Musculus sphincter ani externus
The external anal sphincter is a ring of skeletal muscle surrounding the anus. It originates from the anococcygeal ligament and surrounding skin and inserts as circular fibers around the anal canal, blending with the bulbospongiosus ventrally. Its action is voluntary closure of the anus, maintaining fecal continence. Innervation is via the caudal rectal nerve (a branch of the pudendal nerve, roughly S3–S5). Loss of sphincter tone or function is an important clinical indicator of cauda equina syndrome or sacral nerve damage.
Origin: Anococcygeal ligament and skin surrounding the anus
Insertion: Circular fibers encircling the anal canal; blends with bulbospongiosus ventrally
Action: Voluntary closure of the anus; maintains fecal continence
Innervation: Caudal rectal nerve (pudendal nerve branch, S3-S5)
Reduced or absent anal tone, along with tail weakness or urinary incontinence, suggests neurologic disease (cauda equina, polyneuritis equi, or sacral fracture). It is assessed during a neurologic examination.
## Overview Cauda equina syndrome is a constellation of neurologic signs caused by disease of the nerve roots that form the cauda equina at the end of the spinal cord. Affected horses show varying co
A horse that cannot control defecation, has a flaccid anus, or shows tail weakness needs prompt veterinary evaluation for neurologic disease.
## Overview Cauda equina syndrome is a constellation of neurologic signs caused by disease of the nerve roots that form the cauda equina at the end of the spinal cord. Affected horses show varying combinations of hind-limb weakness, urinary incontinence, fecal retention, and loss of tail tone. Causes include trauma, equine protozoal myeloencephalitis, polyneuritis equi, and space-occupying lesions. ## Causes Trauma to the sacrum, infectious or inflammatory neuritis, protozoal myelitis, and neoplasia can all damage the cauda equina. The specific cause determines treatment and prognosis. ## Diagnosis Neurologic examination demonstrates lower-motor-neuron signs to the tail, anus, bladder, and hind limbs. Rectal examination, cerebrospinal fluid analysis, and advanced imaging help identify the lesion. Testing for Sarcocystis neurona or other infectious agents is often performed. ## Treatment Therapy is directed at the underlying cause when known (antiprotozoal drugs, anti-inflammatories, or surgical decompression). Supportive care includes bladder management, fecal softeners, and prevention of urine scald. ## Recovery and Prognosis Prognosis is guarded and depends on the cause and severity. Some horses with inflammatory disease improve; severe trauma or advanced denervation often leaves permanent deficits. ## Prevention No specific prevention exists beyond minimizing sacral trauma and controlling known infectious risks where possible. Signs include: hindlimb weakness or ataxia; urinary incontinence; fecal retention; reduced tail tone; muscle atrophy of the hindquarters in chronic cases
Reduced or absent anal tone, along with tail weakness or urinary incontinence, suggests neurologic disease (cauda equina, polyneuritis equi, or sacral fracture). It is assessed during a neurologic examination.
A horse that cannot control defecation, has a flaccid anus, or shows tail weakness needs prompt veterinary evaluation for neurologic disease.
See the External Anal Sphincter on our interactive 3D horse model. Open 3D Model