2026 Workshop Handout Bundle-W100J - Late Responses F Wave Responses and H Reflexes

W100J - Late Responses F Wave Responses and H Reflexes - Kesner

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The document reviews three late electrodiagnostic responses: the H reflex, F wave, and A wave. These responses help assess nerve pathways beyond the distal segments measured by routine nerve conduction studies, but each has distinct physiology and limitations.<br /><br />The H reflex is a true monosynaptic spinal reflex involving sensory Ia afferents and motor efferents. It is commonly recorded from the soleus after tibial nerve stimulation at the popliteal fossa. It is largest with submaximal stimulation and typically diminishes as the direct motor response increases. Its latency, side-to-side difference, and H/M amplitude ratio may help assess proximal conduction and motor-neuron-pool excitability. The test can support evaluation of S1 radiculopathy, but abnormalities are not specific to that root, and a normal response does not rule it out. Responses are affected by positioning, muscle activity, stimulation technique, age, and other factors.<br /><br />The F wave is a small, variable motor response produced when antidromic impulses travel to anterior horn cells and a small portion of motor neurons “backfires.” It is not a true reflex and tests motor axons, including ventral roots. Common measures include minimum or mean latency, chronodispersion, persistence, and the F/M amplitude ratio. F waves can be especially useful for detecting early proximal demyelination in acute inflammatory demyelinating polyneuropathy. Their value in radiculopathy, entrapment neuropathy, and plexopathy is limited: abnormalities generally cannot localize a lesion, and normal results do not exclude disease.<br /><br />A waves are small, reproducible late responses that may arise from axonal branching or ephaptic transmission. They can accompany reinnervation or demyelination but are not diagnostic in isolation. Distinguishing them from F waves and other late potentials requires attention to consistency, stimulus intensity, and latency changes.

Keywords

H reflex

F wave

A wave

electrodiagnostic responses

nerve conduction studies

ncs

proximal conduction

S1 radiculopathy

motor axons

demyelination

late responses