, two case reports demonstrated the effectiveness of combining traction devices with dual-approach ESD.⁷⁸^,⁷⁹ In both cases, UESD was used in the first half and a traction device under gas in the second half, showing that the underwater technique allowed closer access to the lesion and a parallel approach to the muscle layer, while minimizing visual field impairment due to partial submersion.

Rare cases with special anatomical features, such as an upside-down stomach (a rare type of esophageal hiatal hernia), can make close access to the lesion difficult under gas insufflation. Although CESD can be performed by deflating the stomach, fluid often accumulates around the lesion and impairs the visual field. A case report using UESD showed that complete submersion provided a clearer view and facilitated access to the lesion.⁸⁰ Similarly, lesions straddling the pyloric ring are difficult to approach on the anal side with CESD. However, one case report demonstrated that a degassed and collapsed lumen in UESD enabled visualization and dissection of the anal side. Furthermore, redouts caused by unexpected bleeding can be managed using gel immersion.⁴⁶

A case report demonstrated the effectiveness of dual-approach ESD for 70-mm early gastric cancer involving the posterior wall of the upper gastric body.⁸¹ Owing to its large size, the lesion extended across both the gravitational and non-gravitational sides. UESD is particularly effective for resecting the edge on the greater curvature side (gravity side), providing improved visualization through complete submersion and facilitating mucosal flap opening by buoyancy.

A case report demonstrated that dual-approach ESD successfully achieved complete resection of a 15-mm early gastric cancer adjacent to a previous ESD scar.⁸² In this case, severe submucosal fibrosis made it difficult to access beneath the mucosal flap with CESD. Therefore, the procedure was switched from CESD to UESD, with the straight hood replaced with a tapered hood (ST hood; Fujifilm Medical). Water pressure from the endoscope facilitated entry beneath the mucosal flap, while the natural underwater zoom effect enabled precise dissection.

PCM with a saline pool in the submucosal pocket is termed water pocket ESD (WP-ESD) of the stomach. A propensity score-matched study conducted by a single endoscopist demonstrated a significantly shorter procedure time and faster dissection speed with WP-ESD than with CESD.⁸³ However, maintaining underwater conditions during gastric ESD is challenging due to frequent bleeding. Despite this challenge, it was not specified whether dual-approach ESD was required. Moreover, aside from this report, no other studies have directly compared CESD and UESD of the stomach. Therefore, further investigations are required to confirm the feasibility of this method in gastric settings.

In summary, the dual-approach ESD is a valuable treatment option for patients with gastric lesions. UESD is particularly helpful in difficult situations, such as sites that hinder endoscope access and gravity-side locations, with the option of switching back to CESD once these challenges have been overcome. The combination of UESD with traction devices or gels may further enhance its effectiveness.

Esophageal UESD

In esophageal ESD, traction methods such as the clip-with-line method and tunneling method have shown favorable outcomes, even under gas insufflation.⁸⁴ However, compared with other organs, the esophagus has a narrower working space, limiting endoscopic maneuverability. Moreover, gravity-related difficulties hinder the procedure, particularly in the left wall. Although concerns persist regarding the risk of aspiration due to reflux, UESD may represent a solution.

A case series (n=9) reported that UESD could be performed without adverse events, with the aspiration risk reduced by using an overtube or endotracheal intubation.⁸⁵ Another case series reported that GI-ESD was attempted in 15 patients with 16 superficial esophageal cancers, but could not be completed in three cases.⁸⁶ Two of these were in the upper esophagus and were incomplete owing to aspiration, although no oxygen saturation drop occurred. Among the cases in which GI-ESD was completed, the median procedure time was 27 minutes, the median dissection speed was 20 mm²/min, the median tumor size was 25 mm, and R0 resection was achieved in all cases without perforation. Most lesions were located on the left or posterior esophageal walls, where the gel tended to collect because of gravity. In contrast, GI-ESD was challenging for a lesion on the right wall, on the opposite side of gravity, which caused visual field impairment owing to the stagnation of air and bubbles. Collectively, these studies suggest that UESD and GI-ESD may be options for selected situations.

