A waves in electroneurography: differential diagnosis with other late responses
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REVIEW ARTICLE
A waves in electroneurography: differential diagnosis with other late responses Liborio Rampello 1
&
Luigi Rampello 1 & Antonio Arcidiacono 2 & Francesco Patti 1
Received: 22 August 2019 / Accepted: 4 August 2020 # Fondazione Società Italiana di Neurologia 2020
Abstract Neurographic studies are an extension of clinical examination and are performed for the functional assessment of peripheral nerves. The study of motor and sensory conduction velocity and the presence, amplitude, morphology and symmetry of the response to electrical stimulation are crucial for the diagnosis and management of peripheral neuromuscular disorders. Neurography also plays an important role in the search for so-called late responses comprising the F wave, H reflex, axonal response and A wave. By analysing the parameters of each late wave, this paper addresses the pathophysiological features and the most common conditions impairing the physiology of late responses, with a special focus on A waves. Keywords A waves . F waves . Axon reflex . H reflex . Neuropathy . Neurography
Introduction Electroneurography is the most widely used and reliable instrumental test in neurophysiology [1]. It serves for the functional assessment of the peripheral nerves by measuring motor (MCV) and sensory (SCV) nerve conduction velocities and the presence, amplitude, morphology and symmetry of the response to electrical stimulation (CMAP and SNAP). Neurography also plays a complementary role in the search for so-called late responses [1]. In addition to triggering the motor response (M), motor nerve stimulation can also activate additional responses known as “late” responses as they present a latency longer than M. These responses yield useful information on the proximal segments in assessing neuropathies and can be distinguished on the basis of their latency that varies depending on the site and intensity of stimulation. They include the F wave (recurrent response), H reflex (reflex response), axonal response (recurrent response) and A wave
* Liborio Rampello [email protected]; [email protected]
(recurrent response). The key features of the late responses are summarized in Table 1 [1].
F wave The F wave is elicited by antidromic excitation of all stimulated motor axons, travelling to the spinal cord with activation of a small proportion of the anterior horn cells and with orthodromic action potentials of one or more motor axons travelling to muscle [2]. The name F wave dates back to the first recordings made in the small muscles of the foot [3]. The F wave is one of the late responses produced by antidromic activation of motor neurons in response to supramaximal stimulation [4]. Unlike the H reflex, F waves are more evident with higher stimulus intensity [5]. The result of intermittent antidromic activation of motor neurons is the onset of intermittent motor action potentials during stimulation. Hence, some F waves may not appear at every stimulation and may vary in morphology and have a low amplitude [6, 7]. Latency, amplitude, mo
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