Polymyositis in cats

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Polymyositis is a relatively common myopathic disorder in dogs, but less common in cats. It has been suggested that polymyositis, masticatory myositis and other clinical variations, such as pharyngeal-oesophageal and focal appendicular myositis, may represent different clinical and pathological expressions of a single primary muscle inflammatory disease. The cause of polymyositis in dogs is not always known, although the responsiveness of the disease to immunosuppressive therapy suggests that the pathogenesis is immune-mediated. In people with polymyositis, the pathogenesis appear to involve cell-mediated immune mechanisms, with the inflammatory cells being mainly CD8+ T cells. 

Polymyositis occurs sporadically in cats, sometimes in association with thymoma. The inflammatory infiltrates are predominantly mononuclear, with small lymphocytes and macrophages. Neutrophils are seen infrequently. Eosinophils are rare. A polymyositis has also been observed in cats usually over 1 year of age, without breed or sex predisposition, and while the cause was not defined, many affected cats were hypokalemic (see hypokalemic myopathy). Pathological findings included myonecrosis, lymphocytic cellular infiltrates, internal nuclei and fibre regeneration. Clinical signs were characterized by a persistent ventroflexion of the neck, appendicular weakness especially in the thoracic limbs, painful muscles and exercise intolerance. Serum levels of CK and aldolase were elevated. Electromyography revealed fibrillation potentials, positive sharp waves and bizarre high-frequency waves. Prognosis was guarded. Some cats recovered spontaneously while others appeared to respond to corticosteroids. Recurrences were observed.

We have seen suspected immune-mediated, mononuclear polymyositis in muscles of several cats, including samples from one cat with myasthenia gravis and thymoma. In muscle samples from another cat with myositis, numerous muscle fibres stained positively with staphylococcal protein A-horseradish peroxidase.