Because the lumen of the cervical esophagus is narrow, securing the visual field under gas insufflation is challenging. However, underwater conditions may facilitate securing the visual field in narrow lumens. A case report showed that superficial esophageal cancer involving one-third of the lumen in the cervical esophagus could be resected en bloc using UESD, while general anesthesia and tracheal intubation were applied to prevent the risk of aspiration.⁸⁷

A distinctive application of UESD using the underwater magnification effect to adjust the depth of dissection has been reported. In esophageal ESD, the submucosal esophageal glands serve as landmarks where superficial squamous cell carcinomas may extend into their ducts; thus, dissection beneath the glands is desirable. Although this can be challenging under gas insufflation, UESD facilitates dissection beneath the glands through buoyancy and the natural zoom effect.⁸⁸ Furthermore, underwater dissection just below the glands may help avoid unnecessary deep cuts and prevent muscle injury,⁸⁹ a known risk factor for post-ESD stricture.⁹⁰

ESD for recurrent lesions around post-ESD scars is challenging because of fibrosis and carries a high risk of perforation. However, the combination of UESD with other techniques may overcome these difficulties. One report described the safe resection of a circumferential lesion with post-ESD scars using UESD with a clip-with-line and the tunneling method, avoiding muscle injury and perforation.⁹¹

In summary, esophageal UESD may be useful for gravity-side lesions and cases of narrow lumens or severe fibrosis. It can also be combined with traction devices or tunneling method to improve its efficacy. However, UESD is challenging on the non-gravity side, such as the right esophageal wall. Therefore, dual-approach ESD is recommended for extensive lesions involving both the gravity and non-gravity sides. To reduce the risk of aspiration, UESD should be performed with an overtube or under general anesthesia.

Laryngopharyngeal UESD

Laryngopharyngeal ESD can be performed under general anesthesia with endotracheal intubation. However, the narrow working space and poor subepithelial visibility due to the complex laryngopharyngeal anatomy hinder this procedure. Moreover, gravity further complicates the dissection when the lesion is located on the posterior wall. Traction methods, such as clip-with-line, double-scope, or laryngeal forceps, have been used to overcome these difficulties. However, the clip-with-line method limits the traction direction, and interference between the endoscopes or forceps can complicate the procedures.⁴⁷ In contrast, the UESD may provide natural traction via buoyancy and water pressure while improving visibility in the narrow space.

A case series (n=21) reported that UESD can be safely and effectively performed for pharyngeal squamous cell carcinoma.⁹² Subcutaneous emphysema occurred in one patient, but laryngeal edema, post-ESD bleeding, and aspiration pneumonia were absent. En bloc resection was achieved in all patients, with an R0 resection rate of 76.2%. The median tumor size was 10 mm, and the median procedure time was 15 minutes.

These case reports further highlight the usefulness of UESD. Combined with clip-with-line traction, UESD was effective for treating lesions in the left pyriform sinus.⁹³ In the limited epiglottic region, UESD also improved the visibility of the dissection plane and procedural efficiency, with nasal intubation using a small-bore tracheal tube to minimize space occupation.⁹⁴ For recurrent lesions adjacent to post-ESD scars, severe fibrosis poses additional challenges, but UESD enables en bloc resection of hypopharyngeal carcinoma adjacent to the post-ESD scar using the natural zoom effect for layer recognition and water pressure traction for subepithelial access.⁹⁵ Moreover, UESD techniques have been applied in endoscopic laryngopharyngeal surgery, and their usefulness has been demonstrated in a case series.⁹⁶

In conclusion, laryngopharyngeal UESD may be feasible under general anesthesia with intubation and useful for challenging lesions. However, as evidence is limited, further studies are needed to validate its safety and optimal application.

DISCUSSION

Since its introduction in 2016, UESD has emerged as an innovative method for the resection of superficial gastrointestinal neoplasms. Initially applied in the colorectum and duodenum, it is now used in the laryngopharynx, esophagus, and stomach. Its distinctive advantages, including buoyancy-assisted mucosal flap opening, improved visualization in gravity-dependent areas, and applicability in difficult settings, such as fibrosis, fatty tissue, narrow lumens, or poor endoscope maneuverability, have broadened the clinical applicability of ESD. UESD can also be effectively combined with traction devices or PCM, and serves as a rescue option when CESD is difficult.⁹⁷

However, UESD presents specific technical challenges, particularly those related to visual field impairments. Bubble formation is inevitable during underwater procedures, even when the gas insufflation system is turned off, and bubbles tend to accumulate on the opposite side of gravity or within the hood, thereby obstructing visualization. To mitigate this, devices and techniques such as wide-hole hoods,¹⁵ automated irrigation,¹⁹ paired-pump immersion systems,²⁵ and flow-assisted dissection¹⁷^,¹⁸^,²⁰ have been proposed. Nevertheless, bubbles rise against gravity and may coalesce or burst,⁹⁸ potentially leading to localized gas accumulations (“air pockets”) that repeatedly hinder complete submersion,³⁰ especially when treating lesions on the opposite side of gravity. Bleeding presents another challenge: severe hemorrhage can cause visual field loss and delayed hemostasis because visualization requires reintroduction of gas insufflation, and bleeding points can be identified. Although novel solutions, such as the underwater compression hemostasis method²⁴ and gel immersion,²¹^,⁴⁶ can be helpful in the management of bleeding, the effectiveness of these solutions needs to be validated in further studies.

