Definition and Etiology
Robert J. MacKay
Polyneuritis Equi (Neuritis of Cauda Equina)
Polyneuritis equi is a progressive granulomatous polyganglio- radiculoneuritis involving the cauda equina and, less commonly, cranial nerves and other spinal nerves.1-4 The disease usually occurs in adult horses,2 although it has been described in a yearling filly.5 No breed or gender predilection has been noted.6 The cause of polyneuritis equi is unknown.
The lesions are characterized by heavy infiltrations of cytotoxic T cells and macrophages with lesser populations of helper T cells, B cells, and plasma cells.1,7 This pathologic process is similar to experimental allergic neuritis of rats, coonhound paralysis of dogs, and Guillain-Barre syndrome in humans, all of which are suspected of having an autoimmune basis.1,8 Other proposed causes include a hypersensitivity reaction after systemic infection, aberrant migration of helminth larvae, and association with equine herpesvirus type 1, equine viral arteritis, or adenovirus infections.5,6,9,10 However, conclusive evidence for any of these theories is lacking.■ Clinical Signs The clinical signs of polyneuritis equi reflect destruction of somatic and autonomic lower motor neurons and sensory neurons at the level of the nerve roots and dorsal root ganglia. Vague signs of dysuria and tail rubbing may precede the neurologic syndrome.1 The onset of acute signs is marked by cutaneous and muscular hyperesthesia around the hindquarters, focused on the tailhead. Rubbing and chewing of this area is characteristic. Over days to weeks, there is progressive desensitization of previously hyperesthetic areas until the skin over the caudal rump, tail, perineum, and caudal thighs becomes anesthetic. At least initially, there may be a zone of residual hypersensitivity around the margins of the desensitized skin.
Paresis of the tail, bladder, rectum, anal sphincter, and penis progresses, with neurogenic atrophy of the muscles of the tail. Fecal retention and overflow urinary incontinence reflect paralysis of rectal and detrusor muscles, respectively. Urine dribbling causes scalding and staining of the perineum (in mares) and hindlimbs. In males, the penis may remain dropped out of the sheath, although penile and preputial sensation is preserved. Impotence resulting from incomplete erection and inability to achieve intromission was the presenting sign in one stallion with polyneuritis equi.11 In some cases, involvement of the lumbosacral nerve roots causes variable weakness and ataxia of the pelvic limbs and atrophy of muscles of the rump or legs. Any spinal nerve root may be affected symmetrically or asymmetrically, and so discrete areas of muscle atrophy, cutaneous desensitization, and sweating are possible anywhere along the spine. Weakness and ataxia have been observed in all four limbs of a few affected horses.12-14Cranial nerves can be involved. The signs depend on the individual nerves affected and the severity of the disease. The nerves most often affected are the motor division of the trigeminal nerve (cranial nerve V; atrophy of chewing muscles), the facial nerve (VII; facial paralysis), and the vestibular nerve (VIII; head tilt and ataxia), although signs due to involvement of cranial nerves XII (tongue weakness and atrophy), III (reduced pupillary light reflex), IX, and X (dysphagia) have also been reported.9,12-14 Signs of cranial nerve involvement are usually asymmetric.
Any disorder that affects the cauda equina may cause similar signs. These include instabilities of the caudal spine caused by luxations or fractures, EHM, sorghum-sudangrass intoxication, rabies, and other infectious diseases that involve the cauda equina or sacrocaudal spinal cord segments, as well as some primary diseases of the lower urinary tract.
■ Diagnosis CSF may have a moderate mononuclear pleocytosis with lesser numbers of nondegenerate neutrophils and high protein concentration. Electromyographic abnormalities occur in polyneuritis equi as a result of denervation of affected muscles. Affected horses have high circulating antibodies to P2 myelin basic protein, but this test is not available commercially and is reported to be nonspecific.15,16 The diagnosis was supported in one case by the finding of an intense mononuclear neuritis in a biopsy sample from the sacrocaudalis dorsalis muscle.1 Thickened sacral nerves were also imaged by transrectal ultrasonography in that horse.
■ Necropsy Findings The cauda equina is thickened, discolored, and covered with edematous tissue and fibrous material. There may be adhesions of the nerve roots or the spinal cord to the meninges and the periosteum of the vertebral canal. Subdural or epidural petechiation and hemorrhage are sometimes present.
At the microscopic level, the major finding is granulomatous inflammation of the extradural and intradural parts of the nerve roots. The nerve sheaths are thickened and infiltrated to varying degrees by lymphocytes, plasma cells, lymphoblasts, macrophages, giant cells, eosinophils, and, in rare cases, neutrophils. Nerves are obliterated by intraneural inflammation, necrosis, and endoneurial thickening. Axonal degeneration and demyelination are present in surviving nerves. Nerve bundles are separated by large amounts of fibrous tissue. There is minimal to moderate evidence of nerve regeneration.
■ Treatment and Prognosis Treatment with corticosteroids at antiinflammatory doses early in the course of the disease may be palliative but is not curative. Immunosuppressive gold salts and cytotoxic drugs such as azathioprine have been used for the treatment of polyneuritis equi, but the efficacy of such approaches has not been reported. General supportive care should be given, including fluid therapy when necessary, and the rectum and bladder must be emptied manually and by catheter, respectively. With careful management, affected horses can be kept alive for months, but the prognosis for extended life is still regarded as hopeless; the usual cause of death is euthanasia for humane reasons.