Disorders of peripheral nerves, NMJ and muscles

Question 1

What are the clinical signs referable to myopathy

Answer 1

Polymyopathies tend to have a bilaterally symmetrical distribution

Spinal reflexes are usually preserved

Pain sensation is usually preserved

Generalized weakness
- short striding
- stilted gait
- protrusion of scapulae
- passive ventroflexion of the neck
- signs generally worsen with exercise

Muscle atrophy is present with some myopathies, but muscle hypertrophy may occur

May observe muscle pain with inflammatory myopathies

May observe increased serum CK levels

Question 2

What would be your differentials for generalized myopathic disease in cats

Answer 2

Inherited
- dystrophin-deficient muscular dystrophy
- alpha-dystroglycan deficient myopathy in Devon Rex and Sphinx cats
- hypokalemic paralysis in Burmese cats

Acquired
- protozoal myositis (i.e. toxoplasmosis)
- immune-mediated
- paraneoplastic (e.g. thymoma, hepatocellular carcinoma)
- idiopathic
- hypokalemia (e.g., secondary to hyperaldosteronism, kidney disease)
- hypernatremia
- hyperthyroidism
- hypoadrenocorticism

Other:
- myositis ossificans
- feline hyperesthesia syndrome
- Vitamin E/selenium responsive myopathy

Question 3

What would be your differentials for localized or regional myopathic disease in cats

Answer 3

Neoplasia:
- primary rhabdomyosarcoma
- local or distant metastatic disease)

Ischemia:
- arterial thrombi
- tumor emboli

Fibrotic myopathy and contractures:
- denervation injury
- chronically malunited fractures

Question 4

What are the key features for dystrophin deficiency (hypertrophic feline muscular dystrophy)

Answer 4

Also known as X-linked muscular dystrophy or Duchenne’s muscular dystrophy

This genettic abnormality results in defective myocyte trans-membrane proteins, which allows the leakage of intracellular components and an excessive inflow of calcium ions
- the defective muscles become hypertrophied and dysfunctional

Clinical signs:
- generalized muscle hypertrophy (including a protruding, hypertrophied tongue
- stilted gait
- regurgitation due to megaesophagus and dysphagia
- cardiomegaly is often observed on thoracic radiographs and echocardiography

Epidemiological factors:
- more males are affected (X-linked disorder)
- cats generally show obvious signs of disease by six months of age

Diagnosis:
- CK levels are usually very high (>10000 IU/L)
- definitive diagnosis is reached by the demonstration of dystrophin absence via immunohistochemistry of muscle biopsies

Question 5

What are the key features of alpha-dystroglycan deficiency in Devon Rex and Sphinx cats

Answer 5

Alpha-dystroglycan is an extracellular membrane-associated protein which is important for the maintenance of sarcolemmal integrity
- this genetic abnormality is inherited via an autosomal recessive pattern with variable phenotypic penetrance

Clinical signs:
- severe weakness including head ventroflexion and head-bobbing
- dorsally protruding scapulae
- high-stepping gait
- signs emerge between 3-23 weeks of age and are slowly progressive
- esophageal weakness and difficulty in swallowing often lead to fatal asphyxiation during eating

Diagnosis:
- CK levels are usually not elevated
- there are minimal changes on electromyography
- diagnosis is reached by immunohistochemistry demonstrating decreased levels of alpha-dystroglycan on muscle biopsies

Question 6

What are the key features of feline idiopathic inflammatory myopathy

Answer 6

Characterized by a sudden onset of weaakness, often accompanied by muscle pain and elevations of CK

Diagnosis:
- essentially a diagnosis by exclusion
- muscle biopsy reveals inflammatory infiltrates (mostly lymphocytes)

Treatment:
- some authors advocate the use of clindamycin (12.5-25 mg/kg, PO, q12h) for one week before the administration of immunosuppressant drugs

Prognosis:
- most cats make a gradual but complete recovery
- relapse is possible