Owing to these challenges, UESD has not yet been widely adopted in clinical practice. However, because CESD and UESD can be switched at any time, dual-approach ESD represents a flexible and effective strategy that leverages the strengths of both CESD and UESD. Rather than considering UESD as a replacement for CESD, it may be more reasonable for clinicians to select each method based on situational advantages and procedural demands. Many case reports reviewed in this article illustrated or adopted a dual-approach ESD strategy.

One of the key determinants in choosing between CESD and UESD is the lesion-gravity relationship. On the opposite side of gravity, CESD is favorable because gravity provides mucosal flap opening and traction, and fluid-related visual field impairment is avoided (Fig. 2D). On the gravity side, UESD is advantageous because submersion is readily achieved and buoyancy offers mucosal flap opening and traction (Fig. 2B). Furthermore, PCM, tunneling method, and device-assisted traction methods have demonstrated better procedure-related outcomes than CESD,⁹⁹ and these methods can be combined with gas insufflation or saline immersion under the same criteria. The adoption of this adaptive approach has the potential to improve procedural efficiency, reduce complications, and enhance resection quality. However, its successful implementation depends not only on operator proficiency in both techniques but also on the development of dual-compatible devices.

Although the introduction of UESD has expanded therapeutic options, its adoption is not without challenges. One major concern is the additional training time required to acquire proficiency in UESD, which may act as a barrier for endoscopists who have already performed CESDs. To address this issue, a revision of the current ESD training paradigm was proposed.¹⁰⁰ Specifically, simultaneous instruction in both conventional and underwater techniques during the early stages of endoscopic education may facilitate the smoother integration of UESD into clinical practice.

Although few studies have investigated the learning curve of UESD by novice endoscopists, an ex vivo study using an excised bovine rectum reported that the procedure time of UESD performed by novice endoscopists was shorter than that of CESD.¹⁰¹ However, the excised bovine rectum does not represent difficulties faced in clinical practice, such as bleeding, fibrosis, and poor endoscope maneuverability. Therefore, further studies on the learning curve of UESD in clinical settings are required.

CONCLUSIONS

In summary, UESD has expanded the therapeutic options available to endoscopists and potentially offers advantages when combined with CESD as a dual-approach strategy. However, most studies have not distinguished between UESD and dual-approach ESD, complicating comparisons with CESD. Future research should clearly differentiate between these techniques to allow for a more accurate evaluation. Furthermore, appropriate procedural selection requires careful consideration of multiple factors, including the positional relationship between the lesion and gravity. With continued refinement of devices, optimization of procedural selection criteria, combination with PCM, tunneling method, and device-assisted traction methods, and adoption of dual-method training paradigms, this integrated approach has the potential to improve clinical outcomes across a wide range of gastrointestinal lesions.

Conflicts of Interest

The author has no potential conflicts of interest.

Funding

None.

Fig. 1.

Classification of lesion position relative to gravity: gravity side, intermediate side, and opposite side of gravity. The latter two are collectively classified as the non-gravity side.

Fig. 2.

Effects of gravity on visual field and natural traction in conventional endoscopic submucosal dissection (CESD) and underwater ESD (UESD). (A) CESD for a lesion on the gravity side. Gravity causes the mucosal flap to droop and impairs the visual field due to partial submergence. (B) UESD for a gravity-side lesion, where complete submergence with buoyancy traction is readily achieved, and bubbles float without impairing the visual field. (C) UESD for a lesion on the opposite side of gravity. Buoyancy hampers the opening of the mucosal flap, while gas and bubbles accumulate around the lesion, impairing the visual field. (D) CESD for a lesion on the opposite side of gravity. Gravity facilitates mucosal flap opening and prevents fluid-related visual field impairment.

Fig. 3.

Comparison of tapered hoods. (A) Short small-caliber-tip transparent hood (8-mm opening, short ST hood, DH-28GR; Fujifilm Medical). (B) ST hood (7-mm opening, DH-33GR; Fujifilm Medical). (C) Calibrated small-caliber-tip transparent hood (4-mm opening, CAST hood; Top).

