Anorectal Manometry (ARM) is a diagnostic procedure utilized to evaluate the function of the muscles and nerves controlling bowel movements. The test involves inserting a small, flexible catheter into the rectum and anal canal to measure pressure changes and coordination patterns. This assessment helps medical professionals understand the physical causes behind symptoms like chronic constipation or fecal incontinence. Abnormal results indicate a dysfunction in the complex process of waste elimination, pointing toward specific issues requiring targeted intervention.
Key Measurements and Defining Normal Function
Anorectal Manometry measures several key parameters. Resting pressure measures the tone of the internal anal sphincter, which provides the majority of continence while the body is at rest; normal values typically fall between 40 and 70 mmHg. Squeeze pressure reflects the strength of the external anal sphincter and surrounding pelvic floor muscles, which are voluntarily contracted to maintain continence during urgency or physical stress. Normal squeeze pressures are significantly higher than resting pressures, often ranging from 100 to 180 mmHg.
The test also measures the Rectoanal Inhibitory Reflex (RAIR), where the internal sphincter temporarily relaxes when the rectum is distended by stool. This reflex allows the sensitive lining of the anal canal to “sample” rectal contents to distinguish between gas, liquid, or solid. Rectal sensation is tested by gradually inflating a balloon to determine the volumes at which a person first senses the presence of contents and feels the urge to defecate. A normal first sensation is typically felt around 30 to 60 milliliters (mL), while the desire to defecate occurs at slightly higher volumes.
Abnormal Results Indicating Fecal Incontinence
Abnormal manometry results indicating fecal incontinence often involve deficits in sphincter muscle pressure. A finding of low resting pressure suggests weakness in the internal anal sphincter, the muscle responsible for passive closure. If pressure is significantly reduced (usually below 40 mmHg), it can lead to leakage, particularly of liquid stool or gas, because the muscle cannot hold the canal closed effectively. This weakness is often associated with age, trauma, or neurological conditions.
Low squeeze pressure indicates a weak or damaged external anal sphincter. Since this voluntary muscle is activated for urgent control, weak squeeze pressure means a person cannot properly tighten the muscle to prevent an impending bowel movement. Low squeeze pressure is frequently seen following obstetric injury, surgery, or chronic straining. Both low resting and low squeeze pressures compromise the barrier to the outside, resulting in the inability to control the passage of stool.
Abnormal rectal sensation thresholds can also contribute to incontinence. If a patient has a reduced sensation threshold, they may feel the urge to defecate at very low volumes, leading to urgency and insufficient time to reach a toilet. Conversely, a hypersensitive rectum perceives small volumes as an urgent need, contributing to sudden leakage. Manometry helps differentiate between muscle weakness and sensory issues to guide treatment selection.
Abnormal Results Indicating Chronic Constipation
Abnormal Manometry results indicating chronic constipation often involve coordination failure during defecation. The most common finding is dyssynergic defecation, where the pelvic floor muscles fail to coordinate properly. Normally, bearing down increases rectal pressure while the anal sphincter muscles simultaneously relax, allowing for easy evacuation.
In dyssynergic defecation, the test shows a paradoxical contraction or inadequate relaxation of the external anal sphincter or puborectalis muscle during simulated defecation. This means the muscles tighten when they should be relaxing, creating a functional obstruction that prevents stool expulsion. This incoordination is categorized based on the adequacy of rectal pushing force and whether anal pressure increases or fails to decrease. The resulting pattern of muscle opposition prevents the successful passage of a bowel movement.
The Balloon Expulsion Test (BET), often performed with manometry, provides a practical assessment of this coordination. Inability to expel a small, water-filled balloon from the rectum within 60 seconds strongly suggests a defecatory disorder. This indicates the patient cannot generate the necessary propulsive force or relax the pelvic floor muscles adequately.
Another finding is rectal hyposensitivity, where the patient requires an unusually large volume of rectal distension (sometimes exceeding 150 mL) to feel the urge to defecate. This blunted sensation can lead to stool retention and impaction because the signal to initiate a bowel movement is delayed.
Treatment and Next Steps After an Abnormal Test
The next steps after an abnormal manometry result depend on the identified dysfunction. For patients diagnosed with dyssynergic defecation, the primary treatment is Biofeedback Therapy. This non-surgical approach uses the manometry device to provide real-time visual feedback, helping the patient learn to coordinate muscle relaxation and the abdominal push required for defecation.
For fecal incontinence caused by weak sphincter muscles (low resting or squeeze pressures), initial steps involve targeted pelvic floor muscle exercises. Biofeedback is also effective for strengthening the external anal sphincter and improving sensory awareness. If muscle weakness is severe due to injury, surgical options may be discussed to repair or augment the damaged sphincter. Medication adjustments, such as fiber supplements or motility agents, may also be incorporated to manage stool consistency, especially when sensory issues like hyposensitivity are present.