Surface electromyography (sEMG) is one of the principal techniques used to evaluate the internal and external anal sphincters and pelvic floor muscles. Paradoxical sEMG testing is used to assess paradoxical contraction of the puborectalis muscle during defecation. The patient is generally placed in the left lateral position. This noninvasive study is performed using surface electrodes. Active electrodes are placed perianally, and a ground electrode is placed on the patient. Sphincter muscle activity is evaluated at rest and during push (strain) and squeeze maneuvers.

Anorectal manometry is a fundamental test of anorectal function used to evaluate patients with constipation, fecal incontinence, or both. It can provide useful information about the pathophysiology of these disorders and other conditions that may cause anorectal pain. The study measures anorectal pressures, muscle tone at rest and during voluntary squeezing, and coordination between the rectum and anal sphincters.

The most important parameters evaluated during testing include maximal voluntary squeeze pressure, resting and relaxation pressures, the rectoanal inhibitory reflex (RAIR) in response to rectal distention, rectal sensory volume thresholds, sphincter length including the high-pressure zone (HPZ), and defecation dynamics assessed through a balloon expulsion study.

The study is performed with the patient in the left lateral position. Bowel preparation is recommended. A four-channel catheter with a compliant balloon at its distal end is inserted approximately 6 cm into the rectum for pressure monitoring. Measurements are obtained at 1 cm intervals.

Maximal voluntary squeeze pressure is measured by asking the patient to squeeze as forcefully as possible for 10–20 seconds. The maneuver is repeated one or two times, with 10–20 seconds of rest or muscle relaxation between attempts. The RAIR is the relaxation of the proximal internal anal sphincter in response to rectal distention. This reflex is evaluated by inflating a balloon within the rectal lumen and observing for a decrease in resting anal pressure. A decrease of at least 50% from the resting pressure following rectal distention in at least one channel is considered a positive reflex.

Rectal sensation testing, also referred to as measurement of rectal sensory volume thresholds, consists of intermittent balloon distention of the rectum. This testing provides information about the rectal volumes that produce sensation and elicit the urge to defecate. Sensory information generated by rectal distention is transmitted to the brain. Fecal continence is maintained through the resting tone and coordinated reflex activity of the internal and external anal sphincters, together with the pelvic floor muscles. Rectal compliance is calculated using data obtained during rectal balloon sensation testing. It reflects the rectum’s capacity and ability to distend.

Balloon expulsion testing may be performed to simulate defecation. The patient’s ability or inability to expel the balloon, as well as the time required for expulsion, is recorded.