Curriculum/Pillar 3 · Needle EMG Physiology

Myopathic vs Neurogenic Patterns

Module 12 of 36·30 min readREAL EMG DATA

Learning objectives

1Assemble the myopathic and neurogenic pattern tables from mechanism
2Integrate spontaneous activity, MUAP morphology, and recruitment into one read
3Recognise chronic reinnervation and active denervation

Every needle examination converges on a pattern. The art of electrodiagnosis is to assemble three independent measurements — spontaneous activity, MUAP morphology, and recruitment — into a coherent diagnosis of where in the motor unit the lesion lies. A neurogenic process attacks the unit from above, removing axons; a myopathic process hollows the unit from within, removing fibres. Because these two mechanisms perturb the same three measurements in opposite, internally consistent directions, their electrical signatures are separable with quantitative confidence when read as a set rather than one finding at a time.

REAL EMG DATAHealthy, myopathic, and neurogenic muscle side by side

Timebase

ms/div

Healthy control

Crisp tri/biphasic MUAPs around 0.5–1 mV that recruit smoothly as effort rises.

Myopathy

Small, short-duration, often polyphasic units; a dense (early/full) recruitment pattern at low force.

Neuropathy

Giant, long-duration MUAPs (>3 mV) from collateral reinnervation; reduced recruitment with rapid firing.

Feature	Healthy	Myopathic	Neuropathic
MUAP amplitude	Normal (0.5–1 mV)	Low (small units)	High (giant units)
MUAP duration	Normal	Short	Long
Recruitment	Normal	Early / full	Reduced
Firing rate at low force	Low	Low (many units)	High (few units)
Polyphasia	<15%	Increased	Increased

Three real concentric-needle recordings of tibialis anterior scrolling together: the healthy 44M (moderate, smooth units), the polymyositis 57M (small, short, low-amplitude units with early/full recruitment), and the chronic L5 radiculopathy 62M (giant >3 mV long-duration reinnervated units with reduced recruitment). The contrast between dense-and-small and sparse-and-giant is the entire lesson in one frame.

The neurogenic pattern

A lower motor neuron lesion subtracts axons, and the three measurements move together. Acutely, fewer surviving units must produce the required force, so recruitment is reduced— few units firing fast. Once Wallerian degeneration is complete (1–3 weeks), the orphaned fibres fire spontaneously, producing fibrillations and positive sharp waves that mark active denervation. Over months, surviving axons sprout to adopt the denervated fibres, packing them into enlarged units that become large in amplitude, long in duration, and polyphasic — the stamp of chronic reinnervation captured in the neuropathy recording, where single units exceed 3 mV. The simultaneous presence of fibrillations (active) and giant units (chronic) dates the lesion as both ongoing and old.

The myopathic pattern

A primary muscle disease sickens or destroys fibres within structurally intact motor units. Each unit, stripped of fibres, becomes small, short, and polyphasic, and — because each is individually weak — the muscle recruits an excessive number of units to meet even trivial force, producing early (full) recruitment. Crucially, fibre necrosis, splitting, and regeneration render the membrane unstable in irritable myopathies (inflammatory, dystrophic, necrotizing, and toxic), so fibrillations may also appear in a myopathic picture. Spontaneous activity therefore does not by itself imply a neurogenic process; it must be interpreted alongside MUAP size, which moves in opposite directions in the two diseases.

Chronic denervation and the convergence trap

Long-standing neurogenic disease can superficially mimic myopathy. After repeated cycles of reinnervation and re-denervation, units may fragment and acquire excessive polyphasia and instability, and severe chronic neuropathy can leave only a few enlarged units whose breakdown introduces short components. Conversely, end-stage myopathy with grouped fibre loss can develop mildly enlarged units. The resolving feature is the direction of the dominant MUAP change: neurogenic disease enlarges units even when polyphasia is florid, whereas myopathy shrinks them. Polyphasia, shared by both, is never the discriminator — amplitude and duration are.

The discriminating pairings

Single findings mislead; pairs resolve. Two combinations carry most of the diagnostic weight: amplitude × recruitment — tall units with sparse (reduced) recruitment is neurogenic, small units with dense (early/full) recruitment is myopathic; and spontaneous activity × MUAP size — fibrillations with large units indicate chronic active neurogenic disease, whereas fibrillations with small units indicate an active (often inflammatory) myopathy. Each pairing yokes a measurement that distinguishes the diseases to one that is shared, forcing the ambiguous finding into an unambiguous interpretation.

A side-by-side comparison

Feature	Healthy	Myopathic	Neurogenic
MUAP amplitude	Normal (~0.5–1 mV)	Low (small units)	High (giant, >3 mV)
MUAP duration	Normal	Short	Long
Polyphasia	<15%	Increased	Increased
Recruitment	Normal (ratio ~5)	Early / full	Reduced
Firing rate at low force	Low	Low (many units)	High (few units)
Spontaneous activity	None	± fibrillations (irritable)	Fibrillations / PSWs (active)
Distribution	—	Proximal, symmetric	Nerve / root / length-dependent

Distribution: the third axis

Pattern alone names the mechanism; distribution names the lesion. Neurogenic processes obey neuroanatomy — the abnormal muscles trace a single nerve, a root myotome, a plexus territory, or a length-dependent distal gradient — so the map of involvement localizes the lesion as precisely as the waveform characterizes it. Myopathies, by contrast, are typically symmetric and proximal(with characteristic exceptions: distal myopathies, inclusion-body myositis with its finger-flexor and quadriceps predilection). Sampling a deliberate set of muscles across nerves, roots, and proximal– distal levels converts a bench-side pattern into an anatomical diagnosis — the step that distinguishes a screening study from a definitive one.

Clinical Pearl

Read the three axes as a vote, not a single ballot. Spontaneous activity tells you whether the membrane is unstable; MUAP amplitude and duration tell you whether units are enlarged (neurogenic) or shrunken (myopathic); recruitment tells you whether units are too few (neurogenic) or excessively many for the force (myopathic); and distribution tells you where. When all four agree the diagnosis is secure; when they conflict, trust amplitude and duration over polyphasia, and trust firing rate over raw pattern density.

Common Pitfall

Equating fibrillations with neurogenic disease is the costliest reflex in pattern recognition. Irritable myopathies — polymyositis, dermatomyositis, necrotizing and dystrophic myopathies — generate abundant fibrillations alongside small, short, early-recruiting units. Reporting such a study as "denervation" inverts the diagnosis and the treatment. Whenever fibrillations are present, immediately pair them with MUAP size: large units confirm neurogenic disease, small units redirect to an active myopathy.

Key points

Neurogenic: reduced recruitment + fibrillations/PSWs (active denervation) + large/long/polyphasic units (chronic reinnervation, often >3 mV).
Myopathic: early/full recruitment + small/short/polyphasic units, ± fibrillations in irritable (inflammatory, dystrophic, necrotizing) myopathies.
Chronic neurogenic disease can mimic myopathy via fragmentation, but units enlarge in neuropathy and shrink in myopathy — amplitude/duration discriminate, polyphasia does not.
Resolve cases with two pairings: amplitude × recruitment, and spontaneous-activity × MUAP-size.
Distribution is the third axis: neurogenic follows nerve/root/length-dependent maps; myopathy is typically symmetric and proximal — it localizes the lesion.