All so-called simple fistulas-in-ano may not have readily detectable primary openings and may possess secondary tracks which preclude their behavior as simple fistulas.

Except for increased cross-sectional asymmetry in the high-pressure zone, which may be a forerunner of adverse outcome, manometric parameters before biofeedback do not predict response to biofeedback therapy. Improvement in continence may be independent of resting and squeeze pressures achieved after biofeedback therapy.

Patients with symptomatic low anal fistula involving minimum sphincter musculature can be treated safely with fistulotomy. In treatment of patients with horseshoe abscesses and high fistulas, aggressive local surgical intervention using a seton permits preservation of the sphincter and good postoperative function.

Proximal rectoanal excitation and inhibitory reflexes, when present, have a constant latency, irrespective of the underlying condition. The different latency values for proximal and distal rectoanal excitatory reflexes in patients with FI may indicate disparate denervation damage to the external anal sphincter.

The internal anal sphincter, the smooth muscle component of the anal sphincter complex, has an ambiguous role in maintaining anal continence. Despite its significant contribution to resting anal canal pressures, even total division of the internal anal sphincter in surgery for anal fistulas may fail to compromise continence in otherwise healthy subjects. However, recently reported abnormalities of the innervation and reflex response of the internal anal sphincter in patients with fecal incontinence indicate its significance in maintaining continence. The advent of sphincter-saving surgery and restorative proctocolectomy has re-emphasized the major contribution of the internal anal sphincter to resting pressure and its significance in preventing fecal leakage. The variable effect of rectal excision on rectoanal inhibitory reflex has led to a reappraisal of the significance of this reflex in discrimination of rectal contents and its impact on anal continence. Electromyographic, manometric, and ultrasonographic evaluation of the internal anal sphincter has provided new insights into its pathophysiology. This article reviews advances in our understanding of internal anal sphincter physiology in health and disease.