Curriculum/Pillar 4 · Neuromuscular Disease Mechanisms

Myopathy Mechanisms

Module 17 of 36·28 min read

Learning objectives

1Explain how fibre loss and splitting generate small, short MUAPs
2Relate membrane instability to myopathic fibrillations and myotonia
3Distinguish the mechanisms of inflammatory, dystrophic, and metabolic myopathy

A myopathy attacks the motor unit from the inside. The innervation is intact and the axon delivers its command faithfully, but the muscle fibres that should answer are diminished — atrophied, necrotic, split, or electrically uncoupled. The electrodiagnostic phenotype of myopathy is therefore the mirror image of neurogenic disease: where denervation subtracts whole units and forces the survivors to enlarge, myopathy hollows units of their fibres while leaving the units themselves in place. Every characteristic finding — small short polyphasic MUAPs, early recruitment, and in some myopathies fibrillations — derives from that single geometric fact, refracted through the particular molecular lesion.

Why fibre loss yields small, short, polyphasic MUAPs

The MUAP is the spatial and temporal summation of the action potentials of all the muscle fibres of one motor unit lying within the recording radius of the needle. Its amplitude is dominated by the few fibres nearest the electrode, its durationreflects the total number and the temporal scatter of contributing fibres across the unit's territory, and its phases index the synchrony of their firing. When a myopathic process removes, atrophies, or functionally silences fibres scattered throughout the unit, fewer fibres contribute to each potential. The summed amplitude falls and, because the temporal spread of the surviving fibres collapses, the duration shortens. Loss of fibres also fractures the smooth summation, so surviving fibres — now sparse and firing slightly out of step, some conducting slowly through regenerating or split segments — generate extra phases and turns, rendering the MUAP polyphasic. The canonical myopathic unit is thus small in amplitude, short in duration, and polyphasic, the precise inverse of the giant long-duration unit of chronic reinnervation.

Early (full) recruitment

Because each myopathic unit has lost fibres, each generates less tension than a healthy unit of the same type. To produce even a small force the muscle must therefore call upon an abnormally large number of units relative to the force generated, recruiting additional motor units at low effort levels and filling the screen with potentials far sooner than the modest force would warrant. This is early (or full) recruitment: a dense interference pattern at minimal contraction, with many small units firing rather than a few large ones. It is the kinematic signature that most reliably separates myopathy from neurogenic disease, in which recruitment is reduced — few units firing rapidly. The contrast is mechanistic and exact: neurogenic disease has too few units for the force; myopathy has too little force per unit, hence too many units for the force.

Why some myopathies fibrillate

Spontaneous activity is not the exclusive property of denervation. Any process that electrically isolates part of a muscle fibre or renders its membrane unstable can generate fibrillation potentials and positive sharp waves. In irritable myopathies — the inflammatory, necrotizing, and dystrophic ones — three mechanisms converge: segmental necrosis can transect a fibre and functionally denervate the surviving portion distal to its endplate; regeneration and inflammation alter the resting membrane and channel complement of immature fibres; and fibre splitting creates membrane domains that fire autonomously. The presence of fibrillations therefore does not, by itself, prove a neurogenic process — a point of enormous practical importance, because abundant fibrillations alongside smallunits indicate an active myopathy, not denervation.

Inflammatory myopathy

The inflammatory myopathies (polymyositis, dermatomyositis, immune- mediated necrotizing myopathy, and inclusion-body myositis) combine immune-mediated fibre necrosis with active regeneration, set against an inflamed, channel-disordered membrane that is intrinsically irritable. Segmental necrosis severs fibres and functionally denervates the orphaned segments, so the needle finds fibrillations and positive sharp waves together with the small, short, polyphasic units and early recruitment of fibre loss. This irritable myopathic pattern — membrane instability layered on myopathic units — is the electrodiagnostic stamp of active inflammation and the basis on which the needle examination both supports the diagnosis and gauges disease activity (mechanistically; the clinical workup belongs to Pillar V).