Fig. 4.

Bleeding-induced visual field loss (red-out) during underwater endoscopic submucosal dissection and the compression hemostasis method according to the lesion-gravity relationship. (A) Red-out on the gravity side. (B) With gas insufflation, blood and saline pool on the gravity side, obscuring the bleeding point. (C) Underwater compression hemostasis method, followed by coagulation with a knife or hemostatic forceps. (D) Red-out on the non-gravity side. (E) With gas insufflation, blood and saline pool on the gravity side, exposing the bleeding point. (F) Under-gas compression hemostasis method, followed by coagulation with a knife or hemostatic forceps.

Fig. 5.

Underwater compression hemostasis method during underwater endoscopic submucosal dissection. (A) Active bleeding occurred on the gravity side during submucosal dissection. (B) The visual field was immediately lost due to active bleeding. (C) The hood tip (yellow arrowheads) was compressed on the area suspected of bleeding. (D) The hood tip was moved slightly to confirm the bleeding point (yellow arrow). (E) Hemostasis was achieved using the DualKnife J (Olympus) and the preciseSECT (Effect 3.0) of the VIO3 (Erbe) in saline while compressing the bleeding point. (F) The compression was released and successful hemostasis was confirmed. Reproduced from Nagata. Endoscopy 2025;57:E257–E258, according to the Creative Commons license.²⁴

Fig. 6.

Comparison of a conventional tapered hood and tapered hood with air bubble outlets. (A) A commercially available tapered hood (ST hood; DH-33GR, Fujifilm Medical) has a narrow slit (blue arrow). (B) Three wide holes as air bubble outlets (yellow arrows) during underwater endoscopic submucosal dissection are created on the side of the conventional ST hood. The distance between each air bubble outlet and between each air bubble outlet to the hood tip opening is at least 2 mm. These holes have an area of 15–25 mm². (C) An endoscopic view under gas conditions when attaching a tapered hood with three air bubble outlets. All three air bubble outlets can be seen (yellow arrows). (D) An endoscopic view in underwater conditions when attaching a tapered hood with three air bubble outlets. Only one air bubble outlet can be seen (yellow arrow) due to refractive index changes. (E) If buoyancy (yellow arrow) has a component vectoring toward the endoscope tip, the air bubbles tend to move toward the hood, and removing air bubbles from the conventional tapered hood tip opening using water pressure from the endoscope becomes challenging. (F) Although buoyancy (yellow arrow) has a component vectoring toward the endoscope tip, the air bubbles can be efficiently removed from the air bubble outlets because their flow does not oppose the direction of buoyancy. Reproduced from Nagata. Dig Endosc 2024;36:225–227, with permission.¹⁶

Fig. 7.

Underwater endoscopic submucosal dissection using a tapered hood with air bubble outlets for a subcircumferential duodenal tumor. (A) The flat elevated subcircumferential duodenal tumor located at the superior duodenal angle is sprayed with indigo carmine. (B) Indigo carmine is pooled at the lesion’s center, indicating that gravity (blue arrow) works in the lesion’s direction and the air bubbles are expected to move toward the endoscope tip in underwater conditions due to buoyancy (yellow arrow). (C) Although air bubble outlets are added, the mucosal flap can be lifted up using the hood tapered tip, preventing the mucosal flap from entering inside the hood. (D) Visual field loss due to arterial bleeding. (E) Pressure hemostasis using the hood tip can be performed (white arrow). (F) Following pressure hemostasis, the use of bipolar hemostatic forceps (Tighturn; RH8C40, Zeon Medical) can control arterial bleeding in underwater conditions. (G) The lesion is resected en bloc without perforation. (H) A resected specimen sprayed with indigo carmine. Pathological examination revealed duodenal cancer, which is 63 mm in size, with negative lateral and vertical margins. Reproduced from Nagata. Dig Endosc 2024;36:225–227, with permission.¹⁶

Fig. 8.

