Myositis Overview and Presentation

Study Notes

Myositis encompasses several types: Dermatomyositis (DM), Polymyositis (PM), Inclusion Body Myositis, Statin-Induced Myopathy, and Rhabdomyolysis.
DM and PM are considered "rheumatic" and often associated with autoimmune and connective tissue diseases (CTD).

Incidence is rare, with 2-10 cases per million people.
Most adult cases occur between ages 45-65, with a female predominance (sex ratio 2-3:1).
DM often overlaps with other rheumatic disorders, such as lupus.

Characterized by progressive proximal muscle weakness, with symptoms developing over weeks to months.
Common areas affected include shoulder girdle, neck flexors, and hip flexors.
Can involve pharyngeal, esophageal, diaphragmatic, and cardiac muscles, although rare.
Minimal or absent pain, often accompanied by fatigue, weight loss, low-grade fever, Raynaud's phenomenon, and occasional lung disease (Interstitial Lung Disease - ILD).
Facial and ocular muscle involvement is rare but indicates potential for other diseases.

DM presents with extra-muscular features that are absent in PM, such as skin rashes and an increased risk of malignancy.
Key cutaneous manifestations include:
- Heliotrope rash (eyelids and malar areas).
- Gottron’s papules (over knuckles).
- V-sign (chest and neck) and Holster sign (proximal thigh).
Regular cancer screenings are essential for adults with DM.

Elevated serum creatinine kinase (CK) levels, often in the thousands, are typical indicators of myositis.
Additional muscle markers that may be elevated include aldolase, AST/ALT, LDH, and myoglobin.
Abnormal electromyography (EMG) can indicate myopathic changes but is not highly specific.
Muscle biopsy remains the traditional gold standard for diagnosis.
Muscle MRI is very sensitive and can identify inflammation in muscle tissue.
Skin biopsy in DM can reveal interface dermatitis, similar to cutaneous lupus, underlining the importance of lesion location in diagnosis.