H reflex In 1918, Hoffmann, after whom the H reflex was named, showed that submaximal stimulation of mixed motor–sensory nerves induces a muscle contraction (H wave, Fig. 2-14A) after a latency that is far longer than that of the direct motor response. This reflex, the electrical representation of the ankle jerk, is based on the activation of afferent fibers from muscle spindles (the same axons that conduct the afferent volley of the tendon reflex). Thus the long delay, typically 28 to 35 ms after the stimulus (adjusted for height and age), reflects the cumulative time required for the impulses to reach the spinal cord via the sensory fibers, synapse with anterior horn cells, and to be transmitted along motor fibers to the muscle (see Fig. 3-1). The H reflex is a useful measure because the impulse traverses both the posterior and anterior spinal roots. The H reflex is particularly helpful in the diagnosis of S1 radiculopathy and of polyradiculopathies. It is difficult to obtain an H reflex from nerves other than the tibial. Stimuli of increasing frequency but low intensity cause a progressive depression and finally obliteration of H waves. In parallel with the Achilles tendon reflex, the H-reflex is transiently obliterated in spinal shock (see Chap. 42).
F response (wave) The F response, so named because it was initially elicited in the feet, was first described by Magladery and McDougal in 1950. It is evoked by a supramaximal stimulus of a mixed motor–sensory or pure motor nerve (Fig. 2-14B). After a latency substantially longer than for the CMAP, a second small muscle action potential is normally recorded at 28 to 32 ms in arms and 40 to 58 ms in legs. This F wave is the result of impulses that travel antidromically in motor fibers to the anterior horn cells, a small number of which are activated and produce an orthodromic response that is recorded in a distal muscle. The F response is representative of proximal motor nerve and root conduction in that it traverses only the ventral root and can be elicited from a number of muscles.
Both responses are lost or delayed in some severe and generalized polyneuropathies (see Chap. 43). The combination of a normal F response and an absent H reflex is found in diseases of sensory nerves and roots. As with the H reflex, the F wave may be absent in the state of spinal shock or destructive diseases of the spinal cord (see Chap. 42). Both of these “late responses” find their main use as corroborative tests that are interpreted in the context of the entire nerve conduction examination.
Blink responses This special nerve conduction test is not in frequent clinical use but it serves a purpose in the diagnosis of certain demyelinating neuropathies and in any process that affects the trigeminal or facial nerve. The supraorbital (or infraorbital) nerve is stimulated transcutaneously and the reflex closure of both orbicularis oculi muscles is recorded with surface electrodes. Two CMAP bursts generated by facial motor neurons are observed: the first (R1) appears ipsilaterally 10 ms after the stimulus and the second (R2), ipsilaterally at 30 ms and contralaterally up to 5 ms later. The amplitudes of the responses vary considerably and are not in themselves clinically important. The first response is not visible as a muscular contraction but may serve some preparatory function by shortening the blink reflex delay. R1 is mediated by an oligosynaptic pontine circuit consisting of one to three neurons located in the vicinity of the principal sensory nucleus of the trigeminal nerve; R2 uses a broader and less-well-defined reflex pathway in the pons and medulla.