Underwater endoscopic submucosal dissection using the continuous low water pressure dissection technique and a tapered hood with air bubble outlets. (A) The laterally spreading tumor in the cecum was sprayed with indigo carmine. (B) As half of the lesion was located opposite to gravity, air bubbles accumulated in the hood. (C) The continuous low water pressure dissection technique effectively minimized and removed the air bubbles. (D) The flushing pump output (OFP-2; Olympus) was adjusted to the weakest setting (yellow arrow) to maintain the appropriate water pressure for dissection power. (E) The lesion was resected en bloc without perforation. (F) The resected specimen was sprayed with indigo carmine. Pathological examination revealed high grade dysplasia (World Health Organization classification), measuring 60 mm in size, with negative lateral and vertical margins. Reproduced from: Nagata. Endoscopy 2024;56:E699–E700, according to the Creative Commons license.¹⁷

Table 1.

Procedure-related outcomes in two single-center RCTs of colorectal ESD (CESD vs. UESD)

Study	Year	Cases (n)	Procedure time (min)	p-value	Dissection speed (mm²/min)	p-value	R0 resection (%)^a)	p-value	Perforation (%)	Post-ESD bleeding (%)	p-value	PECS (%)	p-value
Oh et al.^³²	2024	28/28	75.7/49.5^b)	0.001	15.2/21.9^b)	0.001	100/100		0/0	3.6/3.6		14.3/3.6	0.152
Nagata et al.^³⁰	2025	69/70	55.5/48.3^c)	0.38	17.4/19.9^c)	0.19	97.1/100	0.25	0/0	2.9/0	0.25	4.3/2.9	0.68

Values are presented as CESD/UESD unless otherwise indicated.

RCT, randomized controlled trial; ESD, endoscopic submucosal dissection; CESD, conventional ESD; UESD, underwater ESD; PECS, post-ESD coagulation syndrome.

^a)R0 resection indicates en bloc resection with histologically negative lateral and vertical margins;

^b)mean;

^c)median.

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Underwater endoscopic submucosal dissection in the gastrointestinal tract: technical review and dual-approach endoscopic submucosal dissection

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Underwater endoscopic submucosal dissection in the gastrointestinal tract: technical review and dual-approach endoscopic submucosal dissection

Fig. 1. Classification of lesion position relative to gravity: gravity side, intermediate side, and opposite side of gravity. The latter two are collectively classified as the non-gravity side.

Fig. 2. Effects of gravity on visual field and natural traction in conventional endoscopic submucosal dissection (CESD) and underwater ESD (UESD). (A) CESD for a lesion on the gravity side. Gravity causes the mucosal flap to droop and impairs the visual field due to partial submergence. (B) UESD for a gravity-side lesion, where complete submergence with buoyancy traction is readily achieved, and bubbles float without impairing the visual field. (C) UESD for a lesion on the opposite side of gravity. Buoyancy hampers the opening of the mucosal flap, while gas and bubbles accumulate around the lesion, impairing the visual field. (D) CESD for a lesion on the opposite side of gravity. Gravity facilitates mucosal flap opening and prevents fluid-related visual field impairment.

Fig. 3. Comparison of tapered hoods. (A) Short small-caliber-tip transparent hood (8-mm opening, short ST hood, DH-28GR; Fujifilm Medical). (B) ST hood (7-mm opening, DH-33GR; Fujifilm Medical). (C) Calibrated small-caliber-tip transparent hood (4-mm opening, CAST hood; Top).

Fig. 4. Bleeding-induced visual field loss (red-out) during underwater endoscopic submucosal dissection and the compression hemostasis method according to the lesion-gravity relationship. (A) Red-out on the gravity side. (B) With gas insufflation, blood and saline pool on the gravity side, obscuring the bleeding point. (C) Underwater compression hemostasis method, followed by coagulation with a knife or hemostatic forceps. (D) Red-out on the non-gravity side. (E) With gas insufflation, blood and saline pool on the gravity side, exposing the bleeding point. (F) Under-gas compression hemostasis method, followed by coagulation with a knife or hemostatic forceps.

Fig. 5. Underwater compression hemostasis method during underwater endoscopic submucosal dissection. (A) Active bleeding occurred on the gravity side during submucosal dissection. (B) The visual field was immediately lost due to active bleeding. (C) The hood tip (yellow arrowheads) was compressed on the area suspected of bleeding. (D) The hood tip was moved slightly to confirm the bleeding point (yellow arrow). (E) Hemostasis was achieved using the DualKnife J (Olympus) and the preciseSECT (Effect 3.0) of the VIO3 (Erbe) in saline while compressing the bleeding point. (F) The compression was released and successful hemostasis was confirmed. Reproduced from Nagata. Endoscopy 2025;57:E257–E258, according to the Creative Commons license.24

