Neuromuscular Disorders

Spastic paresis is seen in many breeds of cattle and has been referred to as “contraction of the Achilles tendon,” “straight hock,” and “Elso heel.” (Also see Lameness in Cattle.) It can be divided into two syndromes, one that affects calves and one that affects adults. In calves, the condition appears to be familial and can be seen in many breeds, with signs beginning between 1 wk and 1 yr of age. It is characterized by extension of the stifle and tarsus and by spastic contracture of the muscles of one or both pelvic limbs. Spasticity primarily affects the gastrocnemius and superficial flexor muscles; in some cases, other muscles of the pelvic limb are involved. The leg is usually held in extension behind the calf and does not touch the ground during walking. The disease is progressive but usually responds to neurectomy of the tibial nerve. The cause is unknown. No lesions are seen in peripheral nerves, and the condition is thought to involve excessive activity of the neuromuscular spindle reflex arc. Adult cattle are affected at 3–7 yr of age. Extensor muscles of the back and pelvic limbs are affected, causing lumbar lordosis and caudal extension of the limbs. This condition is also thought to be familial and is usually progressive. Mephenesin (30–40 mg/kg, PO, for 2–3 days) may produce variable control of signs. Quadriceps muscle hypoplasia as a cause of congenital lameness has been described in Holstein calves. Reduced numbers of spinal cord motor neurons suggest that there is failure to innervate the muscle on the affected side.

Hyperkalemic periodic paralysis (see Hyperkalemic Periodic Paralysis) is seen in Quarter horses 2–3 yr old and is due to an inherited mutation of the sodium channel. It causes episodes of muscle tremor and sometimes recumbency, both of which may be precipitated by exercise. Hyperkalemia is usually present during an attack, and electromyography can also be helpful for diagnosis. Acetazolamide (0.5–2.2 mg/kg, PO, bid) or hydrochlorothiazide (0.5 mg/kg, PO, bid) may lessen the frequency and severity of attacks.

Myotonia congenita is an inherited/familial disorder in goats and Shropshire lambs and is occasionally seen in horses. It causes muscle rigidity; marked dimpling on percussion of the muscle belly; and a stiff, stilted gait. Electromyography is a useful aid to diagnosis. This disease results from a mutation in a chloride channel.

Muscular dystrophy is an inherited disease in Merino sheep. It results in a slowly progressive stiffness that affects the limbs and neck from 3–4 wk of age onward. Clinically affected sheep have high resting and postexercise concentrations of serum CK and lactate dehydrogenase.

Porcine stress syndrome or malignant hyperthermia (see Malignant Hyperthermia) is a hypermetabolic and hypercontractile syndrome that, when triggered by anesthesia or stress, produces a sustained increase of intracellular calcium levels within skeletal muscle fibers. This in turn causes muscle stiffness, hyperventilation, hyperthermia, and pale exudative pork. It results from a mutation in a calcium-channel gene that is inherited as an autosomal dominant trait, usually in Landrace pigs.

Hypertrophic neuropathy of Tibetan Mastiffs is an autosomal recessive disease that has been recognized in the USA, Switzerland, and Australia. It causes paraparesis by 8 wk and may progress to tetraparesis. Hyporeflexia is marked, but sensory function is preserved. Demyelination and remyelination are seen on nerve biopsy. Prognosis is guarded. Some puppies regain the ability to walk but remain weak. There is no treatment.

Alaskan Malamute polyneuropathy affects Alaskan Malamutes 10–18 mo old. Clinical signs include exercise intolerance, paraparesis progressing to tetraparesis, muscle atrophy, hyporeflexia, and, in some cases, laryngeal paralysis. Electromyography shows diffuse fibrillation potentials and positive sharp waves. On nerve biopsy, there is axonal necrosis with demyelination. There is no effective treatment, although clinical signs stabilize in some dogs. In most affected dogs, however, progressive disability leads to euthanasia.

Congenital laryngeal paralysis is seen in Bouvier des Flandres (autosomal dominant) and Siberian Huskies, Rottweilers, and Bull Terriers <1 yr old. It results in exercise intolerance and inspiratory dyspnea. Diagnosis is confirmed by visualization on laryngoscopy. Congenital laryngeal paralysis with diffuse peripheral neuropathy is seen in several breeds, including Dalmatians, Rottweilers, and Pyrenean Mountain Dogs (Great Pyrenees). Prognosis is guarded to poor.

