Degenerative Disorders of the Peripheral Nerves and Neuromuscular Junction in Animals

Laryngeal paralysis is common in middle-aged and older dogs. Large breeds, such as Labrador Retrievers, Golden Retrievers, and Saint Bernards, are predisposed, but small-breed dogs and cats can be affected. In most cases, no underlying cause is identified, and the laryngeal paralysis is a component of a generalized polyneuropathy, a syndrome sometimes called geriatric-onset laryngeal paralysis polyneuropathy. A few cases are due to trauma or neoplasia affecting the neck or mediastinum. Hypothyroidism is also a potential cause.

Clinical signs consist of voice change, laryngeal stridor, and a dry cough. In severe cases, exercise intolerance and episodes of respiratory—especially inspiratory—distress and cyanosis occur. Some affected animals have signs of a more generalized polyneuropathy, such as weakness and proprioceptive deficits.

Diagnosis is based on laryngoscopy, with the patient lightly anesthetized. There is a unilateral or bilateral lack of abduction of the arytenoid cartilages and vocal folds during inspiration. Management consists of identifying and treating any underlying disorder.

Treatment of idiopathic laryngeal paralysis consists of surgery, such as prosthetic laryngoplasty (laryngeal tie back). Surgery does not restore normal laryngeal function but usually diminishes severe inspiratory dyspnea. A potential complication of surgery is aspiration of food or liquid.

Canine chronic axonal degeneration, sometimes called geriatric-onset laryngeal paralysis polyneuropathy, affects middle-aged and older dogs and is characterized by axonal degeneration of peripheral nerves. The cause is unknown. Affected dogs have a wide-based, shuffling ataxic gait with tetraparesis, generalized muscle atrophy and decreased spinal reflexes. Laryngeal paralysis is often an early feature. Facial paralysis and dysphagia may also occur. Diagnosis is based on clinical features, electrodiagnostic evaluation showing denervation and decreased motor conduction velocity; and histologic evaluation of nerve and muscle biopsy samples showing loss of large-caliber nerve fibers with axonal degeneration and neurogenic muscle atrophy, respectively. It is important to exclude treatable metabolic causes such as hypothyroidism. The neurologic deficits progress slowly over months. There is no specific treatment.

Dancing Doberman disease is a neuromuscular disease that affects Doberman Pinschers of either sex, approximately 6 months to 7 years old, and is likely inherited in an autosomal recessive pattern. Initially, affected dogs intermittently flex the hip and stifle joints of one pelvic limb while standing. Within several months, most dogs alternately flex and extend both pelvic limbs in a dance-like manner. They often prefer to sit rather than stand. The condition slowly progresses to mild paraparesis, decreased proprioception, and atrophy of the gastrocnemius muscles. The thoracic limbs are not affected. Histopathologic changes have been reported in pelvic limb muscles as well as peripheral nerves, and whether this is a primary muscle or nerve disease remains to be clarified. There is no treatment, and signs do not resolve. However, the disease usually does not result in severe disability and does not appear to be painful.

Distal denervating disease is a common polyneuropathy of dogs in the UK; few cases have been reported elsewhere. The cause is unknown. Any age and breed of dog may be affected. The onset of clinical signs varies from a few days to several weeks. There is progressive tetraparesis, hyporeflexia, and atrophy of proximal skeletal muscles. Sensory deficits are not apparent. Electrodiagnostic evaluation typically shows denervation of limb muscles, relatively normal nerve conduction velocity, and markedly reduced amplitude of M waves. Results of histologic examination of peripheral nerve biopsy specimens are usually normal; however, similar examination of intramuscular nerves samples may be diagnostic; distal intramuscular axons degenerate with collateral axonal sprouting. Treatment is supportive, and the prognosis is excellent, with recovery in 4–6 weeks. Relapse has not been reported.

Distal polyneuropathy of Rottweilers is characterized by paraparesis that slowly progresses to tetraparesis, hyporeflexia, and muscle atrophy. The clinical course can be progressive or can wax and wane. Male and female Rottweilers 1–4 years old have been reported to be affected. The cause is unknown. Electrodiagnostic testing shows denervation in distal muscles of the limbs and decreased motor nerve conduction velocity. Nerve biopsy histopathologic changes consist of axonal necrosis and demyelination, often with infiltrates of macrophages, most severe in distal nerve fibers. Prognosis is poor, although some dogs may temporarily improve with corticosteroid treatment.

Idiopathic facial paralysis is a common disorder that results in unilateral or bilateral paresis or paralysis of the facial muscles in dogs and cats. Cocker Spaniels, Pembroke Welsh Corgis, Boxers, English Setters, and domestic long-haired cats are at increased risk. There is acute onset of unilateral or bilateral inability to blink, drooping ear(s), drooping upper lip, and drooling from the corner of the mouth. Facial sensation (mediated via the trigeminal nerve) remains intact. Diagnosis is based on clinical features and exclusion of other causes of facial paralysis, including ear disease, trauma, and brain-stem lesions. Histopathologic findings consist of degeneration of myelinated axons in the facial nerve. There is no inflammation. The cause is unknown, and there is no specific treatment. Application of artificial tears to the eyes often helps to prevent corneal damage. Partial improvement may occur in a few weeks, but persistent dysfunction is common.

See chapter Laryngeal Hemiplegia in Horses.

Stringhalt (equine reflex hypertonia) in horses is characterized by brisk, involuntary flexion of one or both pelvic limbs during the protraction phase of the gait. Severity ranges from a mild jerk of the limb to flexion so severe that the affected horse can hardly walk. There may be atrophy of the muscles in the distal aspect of the affected limb(s). Two forms of stringhalt occur, ordinary or classic stringhalt is seen sporadically throughout the world, usually as a unilateral problem in individual horses. The cause is unknown. Some cases resolve spontaneously, whereas long digital extensor tenectomy is effective in others.

Australian stringhalt is seen in outbreaks that affect multiple horses in a region and often affects both pelvic limbs. Horses in Australia, New Zealand, and the US have been affected, usually in late summer or autumn. Australian stringhalt is associated with ingestion of Australian dandelion, European dandelion, and mallow, perhaps due to mycotoxins affecting these plants. Histopathologically, the distal aspect of axons in the peroneal and tibial nerves degenerates. Horses with Australian stringhalt usually recover spontaneously when removed from affected pastures.