Muscular dystrophy

The dystrophies are diseases of sarcolemmal fragility. In dystrophinopathy, loss of dystrophin uncouples the cortical actin cytoskeleton from the extracellular matrix by disrupting the dystrophin–glycoprotein complex that normally transmits and buffers the mechanical stress of contraction across the membrane. The unbuffered sarcolemma tears with use, admitting calcium, triggering proteolysis and necrosis, and driving repeated cycles of degeneration and regeneration until the regenerative reserve is exhausted and fibres are replaced by fat and connective tissue. The electrical correlate is, again, a myopathic MUAP picture (small, short, polyphasic units; early recruitment) — frequently irritable, with fibrillations, in the actively degenerating phase, especially in dystrophinopathy and some limb-girdle and dysferlin-related forms.

Metabolic and channelopathic myopathy

Metabolic myopathies are disorders of energy supply — defects of glycogenolysis, glycolysis, fatty-acid oxidation, or mitochondrial oxidative phosphorylation — in which the contractile apparatus is structurally adequate but cannot be powered under demand. Because the fibres are often intact at rest, the needle examination may be normal at rest and between attacks, with abnormalities (or frank electrical silence in the contracture of McArdle disease, a contracture that, tellingly, is electrically silent because it is a failure of relaxation, not a true action-potential-driven cramp) emerging only with provocation. The lesson is that a normal resting study does not exclude a metabolic myopathy.

The myotonic disorders are channelopathies of the sarcolemma that produce repetitive, self-sustaining membrane firing. Loss-of-function mutations in the muscle chloride channel CLCN1 (myotonia congenita) remove the large resting chloride conductance that normally stabilises the membrane and clamps the depolarizing effect of potassium accumulating in the T-tubules; deprived of this stabilising current, a single voluntary discharge triggers a run of after-depolarizations and the fibre fires repetitively. Gain-of-function mutations in the sodium channel SCN4A (paramyotonia, sodium-channel myotonias) impair channel inactivation, leaving a persistent inward sodium current that likewise drives repetitive firing. Either lesion generates the waxing-and-waning myotonic discharge— the audible "dive-bomber" — which is a pure membrane phenomenon, independent of nerve or junction, and a direct electrical readout of an unstable sarcolemma.

Clinical Pearl

Read MUAP size and recruitment together and the myopathic signature is unmistakable: small, short, polyphasic units recruited early and fully, producing a dense low-amplitude interference pattern at trivial force. When fibrillations accompany small units, do not retreat to a neurogenic reading — the combination defines an irritable myopathy (inflammatory, necrotizing, or actively dystrophic). And remember that a resting study can be entirely normal in metabolic myopathy: absence of spontaneous activity never excludes it.

Common Pitfall

The costliest myopathy error is to equate fibrillations with denervation and report an irritable myopathy as a neurogenic process — inverting both the diagnosis and the treatment. Whenever fibrillations are present, immediately pair them with MUAP size: small units redirect to an active myopathy, large units confirm neurogenic disease. Polyphasia is shared by both and never discriminates; amplitude and duration do. A second trap is over-reading early recruitment when effort is submaximal, and a third is concluding that a normal resting needle exam excludes a metabolic myopathy — it does not.

Key points

Myopathy hollows motor units of fibres while innervation stays intact — the geometric inverse of neurogenic reinnervation.
Fewer contributing fibres make MUAPs small (lower amplitude), short (collapsed temporal spread), and polyphasic (loss of synchronous summation).
Early/full recruitment arises because each weakened unit generates little force, so many units fire at low effort — opposite to the reduced recruitment of neurogenic disease.
Irritable myopathies (inflammatory, necrotizing, actively dystrophic) fibrillate via segmental necrosis, regeneration, and fibre splitting — fibrillations + small units mean active myopathy, not denervation.
Dystrophy is sarcolemmal fragility from disruption of the dystrophin–glycoprotein complex; metabolic myopathy is energy failure, often electrically normal at rest (McArdle contracture is electrically silent).
Myotonia is a sarcolemmal channelopathy: CLCN1 loss-of-function (reduced stabilising Cl⁻ conductance) or SCN4A gain-of-function (impaired Na⁺ inactivation) drives repetitive firing — the waxing/waning myotonic discharge.