Anorectal Manometry: Current Techniques and Indications

Anorectal manometry measures the pressures of the anal sphincter muscles, sensation in rectum, and the neural reflexes that are needed for normal bowel movements. This test is mainly performed to evaluate patients with constipation or fecal incontinence but is also useful for pre/post surgical evaluation of anal sphincter tone, functional anorectal pain, pelvic floor dyssynergia and diagnosis of Hirschsprung’s Disease. Anorectal manometry is criticized on certain grounds. The equipment is costly and facility is limited to specialist centers. The procedure has poor sensitivity and specificity in anorectal disorders. At times abnormal measurements do not correlate with disease entities or explain the symptoms. Normal range of various parameters measured is highly variable and poorly reproducible. Clinical outcome after intervention does not correlate with alteration in the measurements obtained. However, anorectal manometry provides many useful data regarding anorectal function. Anorectal manometry indicates the prognosis of treatment, particularly in the management of sphincter injuries and may be used in biofeedback treatment of anismus and solitary rectal ulcer syndrome. Appropriate interpretation and clinical correlation of these tests is of paramount importance. Correspondence: Dr. Anil Arora, Senior Consultant, Department of Gastroenterology and Hepatology, Sir Ganga Ram Hospital, New Delhi, India Mob: 9811047384 e-mail: [email protected] INTRODUCTION The anorectum plays an important role in regulation of defecation and in maintenance of continence. The most widely used test for anorectal function is anorectal manometry. Anorectal manometry measures pressures of the anal sphincter muscles, sensation in the rectum, and neural reflexes that are needed for normal bowel movements. This test is mainly performed to evaluate patients with constipation or fecal incontinence but is also useful for pre/post surgical evaluation of anal sphincter tone, functional anorectal pain, pelvic floor dyssynergia and diagnosis of Hirschsprung’s Disease. After establishing a diagnosis, anorectal manometry can also be used therapeutically.Anorectal manometry is often used in conjunction with other assessment tools like defecography, pudendal nerve studies, anal sphincter electromyography (EMG) and endoanal ultrasonography. A comprehensive assessment of anorectal function during anorectal manometry consists of measuring at a minimum each of the following parameters: (1) anal sphincter function, (2) rectoanal reflex activity, (3) rectal sensation, (4) changes in anal and rectal pressures during attempted defecation, (5) rectal compliance and (6) performance of a balloon expulsion test. ANAL SPHINCTER FUNCTION Anal sphincter function is assessed by measurement of resting sphincter pressure, squeeze sphincter pressure, and the functional length of the anal canal. Normal resting pressure is 40-80mmHg, contributed 85% by internal anal sphincter (IAS) and 15% by external anal sphincter (EAS). Squeeze pressure is normally 80-160mmHg and 100% due to the external anal sphincter. The anorectal manometry test measures pressure gradients all along the anal canal both radially and longitudinally. Thus, maximum resting anal canal pressure predominantly reflects IAS function, while voluntary anal squeeze pressure reflects EAS function. Functional anal canal length is defined as the length of the anal canal over which resting pressure exceeds that of the rectum by greater than 5 mmHg or, alternatively, as the length of the anal canal over which pressures are greater than half of the maximal pressure at rest. Maximal resting anal pressure is defined as the difference between intrarectal pressure and the highest recorded anal sphincter pressure at rest, and is generally recorded 1-2 cm from the anal verge. Maximum squeeze pressure is defined as the difference between the intrarectal pressure and the highest pressure that is recorded at any level within the anal canal during the squeeze maneuver. RECTOANAL REFLEX ACTIVITY Rapid distention of the rectum induces a transient increase in rectal pressure, followed by a transient increase in anal pressure associated with EAS contraction (the rectoanal contractile reflex), and in turn, a more prolonged reduction in anal pressure due to relaxation of the IAS (the rectoanal inhibitory reflex). The rectoanal contractile reflex is a compensatory guarding mechanism that allows a positive anorectal pressure gradient to be maintained during transient increases in intra-abdominal pressure (such as coughing), which is essential for preserving continence. In fecal incontinence patients, anal sphincter pressure is not increased over the intra-abdominal pressure during coughing. The presence of rectoanal inhibitory reflex is recorded when the balloon is distended with a 50 mL volume of air.