Anorectal manometry is a diagnostic test that evaluates the function of the rectum and the anal sphincter muscles. These muscles and their nerve connections coordinate to control bowel movements. The procedure measures the pressures exerted by these muscles, determining if they are activating and coordinating as expected to maintain continence and facilitate defecation.
Purpose of Anorectal Manometry
Anorectal manometry investigates various conditions impacting bowel function. It is recommended for chronic constipation (infrequent or difficult bowel movements) and fecal incontinence (trouble holding stool). These symptoms can arise from issues with muscle strength, nerve function, or coordination in the anorectal area.
The test identifies specific disorders like anismus (dyssynergic defecation), where pelvic floor muscles paradoxically contract instead of relaxing during a bowel movement. It also diagnoses conditions like Hirschsprung disease, a congenital disorder affecting the large intestine. Anorectal manometry can also assess muscle function following injury, surgery, or chronic disease affecting anorectal motility.
The Manometry Procedure
Preparation involves ensuring the rectum is empty. Patients refrain from eating for a few hours before the test and may use an enema to clear the lower bowel. Specific medication instructions are provided by the healthcare provider, especially for individuals with diabetes.
During the procedure, the patient lies on their left side with knees bent towards the chest. A lubricated, thin, flexible catheter, roughly the size of a thermometer, is gently inserted into the rectum. Pressure sensors measure muscle activity. The healthcare provider takes baseline pressure readings at various depths within the rectum and anal canal.
The test involves several maneuvers to assess muscle function. The patient is asked to squeeze their anal muscles (as if preventing a bowel movement) and to relax or push (as if having a bowel movement). A small balloon at the catheter tip is inflated to different volumes to assess rectal sensation and reflexes, simulating the feeling of stool. Towards the end, a balloon expulsion test may be performed, where the patient attempts to expel a water-filled balloon. The entire test typically takes about 30 minutes, during which patients might experience mild discomfort or the urge to defecate during balloon inflation.
Interpreting Results
Measurements obtained during anorectal manometry provide specific information about the anorectal system’s function. Anal sphincter pressures are a key parameter, measured at rest and during a voluntary squeeze. Resting pressure reflects the internal anal sphincter, while squeeze pressure indicates the external anal sphincter’s strength. Normal resting anal pressure ranges around 60-70 mmHg, and an appropriate squeeze pressure typically increases this significantly to 120-170 mmHg. Abnormalities can suggest muscle weakness contributing to incontinence or excessive tightness leading to constipation.
Rectal sensation is evaluated by inflating the balloon inside the rectum to different volumes. The volume at which a patient first senses the balloon, feels an urge to defecate, and reaches a maximum tolerated volume are recorded. Variations in these sensation volumes can indicate issues with nerve function or rectal sensitivity. Rectal compliance, measuring the rectum’s ability to stretch and hold volume, is also assessed.
The balloon expulsion test measures the time it takes for a patient to expel a 50 mL water-filled balloon. A normal expulsion time is generally less than 60 seconds. A prolonged expulsion time can suggest a defecatory disorder where muscles do not coordinate properly for evacuation. All measurements are interpreted by a medical professional within the context of the patient’s symptoms and other clinical findings to reach an accurate diagnosis and guide appropriate treatment.