Myopathy EMG

The myopathic motor unit

Myopathy randomly destroys or shrinks muscle fibres scattered through the motor unit's territory. The surviving fibres are fewer and dispersed, so the recorded motor unit potential becomes small in amplitude, short in duration, and polyphasic. Because each unit now generates less force, the nervous system must recruit additional units far earlier than normal to achieve a given tension — the hallmark of early (or full) recruitment: a dense interference pattern produced at low effort and low force. This recruitment signature, not amplitude alone, distinguishes a myopathic pattern from the reduced recruitment of a neurogenic process, where few units fire rapidly.

Irritable (inflammatory) myopathy

The inflammatory myopathies — polymyositis, dermatomyositis, immune-mediated necrotizing myopathy, and inclusion body myositis — are irritable: segmental necrosis splits muscle fibres and isolates regions from their endplate, rendering them spontaneously active. The needle therefore finds fibrillation potentials and positive sharp waves at rest, the same spontaneous activity seen in denervation but here arising from muscle membrane instability rather than from axon loss. Combined with small, short, polyphasic motor unit potentials and early recruitment, this triad — abundant fibrillations, myopathic units, early recruitment — is the classic irritable myopathy signature and, when proximal and symmetric, points strongly toward an inflammatory or necrotizing process. The density of fibrillations also serves as a crude activity marker, tending to fall with effective immunosuppression.

Myotonic disorders

Myotonic discharges are runs of repetitively firing single muscle fibres whose amplitude and frequency wax and wane, producing the characteristic dive-bomber or revving-motorcycle sound on the loudspeaker. They reflect abnormal sarcolemmal excitability. In myotonic dystrophy (DM1/DM2) they accompany a chronic myopathy with distal-predominant weakness, small units, and frequently a superimposed cardiac conduction risk that makes the diagnosis clinically consequential. In the non-dystrophic channelopathies — chloride-channel myotonia congenita and sodium-channel paramyotonia — myotonic discharges may be the principal finding, and provocative protocols (the short and long exercise tests, with and without cooling) can fingerprint the responsible channel by the pattern of CMAP change after exercise.

Steroid myopathy: the instructive normal study

Exogenous and endogenous glucocorticoid excess produces a non-irritable myopathy of selective type II (fast-twitch) fibre atrophy. Because type II fibres are recruited only at higher force and because the process is atrophy without necrosis, the needle examination is frequently normal: there is no fibrillation (the membrane is stable, not denervated) and routine low-effort sampling under-recruits the affected fibre population, so myopathic units may not be appreciated. This is a high-yield teaching point: a normal EMG does not exclude a steroid myopathy. When a patient on chronic corticosteroids develops proximal weakness, a normal study with a normal or only mildly elevated creatine kinase actually supportssteroid myopathy and argues against a superimposed irritable inflammatory process — a distinction that determines whether to escalate or taper immunosuppression.

Metabolic myopathy

Disorders of glycogen, lipid, and mitochondrial energy metabolism often leave muscle electrically normal at rest, because the defect is one of energy supply under load rather than of structural fibre integrity. The resting and minimal-effort EMG may be unremarkable even when exertional cramps, myoglobinuria, or fatigue are prominent. Forearm exercise testing (measuring the rise in venous lactate and ammonia after ischaemic or non-ischaemic exercise) interrogates the glycolytic pathway: a flat lactate with a normal ammonia rise suggests a glycogenolytic block such as McArdle disease. The lesson is that a normal needle examination narrows but does not exclude myopathy, and that provocative and biochemical testing carries the diagnosis when the resting study is silent.

Muscle selection and sampling pitfalls

Because most acquired myopathies are proximal and symmetric, sampling should target proximal muscles — deltoid, biceps, iliopsoas, vastus — while reserving a paraspinal or a more affected muscle when the proximal limbs are equivocal. Two procedural rules protect interpretation: avoid a muscle that has been needled within the preceding weeks (needle insertion itself produces focal fibrillation-like activity that can be misread as irritability), and avoid recently biopsied sites for the same reason. If a biopsy is planned, the convention is to study one side and biopsy the contralateral homologue, so that the histology is not corrupted by needle trauma.