Please also see the complete
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
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
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.
- Katirji B, Kaminski HJ, Preston DC, Ruff RL,
Shapiro BE. Neuromuscular Disorders in Clinical Practice
. Boston, Mass: Butterworth-Heinemann; 2002.
Brown WF, Bolton CF, Aminoff MJ. Neuromuscular Function
and Disease: Basic, Clinical, and Electrodiagnostic Aspects
. Philadelphia, Pa: WB Saunders Co.; 2002.