Comparative Neuromuscular Laboratory

NEUROMUSCULAR CASE OF THE MONTH - APRIL 2000

Right rear leg lameness in a 2 year old female spayed DSH cat
Contributed by Dr. Gary Ailes
Sierra Veterinary Hospital
Carson City, NV 89706

Clinical History
The cat was presented to the referring veterinarian for a 2 week history of right rear limb lameness that was reported to have begun following surgery for ovariohysterectomy. The cat was painful on muscle palpation. A muscle strain or tear was suspected and strict rest was prescribed for 2 weeks. Since no improvement was noted, the cat was referred for further evaluation.

Physical and Neurological Examination
On physical examination the cat was lame in the right rear leg and unable to support weight as it stepped forward (Go to video segment). Marked atrophy of the right quadriceps muscle was present. A bunny hopping gait was present on running. The remainder of the physical and neurological evaluation was normal. Referral radiographs did not show any abnormalities of hips, stife, hocks, or long bones.

Diagnostic Tests
CBC and biochemical analysis (including T4)
    Absolute lymphocyte - 7260 (1500-7000 /痞
    Absolute monocyte - 1760 (0-850 /痞)
    Total protein - 10.1 (5.9-8.5 g/dl)
    Globulin - 7.1 (3.4-5.2 g/dl)
    Serum creatine kinase (CK) - 192 (64-440 IU/L)
FeLV, FIV, FIP - negative

Muscle Biopsies - Since there was marked atrophy of the right quadriceps muscle, fresh and fixed biopsies were taken from the right (A) and also the left (B) quadriceps for comparison. There was marked myofiber atrophy and endomysial fibrosis within the right quadriceps muscle with lymphocytic infiltration having an endomysial distribution with invasion of non-necrotic fibers. While atrophy and fibrosis were not present within the left muscle, there were multifocal areas of lymphocytic infiltration having an endomysial distribution with invasion of non-necrotic fibers. No organisms were present within the biopsies.

Extensive atrophy, fibrosis and inflammation are present within the right quadriceps muscle, and mild, multifocal areas of lymphocytic infiltration are present within the left quadriceps muscle (B).

Figure 1A. Fresh frozen muscle
biopsy sections from the right quadriceps muscle (H&E stain)

Figure 1B. Fresh frozen muscle
biopsy sections from the left quadriceps muscle (H&E stain)

Conclusion
A diagnosis of an inflammatory myopathy consistent with polymyositis was made on the basis of results of muscle biopsy evaluations. Polymyositis is infrequently diagnosed in cats, and in a review of feline muscle biopsy specimens evaluated at the Comparative Neuromuscular Laboratory, polymyositis in cats was most often secondary to infectious (FeLV, FIV) or paraneoplastic (pre-lymphoma, thymoma) disorders. Serum antibody titers were not evaluated for Toxoplasma or Neospora. No evidence of neoplasia was found on physical examination. Of note in this case was the absence of an elevated serum CK. While serum CK elevation is a good indicator of muscle necrosis, it is a poor indicator of inflammation. Hyperproteinemia associated with hyperglobulinemia may be found secondary to inflammation, secondary to chronic antigenic stimulation, or associated with neoplasia.

Treatment included prednisone (5 mg q24hr for 5 days, 2.5 mg q24hr for 5 days, then 2.5 mg q48hr), L-carnitine (250 mg bid), lipoid acid (30 mg q24hr), and riboflavin (100 mg q24hr). On follow up evaluation 2 months after the initial diagnosis, the gait had markedly improved. The only residual deficit was occasional bunny hopping.