Primary hyperoxaluria (l-glyceric aciduria) is a rare, inherited (autosomal recessive) neurofilament disorder of domestic shorthaired cats that results in renal disease and also produces weakness due to a peripheral neuropathy. Signs develop at 5–9 mo of age. A plantigrade stance is the most prominent sign, and spinal reflexes are sometimes reduced. Urine contains increased oxalate and l-glycerate levels. There is no treatment.

Neuropathy of hereditary hyperchylomicronemia (hyperlipidemia) is a suspected autosomal recessive disorder that causes a generalized peripheral neuropathy in cats. Clinical signs do not develop until at least 8 mo of age. The hyperlipidemia results in deposition of lipid granules within nerves, and there is evidence that the clinical signs can be controlled by a low-fat diet. Blood samples from affected cats have the appearance of “cream of tomato soup.”

Sensory neuropathy of longhaired Dachshunds (probably autosomal recessive) causes pelvic limb ataxia at 8–12 wk of age. Urinary and GI function may also be disturbed. Paw position sense, spinal reflexes, and pain sensation are depressed, and self-mutilation can occur. There is a loss of myelinated fibers in sensory nerves and in selected areas of the spinal cord. There is no treatment, but affected dogs may have a relatively normal quality of life, provided self-mutilation does not occur.

Sensory neuropathy of Border Collies manifests as ataxia, lack of proprioception, and abnormal sensory testing. Onset is usually at 5–7 mo of age and progresses relentlessly. Euthanasia is the common endpoint. The genetics of this disease are not yet known.

Sensory neuropathy in Pointers is seen in English Pointers (autosomal recessive) in the USA and Shorthaired Pointers in Europe. Self-mutilation of the digits is the main clinical sign, and disease onset is before 6 mo of age. Pain perception is absent in the pelvic limbs and depressed in the thoracic limbs. There is neuronal loss in dorsal root ganglia. Prognosis is poor. There is no treatment.

Inherited polyneuropathy of Leonberger dogs is a distal neuropathy, with an age of onset of 1–9 yr in this breed. Clinical signs include weakness, exercise intolerance, change in bark, and dyspnea. Pedigree analysis suggests an X-linked inheritance.

Musladin-Lueke syndrome is a connective tissue disorder affecting muscles, bone, heart, and skin. Clinical signs most prominently reflect muscle fibrosis and contractures. The resulting posture is that of the animal walking on its “tip-toes.” This is an inherited recessive disorder, reported in Beagles, for which the genetic mutation has been identified. The abnormal posture is present shortly after birth. Other clinical signs include thickened cartilage of the ear and wide-set eyes. Seizures may occur concurrently. It is not yet known whether the seizures are related to the primary disorder or are a concurrent disorder. There is no treatment. The human counterpart to this disease is progressive and frequently fatal; however, the disease appears to stabilize in dogs.

Congenital myasthenia gravis (autosomal recessive) has been described in Parson Russell Terrier, Smooth-haired Fox Terrier, and Springer Spaniel puppies. It is due to either a deficiency or dysfunction of the acetylcholine receptor, and there is none of the circulating antireceptor antibody seen in the more common acquired form of the disease. Clinical signs usually start at 5–10 wk of age. The characteristic finding is an exercise-induced weakness, often associated with megaesophagus. The prognosis is more guarded than in acquired myasthenia gravis. The congenital disease has also been described in cats. A presynaptic form is seen in 12- to 16-wk-old Gammel Dansk Hønsehund dogs (autosomal recessive). Treatment consists of anticholinesterase drugs.

Scotty cramp (autosomal recessive) causes episodes of muscular hypertonicity in Scottish Terrier puppies. These episodes are exacerbated by excitement, exercise, stress, and poor health and are characterized by a hypermetric gait and arching of the spine, which can cause the dog to somersault when it runs. The disorder seems to be related to faulty serotonin metabolism. Diazepam and promazines help to relieve signs.

Congenital myoclonus of Labrador Retrievers (familial reflex myoclonus) causes muscle spasms/hypertonicity from an early age. Puppies may be unable to walk or even maintain a sternal position due to extensor rigidity. The prognosis is very poor.

Hypokalemic myopathy of Burmese cats (autosomal recessive) causes periodic paralysis or weakness with ventral flexion of the neck. Cats are affected at 3–4 mo of age. Serum CK is markedly increased. Dietary supplementation of oral potassium usually produces a favorable response (eg, potassium gluconate solution at 2–4 mEq or mmol/cat/day, PO, until serum potassium levels are stable).

