Paraspinal and limb motor neuron involvement within homologous spinal segments in ALS

We studied the involvement of motor neuron groups innervating paraspinal muscles in amyotrophic lateral sclerosis (ALS) and evaluated the value of paraspinal muscle EMG in the diagnosis of the disease.

We used quantitative concentric needle EMG to study denervation and reinnervation in a paraspinal muscle and a limb muscle innervated by the C6 and L5 segments in 32 patients with ALS. As control subjects we studied 11 patients with peripheral neuropathy, and 46 non-neurogenic control subjects.

We found similar abnormalities in motor-unit potentials (MUPs) in paraspinal and limb muscles in these two segments in ALS. Fasciculation potentials (FPs) were more frequent in limb muscles than in paraspinal muscles and fibrillations and sharp waves (fibs-sw) were most frequent in tibialis anterior. In peripheral neuropathy paraspinal muscles were normal but tibialis anterior showed very abnormal motor unit potentials.

These results are consistent with generalised involvement of motor neurons in motor neuron pools in spinal segments in early stages of ALS progression. However, distally predominant fibrillations indicate susceptibility to ongoing denervation in reinnervated distal axons. Complex FPs of similar morphology to MUP analysis in the same early affected muscle suggests a proximal origin for these FPs at this phase.

Our observations emphasize the value of paraspinal muscle EMG in the electrophysiological diagnosis of ALS.