Excellent response rate of anismus to botulinum toxin if rectal prolapse misdiagnosed as anismus (‘pseudoanismus’) is excluded

Aim Anismus causes obstructed defecation as a result of inappropriate contraction of the puborectalis/external sphincter. Proctographic failure to empty after 30 s is used as a simple surrogate for simultaneous electromyography/proctography. Botulinum toxin is theoretically attractive but efficacy is variable. We aimed to evaluate the efficacy of botulinum toxin to treat obstructed defecation caused by anismus. Method Botulinum toxin was administered, under local anaesthetic, into the puborectalis/external sphincter of patients with proctographic anismus. Responders (resolution followed by recurrence of obstructed defecation over a 1‐ to 2‐month period) underwent repeat injection. Nonresponders underwent rectal examination under anaesthetic (EUA). EUA‐diagnosed rectal prolapse was graded using the Oxford Prolapse Grade 1–5. Results Fifty‐six patients were treated with botulinum toxin. Twenty‐two (39%) responded initially and 21/22 (95%) underwent repeat treatment. At a median follow up of 19.2 (range, 7.0–30.4) months, 20/21 (95%) had a sustained response and required no further treatment. Isolated obstructed defecation symptoms (OR = 7.8, P = 0.008), but not proctographic or physiological factors, predicted response on logistic regression analysis. In 33 (97%) of 34 nonresponders, significant abnormalities were demonstrated at EUA: 31 (94%) had a grade 3–5 rectal prolapse, one had internal anal sphincter myopathy and one had a fissure. Exclusion of these alternative diagnoses revised the initial response rate to 96%. Conclusion Simple proctographic criteria overdiagnose anismus and underdiagnose rectal prolapse. This explains the published variable response to botulinum toxin. Failure to respond should prompt EUA seeking undiagnosed rectal prolapse. A response to an initial dose of botulinum toxin might be considered a more reliable diagnosis of anismus than proctography.