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Proliferative Myositis : Intro & Clinical

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Introduction: Proliferative myositis is a rapidly-growing, benign, reactive pseudosarcomatous proliferation that diffusely infiltrates the muscle tissue. It is the intramuscular counterpart of proliferative fasciitis. Like proliferative fasciitis, it is also characterized by the presence of bizarre giant, ganglion-like cells.

Clinical Features: The patient is usually an adult (median age 50 years). It is rare in children. There is no gender or race predilection. The usual location is flat large muscles of trunk and shoulder region (pectoralis, latissimus dorsi, serratus anterior). Some cases involve thigh muscles.

Proliferative myositis presents as a rapidly growing, discrete, palpable nodule or mass in one of the locations mentioned above. Usually there is no pain or tenderness. Due to its rapid growth, it may double in size within a few days.

Gross Pathology: Grossly, proliferative myositis appears as a gray-white, scar-like, poorly-circumscribed area of induration within the involved skeletal muscle.

This low power view shows a fibroblastic proliferation infiltrating muscle fibers. The alternating zones of fibroblastic areas and atrophic skeletal muscle bundles creates a characteristic checkerboard pattern.

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