ABPMR Neuromuscular Medicine common mistakes
ABPMR Neuromuscular Medicine · Understand the traps and how to avoid them.
On needle EMG, the candidate labels every spontaneous discharge as 'fibrillations' and then uses that label to localize the lesion, without distinguishing fibrillation potentials from positive sharp waves, fasciculation potentials, myotonic discharges, complex repetitive discharges, or cramp potentials.
Textbook figures present spontaneous activity as a single category, and candidates memorize 'spontaneous activity = denervation' rather than the firing pattern, waveform shape, and sound that identify each generator. Under time pressure the default label wins.
What to do instead
Before answering any localization question, describe each discharge out loud using three features: firing pattern (regular vs. irregular vs. waxing-waning), morphology (brief biphasic vs. long-duration triphasic vs. spike-and-slow-wave), and auditory signature. Then map each to a generator: fibrillations/positive sharp waves = single muscle fiber, fasciculations = motor unit, myotonic discharges = muscle membrane instability, complex repetitive discharges = ephaptic spread in chronic denervation.
The candidate classifies a nerve as demyelinating purely because the response amplitude is low, or classifies it as axonal because conduction velocity is mildly slow, without checking for conduction block, abnormal temporal dispersion, or whether slowing meets formal criteria in the required number of nerves and segments.
Amplitude is the most visually salient number on a report, and candidates conflate 'small response' with 'demyelination' because both feel like severe disease. Formal demyelinating criteria are multi-part and easy to half-remember.
What to do instead
Use a fixed two-step routine on every case: first ask whether slowing is disproportionate (velocity below roughly 70-80% of the lower limit, distal latency above roughly 125-150% of the upper limit) in the required number of nerves or segments; second ask whether amplitude falls from distal to proximal stimulation with dispersion, which indicates conduction block rather than axonal loss. State explicitly that a low amplitude with normal latency and velocity is axonal until proven otherwise.
The candidate interprets waveforms obtained from a cold limb, or from a limb with unrecognized technical problems (stimulator off the nerve, submaximal supramaximal stimulation, active reference electrode, excessive averaging), and reports a diagnosis such as demyelination or conduction block that the raw data do not support.
Case vignettes rarely state limb temperature and candidates do not think to ask; technical artifacts produce plausible-looking numbers, and there is no habit of confirming supramaximal stimulation or checking for a volume-conducted response.
What to do instead
Make temperature and technical adequacy an explicit checkpoint before interpretation: limb at least 32 degrees Celsius (note that cooling slows conduction velocity by roughly 1.5-2.5 m/s per degree and prolongs distal latency), stimulator output increased until the response stops growing, and the response confirmed as arising from the stimulated nerve by observing an appropriate twitch and checking for anomalous innervation. If a value cannot be trusted, say so rather than building a diagnosis on it.
When needle EMG shows fibrillation potentials in limb muscles, the candidate jumps to radiculopathy without checking paraspinal muscles, without confirming that the involved muscles share a single root rather than a single peripheral nerve or plexus cord, and without noting whether the findings cross plexus boundaries.
Radiculopathy is the most common referral diagnosis, so it becomes the default answer. Root-to-muscle mapping is detailed and candidates learn it as a list rather than as a spatial grid, so they cannot see when a pattern violates root anatomy.
What to do instead
Localize by serial elimination using three tests. First, are the findings confined to muscles innervated by one root across multiple peripheral nerves (radiculopathy) or one nerve or cord across multiple roots (plexopathy or neuropathy)? Second, are paraspinal muscles involved (supports root or anterior horn cell, spares plexus)? Third, is there a sensory nerve action potential that is normal in a numb dermatome (supports root pre-ganglionic lesion)? State all three conclusions before naming a diagnosis.
For suspected ulnar neuropathy, the candidate studies only the below-elbow or only the above-elbow segment, or interprets a low ulnar sensory amplitude as evidence of a lesion at the elbow without excluding a Martin-Gruber anastomosis or an accessory sensory contribution from the median nerve.
Standard ulnar protocols are often remembered as 'stimulate wrist and below elbow', and anomalous innervation is taught as trivia rather than as a routine confounder that changes the interpretation of every ulnar study.
What to do instead
Always stimulate at wrist, below elbow, above elbow, and axilla with the elbow flexed, and compare segments for focal slowing or conduction block. Before concluding a lesion site, check for a Martin-Gruber anastomosis by comparing median-nerve stimulated thenar versus ulnar-innervated hypothenar/adductor responses, and record ulnar sensory studies from the little finger with attention to median-nerve contribution. If values shift when the elbow position changes, repeat with the elbow extended to confirm the focal site rather than assuming it.
The candidate reads a generalized neuropathy study as 'axonal polyneuropathy' without stating whether the pattern is length-dependent and symmetric or non-length-dependent and asymmetric, and without using that distinction to build the differential.
