The value of sphincter asymmetry index in anal incontinence

We tested the value of the sphincter asymmetry index (SAI), an adjunct study of vectormanometry for detecting sphincter defects of difficult clinical diagnosis, in patients with anal incontinence referred for anal manometry. Patients were prospectively classified as having no previous anal trauma (group I, n=13), those with possible trauma (including previous vaginal delivery and anorectal surgery unrelated to the onset of incontinence; group II, n=53), and those with previous anal trauma directly related by the patient to the onset of symptoms (group III, n=39). These were compared to 30 healthy volunteers. Clinical data were compiled to obtain an incontinence score, sphincter defect, mean and maximal pressures, functional anal canal length, and SAI for both resting and squeeze pressure profiles. SAI values for the control group were 7.2+/-2.3% and 5.8+/-2.4% for resting and squeeze pressures, respectively. Female controls had shorter anal canals (P=0.0001) and higher SAI during squeeze (P<0.005) than male controls. Incontinence scores were 6.1+/-3.1, 8.6+/-3.9, and 12.5+/-4.9, in groups I, II, and III, respectively (P<0.001). Mean SAI values at rest were 10.3+/-4.9% in group I, 19.0+/-10.6% in group II, and 23.6+/-14.0% in group III (P<0.001); corresponding values during squeeze were 8.6+/-5.3%, 13.9+/-7.9%, and 16.8+/-8.0% (P<0.01). Pressure profiles both at rest and during squeeze were inversely correlated with SAI; therefore the accuracy of SAI was not affected in patients with severe incontinence. Incontinent patients with a previous history of sphincter trauma thus had more severe incontinence, both clinically and manometrically, and higher SAI values than patients without prior trauma. The analysis of the SAI is a valuable tool for determining a traumatic cause of anal incontinence.