Cases of polymyositis (PM) can be difficult to diagnose. Clinical presentation can include generalized weakness, megaesophagus, dysphagia, stilted gait, and muscle pain. Differential diagnosis includes: toxoplasmosis, neosporosis, Ehrlichia canis, Lyme disease, leptospirosis, Rocky mountain spotted fever, paraneoplastic syndrome (e.g. disseminated mastocytosis), drug-induced disorder, MG, hypothyroid myopathy or neuropathy, lipid-storage myopathies, and lactic acidemias (very painful and require pre- and post- exercise lactate concentrations to confirm).

Elevation of CPK are neither sensitive nor specific for PM. Levels are persistently elevated in ~ 75% of cases, and the degree of elevation depends upon the amount of muscle involved and the degree of muscle necrosis. In some cases, the inflammation is perivascular and not within the muscle tissue itself; these cases may have little to no CPK elevations. Persistent CPK elevations of 40,000-50,000 U/L are more consistent with muscular dystrophies than PM. Newfoundlands are a breed affected with both MG and PM. In the latter case, they typically have megaesophagus and dysphagia with elevated CPK, although ~ 25% have normal CPK concentrations.

Anorexic dogs rarely have elevated CPK concentrations, whereas these are often very high in anorexic cats. In seizuring animals, CPK increases within hours and then decreases again within 48 hours.

Diagnostic confirmation requires an open muscle biopsy as punch biopsy is inadequate. Biopsies from 2 sites are preferred because the disease can have patchy distribution. For patients with generalized weakness, the biopsy sites should be from proximal limb muscles: triceps (forelimb) and vastus lateralis (hind limb). Even with focal signs of PM such as megaesophagus, some histologic changes will be seen in limb muscles. Requirements are a 0.5 cm × 0.5 cm × 1 cm piece of muscle tissue, with the 1 cm taken along the longitudinal length of muscle fibers. Treat fresh biopsy tissue as described above for MMM. If there is muscle atrophy having a distal distribution, this is suggestive of peripheral neuropathy (e.g. as seen in male Leonbergers), and so distal limb muscles such as cranial tibial and extensor carpi radialis should be biopsied.

References: Shelton, J Sm An Pract 39: 368-372, 1998; Lipsitz, Berry, Shelton, JAVMA 215: 956-958, 1999.