Sphincter Ani Muscles

Original Editor - Chioma Clara Okeakpu

Top Contributors - Chioma Clara Okeakpu and Kim Jackson

Introduction

Sphincters are commonly known as circular or ring-like muscles that open or close passages in the body to regulate the flow of substances like urine, faeces or bile through the body. The sphincter ani muscles, also known as the anal sphincter muscles, are the muscles surrounding the anal canal that regulates defecation. They include the internal anal sphincter and the external anal sphincter muscle, and they both play important roles in maintaining faecal continence. Between both sphincters, there is also a longitudinal layer, which consists of muscles of the longitudinal layer of the rectum musculature. It is referred to as the conjoint longitudinal muscle layer (CLML), and these three muscle layer form the anal sphincter complex.

Origin

The internal anal sphincter is a smooth muscle, measuring about 30mm in height and 3mm in thickness, and originates from extensions of the circular layers of the rectum into the anal canal. The autonomic nervous system regulates its activity, which is not affected by human volition, thus making it involuntary in action.

The external anal sphincter is a skeletal muscle made up of striated fibres. Its exact morphology is much more complex as it is composed of three different parts that loop around the anal canal one above the other. They include the upper or deep part, middle or superficial part, and lower or subcutaneous part. The lower subcutaneous part has no skeletal attachment but attaches to subcutaneous tissue anteriorly and posteriorly from the anus in the midline. The middle superficial part makes up the main mass of the muscle and has its attachment also anteriorly and posteriorly from the anus, within the perineal body and in the anococcygeal ligament, respectively, this way indirectly attaching to the coccyx. The upper and deepest part shows a very circular fibre arrangement and, like the subcutaneous part, has no skeletal attachment; however, its exact attachment locations are not described.

Nerve

The internal anal sphincter receives visceral innervation from sympathetic and parasympathetic fibres. The sympathetic fibres are from the inferior pelvic plexus, while the parasympathetic fibers are from the inferior pelvic plexus and pelvis splanchnic nerves.

The external anal sphincter receives somatic innervation mostly from the motor branches of the pudendal nerve(S2-S4), also referred to as the inferior rectal nerves. These nerves pass through the ischioanal fossa to reach the muscle.

Artery

The anal sphincter receives its blood supply from the middle and inferior rectal arteries.

Action

The internal anal sphincter contracts to close the anal canal and inhibit the passage of faeces and relaxes to propel faeces. Like most smooth muscles sphincters in the alimentary canal, the internal anal sphincter remains in continuous contraction and relaxes only in response to inhibitory neural input. It acts through the rectoanal inhibitory reflex (RAIR), which involves the reflexive relaxation of the internal anal sphincter in response to rectal distension .

The external anal sphincter is a skeletal muscle and acts voluntarily like other postural muscles in the body. It responds to volitional commands to contract and maintain continence when faeces are propelled into the rectum. The external anal sphincter maintains its tone to keep the orifice of the anal canal closed and relaxes upon defecation through the defecation reflex.

In summary, while the rectoanal inhibitory reflex causes the internal anal sphincter to relax in the presence of rectal distension, the external anal sphincter works to keep stool in the anal canal until one is ready to defecate.

Clinical relevance

Obstetric anal sphincter injury(OASIS) is associated with third-and-fourth-degree perineal tears. The third and fourth-degree perineal tears are referred to perineal lacerations involving the anal sphincters. It is also associated with neurogenic damage of the pelvic floor nerve supply caused by the stretching and compression of the birth canal during the descent of the foetus, which may lead to demyelination and subsequent denervation of pudendal nerves. OASIS may be asymptomatic in some women and may lead to faecal incontinence due to lack of early management.

Faecal incontinence according to the International Consultation on Incontinence (ICI), is defined as the involuntary passage of liquid or solid stool that is a social or hygienic problem. It occurs when the rectal pressure is higher than the anal sphincter pressure, and its risk factors include bowel disorders like diarrhoea, trauma to the anal sphincter, chronic disease burden, neurological disorders, inflammatory bowel disease, and anatomical disorders of the pelvic floor.

Assessment

Physical examination: This involves inspection for any abnormal skin colour and wrinkling of anus and perianal area. The physician pays attention to where there is presence of perianal skin lesion or injury, pus and blood, mucus, anal fissure, scar, external haemorrhoids, fistula orifice, ulceration, abscess and any form of prolapse.

Digital Rectal Examination: This involves the palpation of the anus and rectum. It is an important diagnostic method for anal, rectal and pelvic diseases. It is performed with the patient staying in positions such as knee-elbow position, left-lateral position, supine position, etc. based on the patient specific condition. The physician or therapist wears gloves and coat right index finger with good amount of lubricant, such as soap, vaseline, liquid paraffin, etc., for palpation. Next, the index finger of the physician is placed on the outer surface of the anus to gently massage and relax the anal sphincter muscles. Slowly and gently, the physician inserts the index finger into the anus to make a full-circle examination of rectum and anus to check for presence of tension in the anus and sphincters, bleeding, and tenderness.

Pelvic floor Physical therapy

Pelvic floor physical therapy (PFPT) is a non-invasive treatment that involves the use of biofeedback, bowel habit guidance ,electrical stimulation (ES), lifestyle modification, and pelvic floor muscle training to strengthen the contraction force of the pelvic floor muscles, such as the external anal sphincter, normalise rectal sensation, and empty the rectum and colon regularly. PFPT has been proven though evidence-based practice as a first-line, conservative option to treat various pelvic floor dysfunctions, including pelvic organ prolapse (POP), faecal ,urinary and anal incontinence, peripartum and postpartum pelvic floor dysfunction .Electrical Stimulation improves recovery after childbirth and eliminates symptoms of faecal or urinary incontinence by promoting neuroregeneration after injury. Also, in the use of biofeedback, combination of nonsurgical treatments of chronic incontinence, such as electrical stimulation and electromyographic biofeedback, referred to as triple-target treatment, appeared to be more effective to biofeedback alone. Pelvic floor muscle training improves urinary and faecal incontinence by strengthening the contraction force of the pelvic floor muscles through the use of Kegel's exercises.