Granulomatous interstitial polymyositis and intramuscular neuritis in a dog.

Background: Granulomatous myositis is a rare condition in both humans and dogs. In humans it is most frequently related to sarcoidosis, where a concurrent granulomatous neuritis has been reported occasionally. Simultaneous granulomatous myositis and neuritis have been diagnosed previously in dogs (unpublished observations), but have not been studied further. Additional investigations are therefore warranted to characterize this disorder. Here we present a detailed description of concurrent idiopathic granulomatous myositis and granulomatous neuritis in a dog with suspected immune-mediated aetiology.

Case Presentation: The dog presented with dysphonia and paresis in the pelvic limbs and tail. In addition to muscle biopsies being taken for histopathology, magnetic resonance imaging, computed tomography and electrodiagnostics were performed. Muscle biopsies displayed granuloma formation with giant cells and epithelioid macrophages in muscle fibres and nerve branches. Microorganisms were not detected. Long-term treatment with glucocorticoids was clinically successful. Two years after the clinical signs started, the dog presented with signs of sepsis and died. Histopathologically, no granulomatous inflammation could be demonstrated in either muscles or nerves at that time.

Conclusions: This case illustrates a granulomatous interstitial polymyositis and intramuscular neuritis that improved clinically and resolved histologically with glucocorticoid treatment. Idiopathic granulomatous myositis and neuritis should be considered as a differential diagnosis in dogs with clinical signs of neuromuscular disorders.