Musculoskeletal Imaging

Polymyositis

a disorder of striated muscle characterized by diffuse, nonsuppurative inflammation and degeneration. The disorder is of unknown cause. When cutaneous disease is also present the term dermatomyositis is used. Polymyositis and dernatomyositis are considered to be collagen vascular diseases because of their clinical, pathological, and radiological features. Diagnostic criteria include:

proximal symmetric muscle weakness that progresses over a period of weeks to months;

elevated serum levels of muscle enzymes or an elevated level of urinary creatinine excretion;

an abnormal electromyogram; and

abnormal muscle biopsy findings that are consistent with myositis.

In some patients with Sjogrens syndrome, polymyositis with slowly progressive weakness of proximal muscles may occur.

Most patients have symmetric involvement of proximal muscles; Raynauds phenomenon is also common.

Radiographs reveal soft tissue and joint abnormalities, notably calcification and oedema of the subcutaneous tissue and muscle. Arthralgia and arthritis may be present but often joint abnormalities are not present on radiographs. Bone erosions and alignment abnormalities have been noted, however, especially radial subluxation or dislocation at the interphalangeal joint of the thumb (floppy thumb sign).

MR imaging is of value in the assessment of polymyositis. Muscle atrophy, fatty replacement of muscles and intramuscular regions of altered signal intensity are evident. Both standard spin-echo sequences and modified imaging techniques, such as fat suppression and short tau (inversion time) inversion recovery (STIR) sequences, may be useful.