2025 Workshop Handout Bundle-W100AS Late Responses Fwave Hreflex

W100AS Late Responses Fwave Hreflex - Kesner
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This document by Dr. Vita G. Kesner provides a detailed overview of two key late responses recorded in electromyography (EMG)—the F-wave and H-reflex—used clinically to assess proximal nerve segments when routine distal nerve conduction studies are normal.<br /><br />The H reflex, first described by Paul Hoffmann, represents a true spinal monosynaptic reflex involving Ia sensory afferents, a direct spinal synapse, and alpha motor efferents. It is most commonly elicited from the soleus muscle via tibial nerve stimulation at the popliteal fossa and can also be recorded from the flexor carpi radialis (FCR) for median nerve studies. The H reflex amplitude is influenced by muscle contraction, antagonist activity, and vibration. Key measurable parameters include latency, amplitude, and the H/M amplitude ratio, which reflects motor neuron pool excitability. Latency is consistently stable due to the monosynaptic pathway. The test aids in evaluating proximal conduction, notably S1 radiculopathy, though abnormalities are not exclusive to this diagnosis.<br /><br />F-waves are late motor responses triggered by antidromic activation of motor axons that backfire anterior horn cells, generating orthodromic impulses to the muscle. These responses test motor axon integrity along the entire nerve pathway, including the ventral roots, and do not involve synapses. F-wave parameters include minimal and mean latencies, chronodispersion (variability of latencies), persistence (frequency of elicited responses), and F/M amplitude ratio. F-wave latencies depend on patient height and limb length. They are especially valuable in detecting proximal demyelination in early Guillain-Barré syndrome and syringomyelia but have limited utility in radiculopathy and entrapment neuropathies.<br /><br />The document also discusses A-waves—late motor potentials occasionally recorded during F-wave studies, believed to arise from axonal branching or ephaptic transmission in reinnervating or demyelinated nerves. A-waves are constant in latency and configuration across stimuli, distinguishing them from variable F-waves.<br /><br />Practical considerations for recording these responses include correct electrode placement, appropriate stimulus intensity and duration, avoidance of antagonistic muscle contraction, and sufficient rest intervals between stimuli to prevent habituation. The presence, absence, or alteration of these late responses must be carefully interpreted within the clinical and neurophysiological context, considering technical pitfalls and limitations.<br /><br />In summary, F-wave and H-reflex studies serve as important adjuncts to standard nerve conduction studies for assessing proximal nerve segments, motor neuron excitability, and certain neuropathic or radiculopathic conditions, albeit with specific interpretative constraints.
Keywords
F-wave
H-reflex
electromyography
EMG
proximal nerve segments
motor neuron excitability
latency
amplitude
A-waves
nerve conduction studies
ncs
radiculopathy