Symmetric distal sensory loss is the prototypical neuropathy taught in medical school, so the pattern question feels already answered. Asymmetric or proximal-predominant patterns require actively comparing limbs, which candidates do not routinely do.
What to do instead
For every neuropathy case, first plot the abnormality: which nerves, which fiber types (sensory vs. motor), and whether the most abnormal values are in the longest nerves (feet and legs worse than hands and arms). A length-dependent gradient points to metabolic, toxic, hereditary, or distal symmetric polyneuropathy; a non-length-dependent or asymmetric pattern points to vasculitis, inflammatory demyelinating neuropathy, hereditary neuropathy with liability to pressure palsies, or multifocal motor neuropathy. Say which pattern you found before offering a differential.
The candidate attributes any focal nerve slowing to entrapment at a common site, even when the slowing is outside a typical tunnel, is multifocal, or is accompanied by conduction block at non-entrapment locations.
Entrapment neuropathies dominate outpatient practice, and the habit of fitting data to the most familiar diagnosis overrides the pattern-recognition step. Conduction block outside a compression site is a finding candidates are not primed to look for.
What to do instead
Check whether the lesion respects anatomical tunnels and whether it is confined to one nerve. Slowing at typical sites with a normal contralateral side supports entrapment; multifocal slowing, conduction block in non-entrapment segments, or a patchy asymmetric pattern supports an immune-mediated or vasculitic process. Also review the history for systemic features and consider whether the pattern is mononeuropathy multiplex, in which case the workup is entirely different.
The candidate diagnoses amyotrophic lateral sclerosis or another anterior horn cell disease in a patient with pure lower motor neuron findings, without excluding multifocal motor neuropathy, spinal and bulbar muscular atrophy, post-polio syndrome, or cervical myeloradiculopathy, and without confirming that upper motor neuron signs are truly present rather than attributed to an unrelated cause.
The vignette describes progressive weakness with fasciculations and atrophy, which pattern-matches ALS immediately; the discriminating features that exclude mimics are subtle, and the candidate stops reasoning once the familiar diagnosis appears.
What to do instead
Before committing to an anterior horn cell diagnosis, require three affirmations: upper motor neuron signs documented on examination (not inferred from imaging), progression that is not confined to motor conduction-block territory, and exclusion of mimics using sensory nerve studies (normal in anterior horn cell disease, abnormal in most plexopathies and myeloradiculopathies), anti-GM1 antibodies and conduction block for multifocal motor neuropathy, and genetic testing where the phenotype suggests a hereditary motor neuronopathy. Note that preserved sensation with weakness is compatible with both ALS and multifocal motor neuropathy, so sensory sparing alone does not settle the question.
The candidate evaluates a suspected neuromuscular junction disorder using a single low-frequency repetitive stimulation in one distal muscle, treats any decrement as diagnostic of myasthenia gravis, and does not consider facilitation testing, proximal muscle selection, or the different physiological signatures of presynaptic versus postsynaptic disease.
Repetitive stimulation is remembered as 'the myasthenia test', and the numeric threshold and the muscle-selection rules are memorized without the physiology of pre- versus postsynaptic failure.
What to do instead
Choose muscles that are clinically weak and, when possible, proximal; test baseline, then low-frequency stimulation (2-3 Hz) looking for a decrement greater than about 10 percent that is maximal by the fourth or fifth response in myasthenia gravis; then add brief maximal exercise or high-frequency stimulation looking for facilitation above roughly 100 percent (or a smaller post-exercise increment depending on criteria) that identifies presynaptic disease such as Lambert-Eaton syndrome or botulism. Interpret a normal study as non-excluding, and pair the electrodiagnosis with antibody testing and, where indicated, single-fiber jitter.
The candidate sees short-duration, low-amplitude, polyphasic motor unit potentials and stops at 'myopathy', without stating whether spontaneous activity is present, without distinguishing myotonic discharges from cramp potentials, and without linking the electrodiagnostic pattern to the surveillance the specific myopathy requires.
Myopathy is treated as a single electrodiagnostic endpoint, when in fact the presence or absence of irritability, the distribution, and the associated cardiac or respiratory risk separate inflammatory, necrotizing, dystrophic, channelopathic, and steroid-induced processes.
What to do instead
Report myopathy in three parts: motor unit potential morphology (brief, small, polyphasic), spontaneous activity (absent, or fibrillations and positive sharp waves indicating an irritable or necrotizing process), and distribution. Then link the pattern to management: irritable myopathy prompts consideration of inflammatory or necrotizing causes and possible biopsy; myotonic discharges prompt evaluation for myotonic dystrophy with cardiac conduction screening and genetic testing; a painless proximal painless weakness pattern in a patient on corticosteroids suggests steroid myopathy and favors reducing the dose rather than adding immunosuppression. Distinguish the waxing-waning 'dive bomber' myotonic discharge from the high-frequency cramp potential, which is generated by the nerve terminal.