WS3.4. Electrodiagnostic Approach to Motor Neuron Disease: Split Hand in ALS

Abstract

Amyotrophic lateral sclerosis (ALS) is characterized by involvement of both upper and lower motor neurons. Electrophysiologic approach includes EMG (denervation potentials and fasciculation potentials) and nerve conduction studies (NCS; low CMAP, decreased motor unit number, and axonal hyperexcitability on nerve excitability testing) for lower motor neuron involvement, and transcranial magnetic stimulation with threshold tracking (decreased short- latency intracortical inhibition) for upper motoneuronal hyperexcitability. Of these, routine NCS can detect the “split hand” pattern, which is a very important indicator for diagnosis, because this symptom has been recognized as being specific to ALS; the thenar and first dorsal interosseous (FDI) muscles are preferentially affected. In contrast, the hypothenar muscle (abductor digiti minimi, ADM) is relatively preserved. FDI and ADM muscles are innervated by the same C8/Th1 spinal segments and the same ulnar nerve. The split hand indicates a potentially specific pathological mechanism, suggesting hyperexcitability of both upper and lower motor neuron/axons. Along with this symptom, wide-spread fasciculations are prominent clinical features in patients with ALS, specifically seen in ALS among disorders with neurogenic muscle atrophy. Fasciculations frequently arise from the motor nerve terminals, and the hyperexcitability of motor axons appears to constitute the pathophysiology of the disease. Neurophysiologic investigations of the upper and lower motor neurons/axons have shown increased excitability. The altered excitability is supposed to relate to motor neuron death.