Electrodiagnostic Medicine: Nerve Versus Muscle, Axon Versus Myelin
Separate the nerve from the muscle
Electrodiagnostic questions are won by deciding three things in order: is there a lesion, is it axonal or demyelinating, and where is it? Nerve conduction studies come first - keep limb temperature above 32 °C, because cooling slows conduction and inflates latencies. Demyelination produces focal slowing, conduction block, or temporal dispersion with relatively preserved amplitude; axonal loss reduces amplitude with only mild latency change. Needle EMG then characterizes the muscle: fibrillation potentials and positive sharp waves indicate active denervation or a necrotizing myopathy, while large, long, polyphasic motor unit potentials with reduced recruitment indicate chronic reinnervation. Fasciculations are nonspecific; myotonic discharges sound like a dive bomber.
Localization traps: cervical radiculopathy may spare paraspinal muscles if the lesion lies distal in the foramen, but paraspinal fibrillations support radiculopathy. In L5 radiculopathy the sural sensory response stays normal because the S1 dorsal root ganglion is spared, whereas a peroneal neuropathy reduces the superficial peroneal sensory amplitude. Carpal tunnel syndrome is a median sensory abnormality across the wrist; ulnar neuropathy at the elbow shows slowing across the elbow segment, not merely at the wrist. A Martin-Gruber anastomosis can mimic ulnar slowing and must be excluded. The H-reflex assesses the S1 root, while the F-wave assesses proximal conduction of many motor fibers. Myasthenia shows decrement on repetitive stimulation with normal conduction studies.
Worked example
Numb lateral fingers with prolonged median distal sensory latency, reduced median sensory amplitude, normal ulnar studies, and a normal needle exam means mild axonal carpal tunnel syndrome - start night splinting.
Rule of thumb
Amplitude tells you how many axons are lost; latency and conduction velocity tell you how much myelin is gone.