The not-so-rare absent RAIR: Internal anal sphincter achalasia in a review of 1072 children with constipation undergoing high-resolution anorectal manometry

Background: Our objective is to describe the prevalence of patients with internal anal sphincter achalasia (IASA) without Hirschsprung disease (HD) among children undergoing anorectal manometry (ARM) and their clinical characteristics. Methods: We performed a retrospective review of high-resolution ARM studies performed at our institution and identified patients with an absent rectoanal inhibitory reflex (RAIR). Clinical presentation, medical history, treatment outcomes, and results of ARM and other diagnostic tests were collected. We compared data between IASA patients, HD patients, and a matched control group of patients with functional constipation (FC). Key results: We reviewed 1,072 ARMs and identified 109 patients with an absent RAIR, of whom 28 were diagnosed with IASA. Compared to patients with FC, patients with IASA had an earlier onset of symptoms and were more likely to have abnormal contrast enema studies. Compared to patients with HD, patients with IASA were more likely to have had a normal timing of meconium passage, a later onset of symptoms, and were diagnosed at an older age. At the latest follow-up, the majority of patients diagnosed with IASA (54%) were only using oral laxatives. Over half of patients with IASA had been treated with anal sphincter botulinum toxin injection, and 55% reported a positive response. Conclusions and Inferences: Patients diagnosed with IASA may represent a more severe patient population compared to patients with FC, but have a later onset of symptoms compared to patients with HD. They may require different treatments for their constipation and deserve further study.