Myotonia congenita is seen in Chow Chows, Staffordshire Terriers, Great Danes, and Miniature Schnauzers (autosomal recessive) and causes signs similar to those seen in myotonic goats. There is often a degree of muscle hypertrophy, and marked stiffness is seen when dogs first rise. Dimpling is seen on percussion of several muscles, including the tongue. Diagnosis can usually be confirmed by electromyography (characteristic “dive bomber” sound); muscle biopsy changes are mild and nonspecific. Prognosis is guarded, although membrane-stabilizing drugs (procainamide, mexiletine) result in significant improvement.

X-linked muscular dystrophies have been described in Irish Terriers, Golden Retrievers, Miniature Schnauzers, Rottweilers, Samoyeds, German Shorthaired Pointers, Groenendaeler Belgian Shepherds, Brittany Spaniels, Rat Terriers, Labrador Retrievers, Japanese Spitz dogs, and also in cats. All are due to mutations in the dystrophin gene. Males show muscle stiffness, dysphagia, and weakness at an early age, along with a plantigrade stance and muscle atrophy as the animal gets older. Initial muscle hypertrophy may be marked, particularly in cats. Diagnosis is facilitated by the initial massive increases in serum levels of CK and by demonstration of hyalinized and mineralized muscle fibers on biopsy. Prognosis is guarded to poor. Currently, there is no treatment. A novel congenital muscular dystrophy has been reported in cats associated with deficiency of merosin (laminin α₂), in which degenerative changes occur in both muscles and peripheral nerves.

Labrador Retriever myopathy (autosomal recessive) causes a stiff gait and marked muscle atrophy in puppies of both sexes. Signs worsen with cold, stress, or exercise, and affected dogs may be unable to keep their heads elevated in a normal position from as early as 3 mo of age. Tendon reflexes are usually absent. Signs stabilize by 6–8 mo of age, and the prognosis is favorable, so affected dogs can make good pets. There is preferential atrophy of type II muscle fibers.

Dermatomyositis of Collies and Shetland Sheepdogs (inherited as a dominant trait with variable expressivity) causes atrophy and weakness of the masticatory and distal limb muscles from a few months of age, sometimes associated with trismus and megaesophagus. These signs are combined with a dermatitis over the face and extremities. The clinical signs may wax and wane and, in general, do not become severely debilitating. Polymyositis and dermatitis are evident on histopathologic examination. This disorder has also been seen in Beauceron Shepherds, Pembroke Welsh Corgis, Australian Cattle Dogs, Lakeland Terriers, Chow Chows, German Shepherds, and Kuvasz dogs.

Glycogen storage diseases can cause muscle weakness and exercise intolerance in young dogs and cats. Examples include glycogenosis types II (Lapland dogs), III (German Shepherds and Akitas), IV (autosomal recessive in Norwegian Forest cats), and VII (English Springer Spaniels).

Mitochondrial myopathy has been described in Clumber and Sussex Spaniels and in Old English Sheepdogs. Mitochondrial myopathies result in exercise intolerance and collapse, and blood lactate and pyruvate levels are often increased after exercise. Ragged red fibers, indicating increased numbers of mitochondria, may be seen on muscle biopsy. Inherited disorders of carnitine metabolism are another cause of mitochondrial myopathy; they may cause accumulation of lipid vacuoles within muscle fibers.

Nemaline rod myopathy (probable autosomal recessive) in cats causes weakness and later a hypermetric gait at 6–18 mo of age. Patellar reflexes are depressed, and muscle atrophy develops progressively. Large numbers of nemaline rods are found in skeletal muscle fibers. The prognosis is poor. A similar disorder is seen sporadically in young dogs.

Central core myopathy has been described as a cause of weakness, muscle atrophy, and exercise intolerance/collapse in young Great Danes in the UK. Signs begin at approximately 6 mo of age. Prognosis is poor.

Congenital megaesophagus is inherited in Wirehaired Fox Terriers and Miniature Schnauzers and possibly also in German Shepherds, Great Danes, Irish Setters, Newfoundlands, Chinese Shar-Pei, Greyhounds, and Siamese cats. Clinical signs include regurgitation and aspiration pneumonia. Prognosis is guarded.

Devon Rex cat hereditary myopathy (autosomal recessive) is seen in kittens around 4–7 wk old and is characterized by exercise intolerance and passive ventroflexion of the head and neck, which is especially noticeable during locomotion, urination, or defecation. Some cats assume a “dog-begging” position. Megaesophagus is present. Prognosis is guarded.