Internal anal sphincter achalasia in chronic functional constipation in children: A myth rather than reality.

[AIMS OF THE STUDY] Internal anal sphincter achalasia (IASA) is characterised by chronic constipation and it is diagnosed based on an absent rectoanal inhibitory reflex (RAIR) on manometry despite a ganglionated rectal biopsy. This study evaluates the pathophysiology of IASA in children with chronic functional constipation (CFC) and determines whether it represents a distinct disorder or a functional variant of constipation.

[METHODS] A retrospective review of prospectively collected data was conducted on 205 children aged 1-16 years with symptoms of constipation and soiling, who failed optimised medical management at Evelina London Children's Hospital between May 2011 and June 2024. Patients were diagnosed with constipation based on the Rome IV criteria. They did not respond to conventional medical treatment of constipation and underwent anorectal investigations and botulinum toxin injection into the external anal sphincter under ketamine anaesthesia. All patients underwent high-resolution anorectal manometry (HRARM) using 8- or 24-channel water-perfused catheters and endosonography under ketamine anaesthesia; and rectal biopsy if RAIR was absent or inconclusive. We compared 178 children, who had symptoms of functional constipation and normal RAIR on manometry with 27 patients, who had chronic functional constipation (CFC) and absent/inconclusive RAIR on manometry and had normal ganglionated rectal biopsy. Findings were compared with 10 children diagnosed with Hirschsprung disease (HD), who had obstructive defecation symptoms after Duhamel pull-through surgery.

[RESULTS] Of 205 children, 27 (13 %) had chronic functional constipation (CFC) and absent or inconclusive RAIR and normal rectal biopsy findings. These patients were comparable with 178 children with functional constipation (FC), who had normal RAIR on manometry. Compared to FC patients, children with CFC presented earlier at median 6.5 (0.83-12) year vs 7 (1.1-16.3) year, p < 0.05. However comparing CFC children to HD patients they presented at an older median age 6.5 (0.83-12) year vs. 2.75 (1.16-8.75) year, p < 0.06); had a longer duration of symptoms 3 (0.5-11.5) years vs. 1 (0.25-3) years, p < 0.001); exhibited greater RAIR relaxation on manometry 23 % vs. 0 %, p < 0.001), and had a thicker internal anal sphincter (IAS) on endosonography 0.84 (0.3-1) vs. 0.71 (0.3-1) mm, p < 0.04). Manometry showed paradoxical contraction of the external anal sphincter (EAS) muscles in 8/27, megarectum in 12/27, both in 4/27, and neither in 4/27. Notably, 24/27 (89 %) children with CFC and absent/inconclusive RAIR responded to botulinum toxin injection into the EAS with significant improvement of their preoperative total symptom severity (SS) score from median score of 26 to 15 at 3 and 12 months follow up, (p < 0.01), respectively.

[CONCLUSION] In children with chronic functional constipation (CFC) and soiling and non-relaxing internal sphincter, RAIR on manometry is masked by paradoxical contraction of the EAS muscles or abolished by failure of reflex relaxation of the IAS muscle due to megarectum. The study shows physiological importance of function of thickened IAS on endosonography and presence of RAIR on manometry in the context of non-relaxing IAS muscle. They respond well to injection of botulinum toxin into the external anal sphincter muscles, supporting classification as functional constipation rather than a distinct pathological entity.