Inflammatory Disorders of the Peripheral Nerve and Neuromuscular Junction
Acquired myasthenia gravis is characterized by failure of neuromuscular conduction due to reduction in the 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. Three clinical forms exist in animals. The generalized form, which affects 57% of dogs with acquired myasthenia, is characterized by exercise-induced stiffness, tremors, and weakness that resolve with rest. However, weakness is not always associated with exercise. Megaesophagus is common in the generalized form. Focal myasthenia (43% of affected dogs) presents as facial, pharyngeal, or esophageal weakness without generalized weakness. Least common is fulminant myasthenia, which presents as acute, flaccid paralysis and megaesophagus, which rapidly progresses to respiratory paralysis and is usually fatal.
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. Megaesophagus is managed with upright feeding and in some cases gastrotomy tube placement for feeding and hydration. Aspiration pneumonia is a frequent complication of megaesophagus and leads to death or euthanasia in ~50% of affected dogs. The prognosis is generally good for animals without pneumonia, and ~85% of dogs will undergo spontaneous remission, evident by a decrease in antibody titer, usually within 6 mo.
Acute idiopathic polyradiculoneuritis 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 not been exposed 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. Initially, there is a short-strided gait in the pelvic limbs that progresses within 1–2 days to flaccid tetraparesis or tetraplegia and, in some cases, to 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, showing signs of discomfort on palpation of the trunk or limbs. 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. Histopathologically, there is inflammation, demyelination, and varying degrees of axonal degeneration in the ventral nerve roots and peripheral nerves.
Chronic relapsing idiopathic polyradiculoneuritis is a rare disease associated with inflammation of the nerves and nerve roots. It affects mature dogs and cats. Exercise intolerance, ataxia, and weakness develop slowly throughout 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 help in some cases, but the disease tends to slowly wax and wane, gradually becoming more severe throughout months to years.
Chronic inflammatory demyelinating polyneuropahy is a fairly common disorder in adult dogs and cats. The cause is unknown. Onset of tetraparesis with hyporeflexia is insidious and 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 administration of corticosteroids (eg, prednisone 1–2 mg/kg/day), although signs may relapse when therapy is stopped.
Neuritis of the cauda equina (polyneuritis equi) 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 increased 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. Histopathologically, there is granulomatous inflammation primarily affecting the extradural portions of the sacrocaudal nerves.
Protozoal polyradiculoneuritis occurs in dogs, especially puppies, and is caused by infection with Toxoplasma gondii (see Toxoplasmosis) or Neospora caninum (see Neosporosis). Transplacental infection is most common, and multiple puppies in a litter can be affected. Affected puppies are normal at birth but by 3–8 wk develop paraparesis and a "bunny-hopping" gait with weak or absent spinal refexes. Over a period of several weeks, the pelvic limbs develop severe extensor rigidity and muscle atrophy. Without treatment, the disease can progress to the thoracic limbs, eventually leading to dysphagia and fatal respiratory paralysis. Serum CK concentration is often increased. Analysis of CSF usually shows increased protein and leukocytes (neutrophils, mononuclear cells, and eosinophils). 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) for 4–6 wk may be effective. The prognosis is poor in dogs with pelvic limb rigidity.
Sensory ganglioneuritis affects dogs of any breed, although Siberian Huskies are at increased risk. At 1–6 yr of age, there is ataxia of all limbs with no paresis, dysphagia, regurgitation, and difficulty prehending food. Hyperesthesia and self-mutilation occur in some cases. There is decreased proprioceptive positioning, weak to absent patellar reflexes, and decreased to absent pain perception in the limbs and face. Diagnosis is based on clinical signs, electrodiagnostic testing that demonstrates slowed sensory nerve conduction, and biopsy of a mixed or sensory nerve that shows loss of myelinated axons and endoneural fibrosis. The cause is unknown, but loss of neurons in dorsal root ganglia with infiltration of lymphocytes and macrophages is seen at necropsy. There is no effective treatment; the disease typically progresses and leads to euthanasia.
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 syndrome, facial paresis, and decreased facial sensation are also possible. The cause is unknown. Histopathologically, 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.