EDX-01 · Neurophysiology Fundamentals

Neurophysiology Fundamentals. Part of the Electrodiagnostics series of the Reflex PM&R Board Review podcast. Audio correction: In the audio: Because it remains attached to its cell body, the distal sensory axon never undergoes Wallerian degeneration. — Correct: The sparing is conditional, not absolute. When a root lesion sits proximal to an intact dorsal root ganglion, the peripheral sensory axon does stay in continuity with its cell body, so it is spared Wallerian degeneration and the SNAP usually remains normal despite dense dermatomal numbness. That holds only while the ganglion and the axon distal to it are intact. Foraminal root disease that involves the DRG can reduce sensory amplitudes, which is documented in a prospective series of lumbosacral monoradiculopathy from disc herniation. The episode goes on to call a normal SNAP with dermatomal numbness proof of a root lesion, "full stop"; treat it instead as a localization clue that supports a preganglionic lesion without establishing one. Reduced SNAPs suggest a lesion at or distal to the DRG once technique and coexisting neuropathy are considered, and mild, early axonal, small-fiber or focal demyelinating postganglionic lesions can leave routine distal SNAPs normal. The companion chapter states the qualified version. 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-01.