Acquired Myasthenia Gravis

Disease is characterized by failure of neuromuscular conduction due to reduction in number of acetylcholine receptors at the neuromuscular junction. It is caused by the development of circulating antibodies directed against the acetylcholine receptors at the neuromuscular junction. It is fairly common in mature dogs, especially German Shepherds, Golden Retrievers, and Labrador Retrievers, but is uncommon in cats. The classic presentation is exercise-induced stiffness, tremors, and weakness that resolve with rest. However, weakness is not always associated with exercise. Facial, pharyngeal, or esophageal weakness is common, and in many cases there is megaesophagus without generalized weakness (focal myasthenia). Regurgitation and aspiration pneumonia are frequent complications.

Generalized weakness often resolves quickly after IV administration of edrophonium chloride (0.1–0.2 mg/kg), which is often used as a diagnostic test. Definitive diagnosis is based on the detection of antibodies in serum. Treatment consists of anticholinesterase drugs, eg, pyridostigmine (1–3 mg/kg, PO, bid-tid) or neostigmine (0.04 mg/kg, SC, qid). Immunosuppressive dosages of prednisone and other immunomodulating drugs are recommended in animals that do not respond to anticholinesterase therapy. The prognosis is generally good, and many dogs will undergo spontaneous remission, evident by a decrease in antibody titer. The prognosis is guarded for animals with aspiration pneumonia or persistent weakness.

Acute Idiopathic Polyradiculoneuritis

Disease primarily affects the ventral nerve roots and peripheral nerves. It is common in dogs and rare in cats. Clinical signs often develop 7–14 days after a raccoon bite or scratch (Coonhound paralysis); however, other affected animals have no exposure to raccoons. A similar syndrome can develop in dogs and cats within 1–2 wk of vaccination. An immune-mediated reaction to raccoon saliva or other antigen is suspected. Typically, flaccid paresis begins in the pelvic limbs and progresses within 1–2 days to tetraparesis and, in some cases, facial and laryngeal weakness. Occasionally, the thoracic limbs are initially affected. Death from respiratory paralysis can occur in severe cases. Spinal cord reflexes are weak to absent, and severe muscle atrophy is evident within 10–14 days. Pain perception is intact and some dogs may appear hyperesthetic. Mentation and appetite are not affected. Urination, defecation, and tail movement usually remain normal.

Analysis of CSF collected from the lumbar subarachnoid space shows increased protein with a normal cell count. Electromyography shows denervation, and nerve conduction studies show marked dispersion and prolonged latency of F-waves, indicative of slowed conduction in the ventral roots. There is no effective treatment other than nursing care, and corticosteroids are not helpful. Most affected animals begin to improve spontaneously within 3 wk, with complete recovery by 2–6 mo. Animals with severe signs and marked muscle atrophy may recover incompletely. Relapses can occur, especially in hunting dogs that frequently encounter raccoons. Pathologically there is inflammation, demyelination, and varying degrees of axonal degeneration in the ventral nerve roots and peripheral nerves.

Chronic Relapsing Idiopathic Polyradiculoneuritis

This rare disease is associated with inflammation of the nerves and nerve roots. It affects mature dogs and cats. Exercise intolerance, ataxia, and weakness develop slowly over several months. Some animals have spontaneous temporary remissions. Spinal cord reflexes are decreased, and cranial nerves may be affected. In severe cases, decreased sensation is evident. Diagnosis is based on nerve biopsy. There is nonsuppurative inflammation; axonal degeneration; and demyelination of nerves, nerve roots, and, in some cases, dorsal root ganglia. The cause is unknown, although immune-mediated mechanisms are suspected. Corticosteroids are helpful in some cases, but the disease tends to slowly wax and wane, gradually becoming more severe over months to years.

Chronic Inflammatory Demyelinating Polyneuropathy

This disorder is fairly common in adult dogs and cats. The etiology is unknown. Onset of tetraparesis with hyporeflexia is insidious and is sometimes accompanied by cranial nerve dysfunction. Electromyography is usually normal, but nerve conduction velocities are slowed with temporal dispersion. Nerve biopsy shows multifocal paranodal demyelination. Clinical signs usually improve with the administration of corticosteroids (eg, prednisone 1–2 mg/kg/day), although signs may relapse when therapy is stopped.

Polyneuritis Equi

Neuritis of the cauda equina is characterized by inflammation of the sacrocaudal nerves, and occasionally other nerves. It is seen in adult horses of all breeds in Europe and North America. The cause is unknown, although an immunologic reaction incited by a viral infection is possible. Affected horses have circulating antibodies against P2 myelin protein. The most consistent clinical signs reflect involvement of the sacrocaudal nerves and include urinary and fecal incontinence, tail paralysis, perineal paresthesia or analgesia, atrophy of the gluteal muscles, mild pelvic limb ataxia, and in male horses, penile paralysis. Affected horses may rub the tail. The thoracic limbs and cranial nerves may also be affected. Diagnosis can usually be based on clinical findings. CSF may be xanthochromic with elevated protein content and mononuclear pleocytosis. Sacral fracture should be excluded by rectal examination and radiography. There is no treatment, and the prognosis for recovery is poor. Pathologically, there is granulomatous inflammation primarily affecting the extradural portions of the sacrocaudal nerves.

Protozoal Polyradiculoneuritis

Disease occurs in dogs, especially puppies, and is caused by infection with Toxoplasma gondii (see Toxoplasmosis) or Neospora caninum (see Neosporosis). Inflammation of nerve roots, peripheral nerves, and skeletal muscle results in progressive paralysis and rigidity of the pelvic limbs. Serum CK concentration is often increased. Analysis of CSF usually shows elevated protein and leukocytes (both neutrophils and mononuclear cells). Serum or CSF antibodies or identification of the organism on muscle biopsy are helpful in diagnosis. Early treatment with clindamycin (15–20 mg/kg, IM, PO, bid) or trimethoprim/sulfadiazine (15 mg/kg, bid) and pyrimethamine (1 mg/kg/day) may be effective. The prognosis is poor in dogs with pelvic limb rigidity.

Trigeminal Neuritis

Idiopathic trigeminal neuropathy is common in dogs and uncommon in cats. It is characterized by acute onset of flaccid jaw paralysis. Affected animals cannot close the mouth and have difficulty eating and drinking. Horner's syndrome, facial paresis, and decreased facial sensation are also possible. The cause is unknown. Pathologically, there is bilateral nonsuppurative inflammation and demyelination in the motor branches of the trigeminal nerve. Affected animals usually recover spontaneously within 3–4 wk. Fluid and nutritional support may be necessary.

Last full review/revision July 2011 by William B. Thomas, DVM, MS, DACVIM (Neurology)