2022 Workshop Handout Bundle-Late Responses: F Waves Responces and H Reflexes

Late Responses: F Waves Responces and H Reflexes

Pdf Summary

The F-response and H-reflex are late responses frequently used in clinical practice to assess proximal nerve segments. They are particularly useful when other nerve conduction studies show normal results. The F-response and H-reflex have different mechanisms, advantages, and limitations. The H-reflex is a true spinal reflex that involves activation of Ia afferent fibers, orthodromic conduction along sensory axons to the spinal cord, a monosynaptic reflex, and orthodromic conduction distally along an alpha motor neuron. The H-reflex is largest at submaximal nerve stimulation and inhibited by stronger stimulation. The H-wave amplitude is influenced by factors such as muscle contraction, antagonists, and vibration. The latency of the H-reflex is longer than that of the M response. The F-response, on the other hand, is a late motor response that occurs after the CMAP. It is evoked by supramaximal stimulation of the nerve and involves activation of distal motor axons, antidromic conduction along motor axons to anterior horn cells, backfiring of a small portion of anterior horn cells, and orthodromic conduction distally along the same motor axon. F-waves can be used to estimate F-wave conduction velocities and proximal conduction times. The presence of A-waves in F-wave recordings may indicate reinnervation following axonal loss or demyelination. A-waves can be confused with F-waves but can be distinguished by their consistent configuration and superimposition. F-waves and A-waves have limited clinical applications and are most helpful in cases of specific diagnoses such as acute inflammatory demyelinating polyneuropathy and syringomyelia.

Keywords

F-response

H-reflex

late responses

clinical practice

nerve conduction studies

spinal reflex

A-waves

proximal conduction times

axonal loss

demyelination

ncs