G-POEM and POEM techniques are the same; therefore, the principles of G-POEM are similar to those of POEM, consisting of submucosal injection, mucosal incision, submucosal tunneling, myotomy, and closure of mucosal entry [10,17]. The procedure can be performed under general anesthesia or conscious sedation. Carbon dioxide (CO₂) gas insufflation is necessary to minimize the risk of tension pneumoperitoneum, and may require percutaneous or surgical decompression because CO₂ is absorbed from the gastrointestinal tract approximately 160 times faster than nitrogen, the major component of room air.18 During the procedure, the patient can be in either lateral decubitus or supine position.

Before starting mucosal injection, food residue in the stomach should be cleaned by irrigation, suction, or lavage. A conventional gastroscope can be used, but a water-jet-integrated gastroscope is preferred to obtain a clearer view. Use of a transparent cap is also recommended to enhance image stabilization. After proper preparation, a submucosal injection of 3–5 cc of hypertonic saline mixed with methylene blue or indigo carmine is made at the greater curvature or anterior wall of the antrum, about 5 cm from the pyloric ring.

As with POEM, creation of a 1.5 to 2 cm mucosal entry is performed at the site of the initial submucosal bleb using an endoscopic needle knife or DualKnife. A longitudinal incision is preferred because closure with clips is easier; however, a transverse approach is also used. A blended-current electrosurgical unit setting is preferred (e.g., VIO 300D [ERBE, Tübingen, Germany] EndoCut I 2:3:3 or 2:3:2, or EndoCut Q 3:1:1). Following incision, the scope is inserted gently into the submucosal layer. This initial entry is not easy and should not be rushed, to prevent early bleeding or muscle injury, which impede subsequent steps. After the scope is inserted, submucosal tunneling is made towards the pylorus using the same blended current or spray coagulation (effect 2, 50 W). Injection of sufficient solution is required for safe and fast tunneling and avoidance of mucosa-side injury, as with POEM. Unlike tunneling in the esophagus, identifying the correct direction of the pylorus is not easy, and several attempts at tunnel exit and reentry may be needed. Small vessels can be managed using spray coagulation; however, large blood vessels need to be coagulated preemptively with a coagulation forceps (Coagrasper, FD-410LR; Olympus, Tokyo, Japan) and a soft coagulation setting (Soft Coag 30–50 W). The sphincter can be identified as a whitish round muscular bundle when the scope reaches the pylorus. The length of myotomy ranges between 1.5 and 3 cm in most reports. Considering that the thickness of the pyloric ring is generally less than 1 cm, an additional 1- to 2-cm myotomy of the gastric antrum is made. The direction of myotomy can be either anterograde or retrograde. Cutting only the inner circular muscle of the pylorus and gastric antrum is recommended, without performing full-thickness myotomy. As with POEM, the entry site and mucosal incision is closed with either clips or endoscopic sutures (Fig. 1) [17].

Clinical outcomes have been reported in about 130 patients in 5 studies [2,16,19-21]. All studies were retrospective, and there has been no prospective study or randomized controlled trial (Table 1).

Shlomovitz et al. reported the first case series in 7 patients with GP (5 idiopathic and 2 postoperative) [2]. All procedures were technically successful without immediate procedural complications. A few perioperative complications were reported, including 1 case of upper gastrointestinal bleeding 2 weeks postoperatively, 1 case of swallowing difficulty delaying discharge by 1 day, and case of 1 hospital-acquired pneumonia. Six of the 7 patients experienced significant symptomatic improvement. Three-month follow-up nuclear medicine solid-phase gastric-emptying scan (GES), was available for 5 of the 7 patients. Normal gastric emptying at 4 h was noted in 4 of the 5 patients (80%). Khashab et al. reported the results from the first multicenter study [20].

A total of 30 patients with refractory GP (11 diabetic, 12 postsurgical, 7 idiopathic) underwent G-POEM without technical failure, and the technical details included a mean procedure time of 72 min and a mean myotomy length of 2.3 cm. Two adverse events occurred in 2 (6.7%) patients, including 1 case of capnoperitoneum and 1 prepyloric ulcer. Clinical response rate was 86% during a median follow-up of 5.5 months. Four patients (2 diabetic, 1 postsurgical, 1 idiopathic cause) did not respond to G-POEM. Among 17 of 30 patients, there was improvement in gastric emptying time in 82.4% (14/17), including normalization in 47% and partial improvement in 35%. Gonzalez et al. also reported favorable mid-term efficacy in 29 GP patients [22]. The GP Cardinal Symptom Index (GCSI) at 3 and 6 months and the change in GES were outcomes. The technical success rate was 100% (average 47 min) with minor complications (1 bleeding and 1 abscess). The clinical success rate was 79% at 3 months and 69% at 6 months, and the GES normalized in 70% of cases. Factors associated with poor response were investigated and diabetes and female sex were significantly associated with risk of failure in univariate analysis. Dacha et al. reported clinical outcomes including QoL assessment in 16 patients [21]. G-POEM was technically successful in all cases [21]. They reported 81% improvement in the mean GCSI and QoL after G-POEM without adverse events during mean a follow-up of 7.7 months. Improvement in mean 4-h gastric retention on GES was sustained for 12 months. Rodriguez et al. reported the outcome in 47 patients (57.4% idiopathic, 25.6% diabetic, 17% postoperative) [16]. The average percentage of retained food at 4 h was improved from 37% to 20% and GCSI was also improved significantly. Overall, G-POEM can be safely performed in experienced hands and the mid-term clinical response ranges from 70%–90%.