-inflammatory infiltrate within a muscle, chronic inflammation

-primarily lymphocytes, but can be other immune cells too

-association with other autoimmune disease suggest an autoimmune pathogenesis, but the exact etiology is unknown

-can be caused by infections, drugs, toxins

-progressive symmetric proximal weakness

-elevation of muscle enzymes (CPK & aldolase)

-histological features demonstrating mononuclear inflammatory cell infiltrates and muscle fiber necrosis 

-characteristic electromyographic abnormalities

-bimodal age distribution (10-15 years, then 40-60)

-M>F, AA 3-4x higher incidence 

-onset typically insidious w/proximal weakness

-difficulties getting up from chair, combing hair

-commonly involve upper esophageal muscles producing dysphagia and occasionally aspiration

-myalgias generally mild 

-no skin features 

-assoc. with anti-Jo-1 and ILD, Raynaud's 

-T cells cytotoxic to muscle on biopsy

-characteristic skin eruption occurs, usually preceding disease

-rash has several distinct varieties: Gottron's lesions, Poikilodermatous eruptions, Helitrope rash

-perifascicular atrophy is more common here

-T cells cytotoxic to muscle on biopsy

-immunoglobulin deposition and complement components in capillaries and small arterioles

-seen in DM

-erythematous, often scaly eruption that occurs in symmetric fashion over MCP and interphalangeal joints (can be confused with psoriasis)

-Sign: scaling, macular erythema over the extensor surface of the elbow 

-violaceous eruption on upper eyelids, often with eyelid swelling 

-lilac colored

-malar-like rash of DM involves nasolabial area (distinguishing feature from SLE rash in which this area is spared)

-lesions are very pruritic

-hyperkeratosis and cracking

-characteristically seen in patients with positive anti-Jo-1

-pulmonary involvement may take many forms such as resp. muscle weakness, ILD, pulm. vasculitis, pulm. HTN, aspiration

-cardiac problems such as heart block, arrhythmias, cardiomyopathy, MI

-arthropathies can occur: nonerosive, symmetrical, small joint

-GI: dysphagia, nasal regurgitation

-PV: Raynaud's, systemic vasculitis 

-these are patients with Anti-Jo-1

-present with fever, myositis, polyarthritis, ILD, Raynaud's 

-includes assessment of muscle strength including active resistance

-have pt. perform special tasks such as getting out of chair, lifting head from table while supine

-assessment of motor power may be confounded by fatigue and pain of arthritis or myalgia 

-history and PE

-muscle enzyme elevation

-confirmed by muscle biopsy, involved but not atrophic one 

-electron microscopy if wanting to diagnose inclusion body myositis 

-characterized by presence of intracellular vacuoles w/ inclusions under light microscopy

-onset over a period of years with proximal and distal muscle involvement is suggestive

-mainstays are corticosteroids, high dose prednisone until enzyme levels are normal or weakness has resolved

-methotrexate or azathioprine are second line agents in case of CS intolerance or resistance 

-rehab and PT for muscle