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Firing pattern of fasciculations in ALS

Evidence for axonal and neuronal origin

From the Department of Clinical Neurophysiology (B.U.K., D.F.S., M.J.Z.) and Neuromuscular Centre Nijmegen (H.J.S.); Institute of Neurology, Radboud University Nijmegen Medical Centre, Nijmegen; Faculty of Human Movement Sciences, VU University Amsterdam (D.F.S); and Netherlands ALS Centre (H.J.S.), Amsterdam, Utrecht, and Nijmegen.

Address correspondence and reprint requests to Dr. Bert Kleine, 314 Department of Clinical Neurophysiology, Institute of Neurology, Radboud University Nijmegen Medical Centre, PO Box 9101, 6500HB Nijmegen, The Netherlands b.kleine{at}neuro.umcn.nl

Background: In amyotrophic lateral sclerosis (ALS), the origin of fasciculations is disputed. We hypothesized that the discharge pattern of fasciculation potentials (FPs) would be different for FPs arising in the motor axon or in the spinal motor neuron.

Method: FPs were recorded by high-density surface EMG of the biceps brachii or vastus lateralis muscle for 15 minutes in 10 patients with ALS. Records were decomposed into different FP waveforms and their firing moments. Interspike interval (ISI) histograms were constructed for FPs that fired more than 100 times.

Results: Two types of ISI histograms were found. 1) In 23 of 30 different FPs with a total of 8,597 ISIs, the refractory period was 3 to 4 msec. ISIs longer than 15 msec had a Poisson distribution. Five of these 23 FPs discharged doublets with an ISI of approximately 5 msec, indicative of supernormality. This is consistent with the FPs arising in motor axons. 2) In the other 7 FPs, accounting for 11,266 ISIs, the refractory period was 17 to 46 msec. The preferred ISI duration was around 80 msec. Both timing factors are consistent with origin in the spinal motor neuron.

Conclusions: Firing pattern analysis, based on high-density surface EMG, can detect fasciculation potentials (FPs) of axonal and neuronal origin in amyotrophic lateral sclerosis. The two FP types coexist within the same muscle. The recognition that clinically identical fasciculations conceal the existence of two types of FP that can be studied in a noninvasive manner will introduce a new aspect in the research of motor neuron disease.

GLOSSARY: ALS = amyotrophic lateral sclerosis; FP = fasciculation potential; ISI = interspike interval; MU = motor unit.

Supported by The Netherlands Organisation for Health Research and Development (grant 940-37-034).

Disclosure: The authors report no conflicts of interest.

Received May 14, 2007. Accepted in final form July 31, 2007.