NEUROMUSCULAR CASE OF THE MONTH - JUNE 2004

Episodic muscle swelling and lameness in a Labrador retriever
Contributed by Drs. Stephanie Kube, Tammy Stevenson, Karen Vernau, Peter Dickinson and Rick LeCouteur
Veterinary Medical Teaching Hospital, University of California, Davis, CA

Clinical History
A 6 year old female, spayed Labrador retriever (A) was presented for an 8 month history of episodic muscle swelling and lameness following exertion and mild trauma. Lameness and swelling began initially in the thoracic limbs and later involved the pelvic limbs from the proximal femur to the stifle. Episodes were usually associated with prolonged exercise (hiking, walking up hill, swimming) as well as with impact (jumping into or out of the car). Episodes usually resolved on their own within 24 hours without therapy. Radiographs of the elbows, shoulders, and cervical vertebral column were within normal limits. Results of in-house serum chemistry performed 2 months prior to referral showed mildly elevated ALT and AST concentrations and markedly elevated CK concentration (32,648 IU/L; reference range:10-200 IU/L).

Physical and Neurological Examination
The dog was well-muscled with asymmetry of the pelvic limb musculature. The right quadriceps, semimembranosus, and semitendinosus muscles were larger than the left (B). The swellings were firm but did not appear to be painful to the dog. No other specific abnormalities were found on physical and neurological examination. The problem was localized to the motor unit and a myopathy was suspected.

Diagnostic Testing
CBC – No significant abnormalities
Serum Chemistry Profile – ALT 143 IU/L (reference range: 19-67); AST 182 IU/L (reference range: 21-54 IU/L); CK 4914 IU/L (reference range: 36-414 IU/L)
Thoracic radiographs (3 views) – Within normal limits
Abnormal ultrasound examination – Within normal limits


Electrodiagnostic Testing
Electromyography showed mild abnormalities consisting of fibrillation potentials and increased insertional activity in the cranial tibial and lumbar paraspinal muscles (C). Motor and sensory nerve conduction velocity determinations, and results of repetitive stimulation testing, were within normal limits. Latencies and configurations of F-waves and H-waves were within normal limits.

 C
 

Muscle and Nerve Biopsy
A biopsy from the quadriceps muscle showed multifocal areas of mononuclear cell infiltration having an endomysial and perimysial distribution with invasion of non-necrotic fibers (D, H&E stain). No organisms were identified. Immunophenotypic analysis of cellular infiltrates performed at the Comparative Neuromuscular Laboratory, University of California, San Diego, showed a predominance of CD3+ (E) and CD8+ (F) lymphocytes with smaller numbers of CD4+ (G) lymphocytes. These findings are consistent with an immune-mediated inflammatory myopathy (polymyositis).

D	E
F	G

Conclusion
Based on the diagnosis of an immune-mediated inflammatory myopathy, treatment was initiated with immunosuppressive dosages of corticosteroids followed by gradual tapering. Five months following the diagnosis, the dog was exercising and acting normally, and maintained on low dose alternate day corticosteroids. Follow-up CK concentrations have been within the reference range. This case demonstrates an unusual presentation for an inflammatory myopathy and should alert clinicians to the possibility of this diagnosis in cases with intermittent muscle swelling and lameness..

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