EDX · EP 12 · ELECTRODIAGNOSTICS
Before You Listen
Before You Listen
- Prerequisites: the basic motor unit (alpha motor neuron, axon, neuromuscular junctions, innervated muscle fibers); components of a motor unit action potential (MUAP = motor unit action potential) and its measurement (duration, amplitude, polyphasia); the principle that compound muscle action potential (CMAP) amplitude reflects total mass of activated muscle fibers; fibrillation potential and positive sharp wave morphology; the difference between sensory nerve action potential (SNAP) and CMAP recordings; a working sense of recruitment.
- Runtime: approximately 1 hour 8 minutes.
- Topic in one line: the cardinal short-duration low-amplitude polyphasic MUAP with early recruitment that defines myopathy on needle electromyography (EMG); the rule that nerve conduction studies (NCS) are normal except for prolonged CMAP duration in critical illness myopathy (CIM); myotonic discharges in DM1, DM2, myotonia congenita, paramyotonia congenita, hyperkalemic periodic paralysis, and Pompe paraspinals; the muscular dystrophies anchored by Duchenne/Becker (Xp21 dystrophin) and the DM1-vs-DM2 distinction; the five-subtype inflammatory myopathy classification and the unique mixed myopathic-neurogenic pattern of inclusion body myositis (IBM); the metabolic pearls of electrical silence in McArdle and paraspinal myotonic discharges in Pompe; the steroid-myopathy triad (normal creatine kinase [CK], no fibrillation potentials, near-normal EMG); the CIM-vs-critical-illness-polyneuropathy (CIP) distinction.
Vignette. A 68-year-old man develops profound weakness and fails extubation after 14 days in the intensive care unit for septic shock complicated by acute respiratory distress syndrome. He received high-dose intravenous methylprednisolone for 5 days and a 72-hour vecuronium infusion early in the course. Sensation is preserved. Tendon reflexes are reduced symmetrically. Motor compound muscle action potential (CMAP) amplitudes are reduced to about 30% of normal in both peroneal and ulnar territories, and each CMAP is markedly prolonged in duration, visibly stretched out across the screen. Sensory nerve action potential (SNAP) amplitudes and conduction velocities are entirely normal. Direct muscle stimulation, performed because the patient cannot cooperate with needle EMG, produces a small low-amplitude muscle response.
What is the precise electrodiagnostic diagnosis, what single CMAP feature has near-100% specificity for it when present in more than one nerve, what does the normal SNAP exclude, and how does the direct-muscle-stimulation response distinguish this disorder from its closest mimic?
(Answer at the end of this chapter)
Section 1: The Cardinal Myopathic EMG Pattern and Why NCS Are Normal
Bottom line: in myopathy each motor unit loses muscle fibers but the alpha motor neuron, axon, and neuromuscular junction are intact, so motor unit action potentials become short in duration (the most reliable feature), low in amplitude, and polyphasic, and the nervous system compensates with early recruitment; nerve conduction studies are usually normal because compound muscle action potential amplitude reflects total muscle bulk.
The cardinal principle of myopathic electromyography lives in the muscle fiber itself. The alpha motor neuron, axon, and neuromuscular junction are intact; necrosis, atrophy, vacuolar degeneration, or membrane dysfunction reduces the number of functioning fibers within each motor unit. When fibers drop out, the motor unit action potential (MUAP) becomes smaller in amplitude (fewer fibers contributing) and shorter in duration (less spatial and temporal spread). Short duration is the single most reliable myopathic feature. Duration is measured from the initial deflection to the final return to baseline. Low amplitude follows from the same biology but is more variable because amplitude depends on electrode position and on muscle fiber hypertrophy, which can paradoxically raise amplitude in some dystrophies. Increased polyphasia (more than 4 phases) is common in myopathy but non-specific: up to 12 to 15% of MUAPs in normal muscle may be polyphasic, and polyphasia accompanies neurogenic reinnervation. Increased turns add specificity when combined with short duration and low amplitude.
The recruitment pattern is the second pillar. Each myopathic motor unit generates less force, so the nervous system activates additional units at a lower force threshold. The result is early recruitment: a full interference pattern fills the EMG screen at minimal effort, with many small units firing at low individual rates. The contrast with neurogenic disease is sharp: when units have been lost (denervation), surviving units must fire at higher rates to compensate, producing reduced recruitment with a few large units firing above 15 to 20 hertz even at maximal effort. A single motor unit normally fires at 10 to 15 hertz before a second is recruited; many units below this rate at low force is myopathic, few units firing fast is neurogenic.
Nerve conduction studies are usually normal in myopathy, and that fact is itself diagnostic. Sensory nerve action potentials (SNAPs) are always normal in pure myopathy because sensory nerves and dorsal root ganglia are unaffected. An abnormal SNAP should redirect the workup toward a neuropathic or combined disorder. Motor NCS are typically normal because compound muscle action potential (CMAP) amplitude reflects the total number of functioning muscle fibers within the recording-electrode pickup area; in most myopathies, fiber loss is diffuse and partial early in the course and overall muscle bulk is preserved. Exceptions: critical illness myopathy produces a hallmark prolonged CMAP duration approaching 100% specificity when present in more than one nerve (sodium channel dysfunction; Section 5). Severe distal and end-stage myopathies can reduce CMAP amplitudes globally. Myotonic dystrophy may produce diffusely low CMAPs from combined myopathy and membrane abnormality.
The American Association of Neuromuscular and Electrodiagnostic Medicine (AANEM) minimum study for suspected myopathy: needle EMG of two limbs including proximal muscles, two motor and two sensory NCS, and up to two repetitive nerve stimulation (RNS) studies to exclude neuromuscular junction (NMJ) disorders. Proximal sampling matters because most myopathies preferentially damage proximal limb girdles (deltoid, biceps, iliopsoas, vastus lateralis); distal-only sampling risks a false-negative study.
High Yield — Cardinal myopathic pattern and NCS rules
- Short duration is the single most reliable MUAP feature of myopathy. Low amplitude and increased polyphasia accompany it but are more variable.
- Early recruitment: full interference pattern at minimal effort with many small units firing at low rates. Compare with neurogenic reduced recruitment (few units at >15-20 Hz).
- Normal motor units fire at 10-15 Hz before a second unit is recruited.
- SNAPs are always normal in pure myopathy.
- CMAP amplitude usually normal; exceptions = severe distal/end-stage myopathy, myotonic dystrophy, and CIM.
- Prolonged CMAP duration approaches 100% specificity for CIM when present in more than one nerve.
- AANEM minimum: EMG of 2 limbs (including proximal muscles), 2 motor + 2 sensory NCS, up to 2 RNS studies.
Board Trap — Increased polyphasia is NOT specific for myopathy
A vignette describes increased polyphasia and asks whether the pattern is myopathic. Polyphasia alone is non-specific: up to 12-15% of MUAPs in normal muscle are polyphasic, and reinnervation in neurogenic disease produces polyphasia readily. The discriminators are duration (short = myopathic, long = neurogenic) and recruitment (early = myopathic, reduced = neurogenic). Pick polyphasia as the answer only when it co-occurs with short duration and low amplitude.
The needle detects the electrical activity of the muscle fibers closest to it very clearly. But it also detects the activity of fibers much farther away at the periphery of the motor unit territory. And those distant peripheral fibers contribute to the very beginning and the very end of the electrical waveform.
— EDX-12 podcast, ~06:18