Summary: These are two forms of localized inflammatory myopathy, namely the focal myositis and localized nodular myositis. Their clinical presentation and histopathology are very similar. The claimed difference [Smith AG et al., 2000] is that focal myositis does not progress to polymyositis but some localized nodular myositis progress to polymyositis, particularly those with elevated CPK and ESR. They may well be in the spectrum of a same disease.

Localized nodular myositis presents as solitary mass in young or middle aged adults with the thigh as the most common site. Patient may or may not have general symptoms. Clinical features may simulate a soft tissue tumor or even sarcoma. Histologically, there are multifocal chronic inflammatory cell infiltration, endomysial inflammatory cell infiltration, myopathic and regenerating fibers, and fibrosis. Some of them, especially those with elevated ESR and CPK, may eventually evolve into polymyositis. [Cumming WJ et al., 1977; Toti P et al., 1994; Flaisler F et al., 1993; Smith AG et al., 2000; Yanmaz Alnigenis MN et al., 1999]

Focal myositis: Focaly myositis rarely progresses to involve other muscle but some localized nodular myositis will progress to polymyositis. CPK and ESR are normal or slightly elevated. The histopathologic features of these two entities are very similar. It is quite difficult to distinguish these two entities at the initial presentation. [Toti P et al., 1994; Flaisler F et al., 1993; Smith AG et al., 2000; Yanmaz Alnigenis MN et al., 1999]

Clinical features:

· Age and sex: Affect young and middle-aged adult with no sex predilection.

· Location: The thigh and lower leg are the most common locations but the shoulder, arm, back, and even the tongue have all been affected.

· Presentation: Localised nodular myositis usually presents as a painful, sometimes relapsing and remitting, nodular mass. The growth may be rapid and may clinically suggest a tumoral or a thrombophlebitic disease.

· ESR and CPK: They may or may not be elevated in localized nodular myositis. Focal myositis are usually not associated with elevated ESR and CPK.

· EMG: The three cases of localized nodular myositis described by Cumming WJ et al., (1977) showed a myopathic pattern.

Prognosis: Localized nodular myositis, particularly cases with elevated ESR and CPK, may progress to polymyositis. Focal myositis usually does not progress to polymyositis.

HISTOPATHOLOGY AND IMMUNOHISTOCHEMISTRY: Head

There is intense, predominantly interstitial chronic inflammation associated with scattered foci of muscle fiber destruction and regeneration which resemble muscle infarcts. Endomysial chronic inflammatory cell infiltration is present. The lesion may also have substantial fibrosis.

DIFFERENTIAL DIAGNOSIS: Head

Polymyositis: look for involvement in other muscle.

Other forms of myositis.