[Electroneuromyographic criteria of amyotrophic lateral sclerosis]

Electrophysiological study plays a key role in the diagnosis of amyotrophic lateral sclerosis (ALS). ENMG demonstrates lower motor neuron involvement in muscles clinically involved but also in non clinically involved territories. Conventional EMG shows mixed signs of active denervation, partial chronic denervation and fasciculation, in a variable pattern from one muscle to another. The choice of the muscles to be recorded will privilege muscles innervated by motor neurons from bulbar region, cervical, thoracic and lumbar medulla in order to define the extension of the pathological process. Special ENMG techniques (single fiber EMG, macro-EMG, motor unit firing study, motor unit number estimate) have been proposed for the evaluation and better understanding of pathophysiology. They are not used in routine. Motor conduction study demonstrates motor axons loss and excludes nerve conduction abnormalities of a demyelinating motor neuropathy. Sensory conduction and sensory nerve action potential amplitude are normal in ALS, even in severely motor involved territory. Electrophysiological study can show upper motor neuron involvement when studying motor evoked potentials using transcranial magnetic stimulation of motor cortex. Several parameters have been studied. They are diversely affected and so their diagnostic sensitivity also varies.