Physical and Neurological Examination
The dog was generally bright, alert, and responsive on presentation with no detectable abnormalities on routine physical examination. The dog could ambulate for only short distances and then would sit down.( Go to Video Clip)The pelvic limbs were more affected than the thoracic limbs. Segmental reflexes were normal to decreased and there was decreased withdrawal reflexes bilaterally. Placing reflexes were normal in the thoracic limbs and slow in the pelvic limbs. Mentation was normal. The neuroanatomical localization was a lower motor neuron disorder.

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Diagnostic Testing

CBC and Urinalysis: No significant abnormalities

Serum Chemistry: Mildly elevated AST and ALT. Elevated creatine kinase (CK) 2512 IU/L (reference 59-895)

Acetylcholine Receptor Antibody: 0.04 nmol/l (canine reference <0.6 nmol/l)

Bile Acids: Non-remarkable

CSF: Clear and colorless with normal white blood cell count and elevated protein at 119 mg/dl (reference 15-35)

ACTH Stimulation: Non-remarkable

Thyroid: T3(RIA) and T4(RIA) non-remarkable

MRI Lumbar Spine: Degenerative L2-3 disc with very mild bulging

Muscle and Peripheral Nerve Biopsies: Biopsies from the quadriceps and triceps muscles were evaluated. Marked mononuclear cell infiltration was present within both muscles (A). Immunophenotyping was performed and showed a mixed population of T cells (anti-CD3 with immunoperoxidase localization, B), dendritic cells (anti-CD11c, C), and macrophages (anti-CD11b, D). B cells were not identified (not shown, anti-CD21). The nerve biopsy was not abnormal.

Diagnosis and Treatment
A diagnosis of generalized inflammatory myopathy was made consistent with an immune-mediated polymyositis. Prednisone therapy was initiated at 0.5 mg/kg PO BID. The dog improved and was able to walk longer distances and squat to urinate. The CK concentration returned to the reference range. Prednisone therapy continued at tapering doses over a 5 month period. At approximately 4 months following the diagnosis of polymyositis, the dog developed a generalized lymphadenopathy (see picture below). A lymph node biopsy confirmed a large cell type malignant lymphoma. The dog is currently on a course of chemotherapy including vincristine, cytoxan and prednisone. Periodic updates will be given on how this dog progresses.

Comments
This case has many similarities to one of our previous Cases of the Month (January 2001). There was no evidence of neoplasia at the time of initial muscle biopsy. The dog responded to prednisone therapy, and several months following the diagnosis of polymyositis, lymphadenopathy developed. Phenotypes of cell populations within the muscle biopsy specimens differed from that of cell populations in the lymph node biopsy taken months later which showed 99%of the cells large B cells(lymph node immunostaining performed by Dr. Emily Walder). This case study further highlights the importance of periodic evaluation for neoplasia in dogs with a diagnosis of presumed immune-mediated polymyositis