Does an axonal polyneuropathy necessarily display increased insertional activities? If nerve conduction velocity studies are consistent with an axonal polyneuropathy, can such a diagnosis be made even without fibrillation or positive waves?
Response from Orly Avitzur, MD, MBA
Lecturer, Department of Neurology, Yale University School of Medicine, New Haven, Connecticut; Assistant Professor, Department of Neurology, New York Medical College, Valhalla, New York
Absolutely. The electrophysiologic characteristics of neuronopathies and axonopathies depend on how long the condition has been present. In the acute states, there is moderate-to-severe denervation, which may include increased insertional activity, positive sharp waves, and fibrillation potentials. But this is often not the case for chronic motor neuronopathies or axonopathies, nor, of course, for sensory or autonomic axonopathies.
Generally, in axonal neuropathies, nerve conduction velocities are normal or slightly decreased; distal latencies are normal or slightly prolonged; but motor potentials and sensory amplitudes are significantly reduced. These changes are more apparent in the lower extremities where a distal pattern is greater than a proximal pattern. Needle electromyography (EMG) studies may detect spontaneous activity, such as increased insertional activity, positive sharp waves, and fibrillation potentials, within weeks, although various patterns develop depending on the tempo of progression. In Chapter 7 ("Clinical Electromyography") of Neuromuscular Disorders in Clinical Practice , Katirji and associates state:
. . . if the process is relatively active and progressive, a combination of denervation with reduced recruitment of reinnervated motor unit action potentials (MUAPs) will be seen, most prominent distally. If, on the other hand, the polyneuropathy is chronic and only very slowly progressive, reinnervation may completely keep pace with denervation. . . and the sole finding on needle EMG is reduced recruitment of reinnervation MUAPs.
Brown and coworkers, in Chapter 19 ("Negative Signs and Symptoms of Peripheral Nerve and Muscle Disease") of their textbook Neuromuscular Function and Disease, Basic, Clinical, and Electrodiagnostic Aspects , note that "chronic motor neuronopathies or axonopathies typically exhibit little or no abnormal insertional or fibrillation potential activity or evidence of instabilities of axonal or neuromuscular transmission and their M-potentials may be relatively well preserved, even in the face of severe motor axon losses." They also explain that in such situations, reinnervation probably keeps pace with the rate of axonal loss until the final stages.