Fig. 6. Comparison of a conventional tapered hood and tapered hood with air bubble outlets. (A) A commercially available tapered hood (ST hood; DH-33GR, Fujifilm Medical) has a narrow slit (blue arrow). (B) Three wide holes as air bubble outlets (yellow arrows) during underwater endoscopic submucosal dissection are created on the side of the conventional ST hood. The distance between each air bubble outlet and between each air bubble outlet to the hood tip opening is at least 2 mm. These holes have an area of 15–25 mm2. (C) An endoscopic view under gas conditions when attaching a tapered hood with three air bubble outlets. All three air bubble outlets can be seen (yellow arrows). (D) An endoscopic view in underwater conditions when attaching a tapered hood with three air bubble outlets. Only one air bubble outlet can be seen (yellow arrow) due to refractive index changes. (E) If buoyancy (yellow arrow) has a component vectoring toward the endoscope tip, the air bubbles tend to move toward the hood, and removing air bubbles from the conventional tapered hood tip opening using water pressure from the endoscope becomes challenging. (F) Although buoyancy (yellow arrow) has a component vectoring toward the endoscope tip, the air bubbles can be efficiently removed from the air bubble outlets because their flow does not oppose the direction of buoyancy. Reproduced from Nagata. Dig Endosc 2024;36:225–227, with permission.16

Fig. 7. Underwater endoscopic submucosal dissection using a tapered hood with air bubble outlets for a subcircumferential duodenal tumor. (A) The flat elevated subcircumferential duodenal tumor located at the superior duodenal angle is sprayed with indigo carmine. (B) Indigo carmine is pooled at the lesion’s center, indicating that gravity (blue arrow) works in the lesion’s direction and the air bubbles are expected to move toward the endoscope tip in underwater conditions due to buoyancy (yellow arrow). (C) Although air bubble outlets are added, the mucosal flap can be lifted up using the hood tapered tip, preventing the mucosal flap from entering inside the hood. (D) Visual field loss due to arterial bleeding. (E) Pressure hemostasis using the hood tip can be performed (white arrow). (F) Following pressure hemostasis, the use of bipolar hemostatic forceps (Tighturn; RH8C40, Zeon Medical) can control arterial bleeding in underwater conditions. (G) The lesion is resected en bloc without perforation. (H) A resected specimen sprayed with indigo carmine. Pathological examination revealed duodenal cancer, which is 63 mm in size, with negative lateral and vertical margins. Reproduced from Nagata. Dig Endosc 2024;36:225–227, with permission.16

Fig. 8. Underwater endoscopic submucosal dissection using the continuous low water pressure dissection technique and a tapered hood with air bubble outlets. (A) The laterally spreading tumor in the cecum was sprayed with indigo carmine. (B) As half of the lesion was located opposite to gravity, air bubbles accumulated in the hood. (C) The continuous low water pressure dissection technique effectively minimized and removed the air bubbles. (D) The flushing pump output (OFP-2; Olympus) was adjusted to the weakest setting (yellow arrow) to maintain the appropriate water pressure for dissection power. (E) The lesion was resected en bloc without perforation. (F) The resected specimen was sprayed with indigo carmine. Pathological examination revealed high grade dysplasia (World Health Organization classification), measuring 60 mm in size, with negative lateral and vertical margins. Reproduced from: Nagata. Endoscopy 2024;56:E699–E700, according to the Creative Commons license.17

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Underwater endoscopic submucosal dissection in the gastrointestinal tract: technical review and dual-approach endoscopic submucosal dissection

Study	Year	Cases (n)	Procedure time (min)	p-value	Dissection speed (mm²/min)	p-value	R0 resection (%)^a)	p-value	Perforation (%)	Post-ESD bleeding (%)	p-value	PECS (%)	p-value
Oh et al.³²	2024	28/28	75.7/49.5^b)	0.001	15.2/21.9^b)	0.001	100/100		0/0	3.6/3.6		14.3/3.6	0.152
Nagata et al.³⁰	2025	69/70	55.5/48.3^c)	0.38	17.4/19.9^c)	0.19	97.1/100	0.25	0/0	2.9/0	0.25	4.3/2.9	0.68

Table 1. Procedure-related outcomes in two single-center RCTs of colorectal ESD (CESD vs. UESD)

Values are presented as CESD/UESD unless otherwise indicated.

RCT, randomized controlled trial; ESD, endoscopic submucosal dissection; CESD, conventional ESD; UESD, underwater ESD; PECS, post-ESD coagulation syndrome.

R0 resection indicates en bloc resection with histologically negative lateral and vertical margins;

mean;

median.