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

Colorectal Dis. 2012 Feb;14(2):224-30. doi: 10.1111/j.1463-1318.2011.02561.x.


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.

MeSH terms

  • Adult
  • Anal Canal / physiopathology
  • Anus Diseases / diagnostic imaging*
  • Anus Diseases / drug therapy*
  • Anus Diseases / physiopathology
  • Botulinum Toxins, Type A / therapeutic use*
  • Defecation
  • Defecography
  • Diagnostic Errors*
  • Female
  • Humans
  • Logistic Models
  • Male
  • Manometry
  • Middle Aged
  • Multivariate Analysis
  • Muscle Contraction
  • Neuromuscular Agents / therapeutic use*
  • Proportional Hazards Models
  • Rectal Prolapse / diagnosis*
  • Recurrence


  • Neuromuscular Agents
  • Botulinum Toxins, Type A