Abnormal Findings and Clinical Uses of FResponse and HReflex Studies

F-response and H-reflex studies are sometimes useful in the evaluation of patients with peripheral neuropathies,

Figure 24-23 Diagrammatic representation of the relationship between the direct (M) response, F response, and H reflex, and the intensity of the eliciting stimulus. With low-intensity stimulation of the tibial nerve, an H reflex is elicited from the soleus muscle. As the intensity of stimulation increases, the H reflex declines and a small M wave is seen. With a higher stimulus intensity, the H reflex disappears and the M wave increases in size until it is maximal. Following the maximal M wave, a small F response is sometimes seen. (From Aminoff MJ: Electromyography in Clinical Practice, 3rd ed. New York, Churchill Livingstone, 1998.)

particularly when the pathological process is so proximal that conventional nerve conduction studies fail to reveal any abnormalities. A prolongation of the minimum F-response latency may occur in patients having polyneuropathies with involvement of motor fibers, sometimes when conventional nerve conduction studies are normal. Abnormalities are sometimes encountered with radiculopathies, but these patients generally have abnormalities on needle electromyography so that the added F-wave abnormality does not influence management. H-reflex abnormalities (lost or delayed responses) may also be found in patients with polyneuropathies or with a lesion anywhere along the pathways subserving the reflex.

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