Cause:Multiple autoimmune, usually involving CD8 killer T cells (Nejm 1991;324:877); plastic surgical bovine collagen injections (Ann IM 1993;118:920)
Pathophys:Perhaps autoimmunity to own muscle protein. In DM, immune complex activation and vessel damage via complement (Nejm 1986;314:329); in PM and IBM, autoimmunity is T-cell mediated
Incidence of all 3 about 1/100 000/yr
Dermatomyositis (DM): females > males in both children and adults
Polymyositis (PM): adults only
Inclusion body myositis (IBM): male/female = 3:1; Caucasians > blacks; most patients >50 yr old
Sx: Fever ± for weeks; proximal muscle weakness manifest by falling, trouble standing up, or with stair climbing, gradually progressive over weeks to months; painless (if painful, r/o polymyalgia rheumatica); dysphagia and regurgitation in 50%; never eye muscle involvement; Raynaud’s syndrome. IBM may affect fine distal motor early as well; finger flexor or toe extensors impaired in 50%
Si: Proximal > distal muscle weakness, palatal paralysis; periungual hyperemia and telangiectasias; minimal joint involvement; reflexes preserved unless muscle totally gone as often is the case with quadricep in IBM
in DM, above plus facial edema and dusky erythema, esp in sun-exposed areas; heliotrope (purplish) eyelid, periorbital, and facial coloring; rash w macular scaling plaques, esp in exposed areas and knuckles; scalp involvement (Jama 1994;272:1939)
Respiratory failure from muscle weakness; amyloidosis; 10% get interstitial pneumonitis and fibrosis (Am Rev Respir Dis 1990;141:727); myocardiopathy; heart block (Ann IM 1981;94:41). Cancer associated in 10% of PM and 15% of DM patients (Nejm 1992;326:363 vs 20% and 40%—Ann IM 2001;134:1087). DM appears in overlap syndromes with other connective tissue diseases
r/o other causes of muscle weakness including colchicine myopathy/neuropathy (Nejm 1987;316:1562); drug-induced myopathies (Semin Arth Rheum 1990;19:259) from penicillamine, AZT, ipecac, cimetidine, chloroquine, steroids, statins; and infectious myopathies from parasitic diseases, Lyme, legionella, and viral myositis
Lab:
Chem:AST (SGOT), LDH, aldolase, and CPK increased
Noninv:EMG, diagnostic small-amplitude action potentials and fibrillations
Path:Muscle bx shows segmental necrosis, enlarged central nuclei, regeneration (myoblasts, myocytes, basophilic myofibrils), focal inflammation, esp around vessels
Serol:ANAs positive (83%); anti-Jo-1 in 40% dermatomyositis and if positive, >50% have interstitial lung disease; anti-KJ; anti-Mi-2, etc (Ann IM 1995;122:715)
Rx:
(Ann IM 1995;122:715)
Steroids, eg, 60 mg prednisone qd × 3 mo, then taper to 5-10 mg q 1 mo as muscle strength increases and enzymes decrease; helps 80% (D. Dawson, HMS 3/85)
Azathioprine if steroids fail; also chlorambucil and cyclophosphamide
Methotrexate if lungs will tolerate it
Immunologic w immune globulin 2 gm/kg iv q 1 mo; expensive but clearly helps DM and may help PM and IBM (Nejm 1993;329:1993); plasma exchange and leukophoresis no help (Nejm 1992;326:1380)