2023 Workshop Handout Bundle-Late Responses - F-Wave Responses and H Reflexes

Late Responses - F-Wave Responses and H Reflexes
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Late responses like the F-wave response and H reflex are commonly used in clinical practice to study the proximal nerve segments when nerve conduction studies of distal segments are normal. The H reflex is a true spinal reflex that involves the activation of sensory fibers, orthodromic conduction along sensory axons to the spinal cord, a monosynaptic reflex, and orthodromic conduction distally along an alpha motor neuron. The H-wave amplitude is influenced by factors such as muscle contraction, antagonists, and vibration. The F-wave response occurs after the CMAP response and is generated by the activation of distal motor axons, antidromic conduction along motor axons to the anterior horn cells, backfiring of a small portion of anterior horn cells, and orthodromic conduction distally along the same motor axon past the stimulation site to the muscle. F-waves can provide information about axonal loss and reinnervation. A-waves can also be observed during F-wave recordings and are small motor potentials that are identical in onset latency, amplitude, and configuration with each successive stimulus. They are thought to be signs of reinnervating nerves. F-waves and A-waves have various applications in diagnosing and assessing peripheral neuropathies and radiculopathies. However, there are limitations and pitfalls to consider when interpreting these late responses, and additional factors and techniques must be taken into account to ensure accurate and reliable measurements.