PURPOSE:Fecal leakage after open lateral internal anal sphincterotomy for chronic anal fissure is common, but underreported. The aim of this study was to prospectively assess the physiologic and morphologic effects of sphincterotomy, comparing continent and incontinent patients after surgery. This group was further compared with an unselected group of patients presenting with incontinence after hemorrhoidectomy.METHODS:Between January 1997 and June 1999, 23 patients were prospectively followed up through internal sphincterotomy with conventional and vector volume anorectal manometry, parametric assessment of the rectoanal inhibitory reflex, and endoanal magnetic resonance imaging. Fourteen continent patients were compared with 9 incontinent postoperative cases, 9 patients referred with incontinence after hemorrhoidectomy, and 33 healthy volunteers without anorectal disease.RESULTS:Significant differences were noted between continent and incontinent postsphincterotomy cases for all resting conventional and vector volume parameters and for some squeeze parameters. Although there was a significant reduction in postoperative high pressure zone length at rest, there were no differences between the postoperative groups. There was an increase in sphincter asymmetry of 6.7 percent (± 3.5 percent) in incontinent postsphincterotomy patients and a decrease of 2.8 percent (± 3.2 percent) in continent cases. Significant differences were noted for resting parameters between incontinent postsphincterotomy and posthemorrhoidectomy patients, with a higher resting sphincter asymmetry in the latter group. The area under the rectoanal inhibitory curve was smaller in postsphincterotomy incontinent patients when compared with continent cohorts over the distal and intermediate sphincter zones at rest with a reduced latency of inhibition. There was no difference in the magnetic resonance images of the sphincterotomy site between incontinent and continent postsphincterotomy cases and no posthemorrhoidectomy case had evidence of sphincteric damage.CONCLUSION:There are complex significant differences in the postoperative physiology of patients undergoing lateral internal sphincterotomy who become incontinent when compared with those who maintain continence. These physiologic changes are not reflected in detectable morphologic sphincteric differences. It is unknown whether these changes predict for long‐term incontinence, and it is suggested that postoperative incontinence after minor anorectal surgery is not necessarily related either to a preexisting sphincter defect or inadvertent intraoperative sphincter injury.