The rectal sphincter is a complex muscular structure that controls the final stage of the digestive process and is responsible for maintaining bowel control. This muscular ring at the end of the digestive tract is fundamental to a person’s quality of life, as its proper function allows for the conscious delay of a bowel movement. When the sphincter system is working correctly, it prevents the involuntary leakage of gas or stool. Understanding the coordinated actions of these muscles and the factors that can disrupt them is important for addressing issues related to continence.
The Two Muscles of the Rectal Sphincter
The anal canal is encircled by two distinct, concentric rings of muscle that work together to regulate the passage of waste. The inner ring is the Internal Anal Sphincter (IAS), a thickening of the smooth muscle layer of the rectal wall. The IAS is entirely involuntary, meaning it operates without conscious thought, and it maintains a state of continuous contraction to prevent leakage at rest.
The Internal Anal Sphincter is responsible for a significant majority of the resting pressure within the anal canal. This constant tone is mediated by the autonomic nervous system, specifically the sympathetic nerves, which work to keep the sphincter contracted. Surrounding this inner layer is the External Anal Sphincter (EAS), which is composed of skeletal muscle. This composition places the EAS under voluntary control, allowing a person to consciously squeeze and contract the muscle to temporarily hold back a bowel movement.
The External Anal Sphincter blends superiorly with the puborectalis muscle, forming a muscular sling that contributes to the anorectal angle, which is also important for continence. The EAS is responsible for a smaller percentage of the resting tone but provides the reserve strength needed for emergency control. It is innervated by the somatic nervous system via the pudendal nerve.
Mechanism of Continence and Defecation
Continence, or the ability to control bowel movements, relies on the continuous, involuntary contraction of the Internal Anal Sphincter. This muscle ensures the anal canal remains closed until the body is ready to defecate. When stool enters the rectum, the stretching of the rectal wall initiates the rectoanal inhibitory reflex (RAIR).
The RAIR involves the transient, reflex relaxation of the involuntary Internal Anal Sphincter in response to rectal distension. This momentary relaxation allows a small sample of the rectal contents—whether gas, liquid, or solid—to descend and be sensed by the specialized mucosa of the upper anal canal. This “anal sampling” mechanism permits a person to differentiate between gas and stool, which is fundamental to maintaining control.
If the contents are identified as gas or a liquid that needs to be held, the voluntary External Anal Sphincter and the puborectalis muscle must contract to maintain continence. Defecation is a coordinated process that requires the conscious relaxation of the External Anal Sphincter and the puborectalis, coupled with the relaxation of the Internal Anal Sphincter and an increase in intra-abdominal pressure.
Common Causes of Sphincter Dysfunction
Fecal incontinence often results from damage to the rectal sphincter system caused by physical or neurological injuries. Obstetric trauma is a frequent cause of damage to one or both sphincter muscles, particularly during vaginal childbirth, with the risk increasing if forceps are used. The stretching or tearing of the muscle tissue during delivery, sometimes involving a third- or fourth-degree tear, can lead to problems later in life.
Surgical procedures in the anorectal area, such as hemorrhoidectomy or operations for an anal fissure or fistula, also carry a risk of accidental sphincter injury. Chronic health issues can compromise function by affecting the nerves, such as diabetic neuropathy, which impairs the ability of the nerves to send signals to the muscles.
Neurological conditions, including spinal cord injuries or diseases like multiple sclerosis, can disrupt the complex nerve pathways controlling the sphincters. Aging contributes to muscle weakening, known as sarcopenia, and a decrease in the elasticity of the involuntary Internal Anal Sphincter. Repeated straining due to chronic constipation can also weaken the muscles over time, potentially leading to a rectal prolapse.
Treatment Options for Restoring Control
Initial management for sphincter dysfunction often begins with conservative, non-invasive approaches to manage symptoms and strengthen existing muscle function. Simple lifestyle adjustments, such as dietary changes to regulate stool consistency, and medications like antidiarrheals or fiber supplements, can significantly reduce episodes of leakage. Pelvic floor muscle exercises, commonly known as Kegels, aim to strengthen the voluntary External Anal Sphincter and the surrounding pelvic floor muscles.
Biofeedback therapy is a physical rehabilitation technique that helps patients regain control by improving muscle awareness. During this process, sensors provide real-time feedback on muscle contraction strength, helping the patient learn to properly isolate and strengthen the External Anal Sphincter. For cases where nerve function is impaired, sacral nerve stimulation (SNS) is an option that involves implanting a device to send mild electrical pulses to the sacral nerves. This stimulation helps the nerves and muscles communicate more effectively, improving both sphincter function and rectal sensation.
If conservative treatments fail, surgical options are considered, especially when a clear muscle tear is identified, most often due to childbirth. Sphincteroplasty is a common procedure where the torn, separated ends of the muscle are dissected and sutured back together to restore the continuous ring. For severe, non-repairable damage, an artificial anal sphincter can be implanted, which consists of a cuff placed around the anus that the patient can manually inflate and deflate to control bowel movements.