Rectal tone refers to the resting tension of the muscles in and around the rectum, particularly the anal sphincters. This inherent muscular tension is fundamental for maintaining fecal continence and facilitating controlled bowel movements.
The Role of Rectal Tone
The anal canal features two primary sphincters: the internal anal sphincter (IAS) and the external anal sphincter (EAS). The internal anal sphincter, composed of smooth muscle, is largely responsible for the resting tone of the anal canal, accounting for 80% to 85% of its involuntary resting pressure. This involuntary muscle remains contracted most of the time, preventing the unintentional passage of stool or gas and thereby maintaining continence.
In contrast, the external anal sphincter consists of skeletal muscle and is under conscious control, allowing for voluntary contraction or relaxation. The nervous system intricately regulates both sphincters. When the rectum distends with fecal matter, stretch receptors stimulate the desire to defecate, leading to the involuntary relaxation of the internal anal sphincter through the rectoanal inhibitory reflex.
If defecation is not desired, conscious contraction of the external anal sphincter can temporarily delay the process. During defecation, the external anal sphincter voluntarily relaxes, the puborectalis muscle also relaxes, and rectal muscles contract, often aided by abdominal muscle contraction, to facilitate stool expulsion.
How Rectal Tone is Evaluated
Healthcare professionals assess rectal tone through methods like the digital rectal examination (DRE) and anorectal manometry. The digital rectal examination is a primary clinical method where a gloved, lubricated finger is gently inserted into the anal canal. During this examination, the clinician evaluates the resting tone of the anal sphincter, noting the resistance encountered upon insertion.
Patients are also asked to voluntarily squeeze and relax their anal muscles around the finger to assess the strength and coordination of the external anal sphincter. While DRE provides valuable subjective information about muscle tone and function, its ability to quantify anal sphincter tone can vary among examiners.
For a more objective and detailed assessment, anorectal manometry is often employed. This test involves inserting a thin, flexible catheter with pressure sensors into the anus and rectum to measure intraluminal pressures. Anorectal manometry can quantify resting and squeeze pressures, evaluate rectal sensation, and assess reflex activity, providing comprehensive data on anorectal function. This objective data helps identify specific abnormalities in muscle function that might not be fully captured by DRE alone.
Conditions Related to Rectal Tone
Abnormal rectal tone can significantly impact bowel function and overall quality of life. High rectal tone, known as hypertonia or hypertonic pelvic floor, occurs when the anal sphincter muscles are in a state of continuous contraction or spasm. This increased tension can lead to chronic constipation, as muscles fail to relax adequately for defecation, causing straining and difficulty passing stool.
Anal fissures, small tears in the anal lining, can also be associated with hypertonia due to the increased pressure and difficulty with bowel movements. Pelvic floor dyssynergia, where the pelvic floor muscles do not coordinate properly during defecation, can also contribute to or result from hypertonia. Causes for hypertonia can include habitual stool holding, injury, prolonged sitting, or other painful conditions.
Conversely, low rectal tone, or hypotonia, indicates decreased muscle strength and can lead to fecal incontinence (involuntary leakage of stool). This condition often results from damage to the anal sphincter muscles or the nerves that control them. Childbirth trauma, particularly vaginal delivery, is a common cause of such damage, potentially affecting both muscle integrity and nerve supply.
Nerve injuries, such as those from spinal cord pathology, or general muscle weakness can also contribute to hypotonia. Addressing these underlying issues, which may involve strengthening exercises for hypotonia or relaxation techniques for hypertonia, is important for managing symptoms and improving bowel control.