Muscle Disorders (Myopathies)
Myopathies
Myopathic weakness is typically proximal, symmetric, and painless, with normal sensation and reflexes unless advanced. Creatine kinase may be elevated. Electrodiagnosis shows short-duration, small-amplitude, polyphasic motor unit potentials with early recruitment; fibrillations and positive sharp waves may occur in inflammatory and necrotizing myopathies, so spontaneous activity does not exclude myopathy. NCS are normal except for reduced CMAP amplitude in severe muscle loss.
Inflammatory myopathies: dermatomyositis has perifascicular atrophy, Gottron papules, heliotrope rash, and anti-Mi-2 or anti-MDA5 antibodies; polymyositis is a diagnosis of exclusion; immune-mediated necrotizing myopathy has very high CK and anti-SRP or anti-HMGCR antibodies; inclusion body myositis affects finger flexors and quadriceps asymmetrically in older adults, with rimmed vacuoles and poor steroid response.
Dystrophies: Duchenne and Becker (dystrophin), myotonic dystrophy type 1 (CTG repeat, distal weakness, myotonia, cataracts, cardiac conduction disease). Toxic myopathy from statins, steroids, and alcohol; channelopathies such as periodic paralysis and myotonia congenita.
Traps: IBM is often mistaken for polymyositis and treated with steroids; distal weakness with finger flexor involvement suggests IBM. Steroid myopathy causes proximal weakness with normal CK and no spontaneous activity. Myotonic discharges sound like dive-bomber potentials and occur in myotonic dystrophy, Pompe disease, and hyperkalemic periodic paralysis.
Worked example: an older man with asymmetric quadriceps and finger flexor weakness, mildly elevated CK, and myopathic EMG has IBM; muscle biopsy shows rimmed vacuoles, and immunosuppression is not effective.
Rule of thumb: early recruitment with small MUAPs indicates myopathy; reduced recruitment with large MUAPs indicates neurogenic disease.