EDX-04 · Late Responses, Repetitive Nerve Stimulation, Single-Fiber EMG, and Autonomic Testing

Late Responses, Repetitive Nerve Stimulation, Single-Fiber EMG, and Autonomic Testing. Part of the Electrodiagnostics series of the Reflex PM&R Board Review podcast. Audio corrections: (1) In the audio: but a normal study definitely rules out MG. — Correct: Normal jitter in an adequately studied clinically weak muscle argues against MG causing that weakness. One normal muscle does not globally exclude MG; interpretation depends on muscle selection, technique and clinical context. (2) In the audio: they hyperpolarize and then spontaneously backfire, generating a brand new orthodromic action potential. — Correct: Anterior horn cells are depolarized (re-excited) by the antidromic volley, not hyperpolarized. In about 1-5% of motor neurons this triggers a new orthodromic action potential, producing the F-wave. (3) In the audio: It indicates a depleted motor neuron pool and is a classic electrodiagnostic hallmark of motor neuron disease. — Correct: Repeater F-waves indicate a reduced pool of excitable motor units, which is not specific to motor neuron disease. In a study of 50 healthy participants and five disease groups of 50 each, repeaters were more frequent in every disease group, including diabetic polyneuropathy, entrapment neuropathies and root lesions, and did not distinguish between those causes (PMID 22431079). A separate ulnar study of healthy volunteers found repeaters also occur normally and vary with stimulation conditions (PMID 27861192). Keep the association with motor-unit loss including ALS, drop the word hallmark, and read repeaters alongside the needle examination and the rest of the study rather than as a diagnosis. The written chapter is correct. Full correction register: https://www.reflexpmr.com/errata. The full companion chapter and a linked board-style Q-bank set for this topic are at https://www.reflexpmr.com/read/EDX-04.