What can be detected in an EMG?

An EMG test is used to distinguish between a nerve disorder (such as ALS) and a muscle disorder (myopathy). The electrical signals recorded by the EMG are altered in both muscle disorders and motor nerve damage. However, an EMG examination can distinguish the electrical signal indicative of motor nerve damage in ALS (“neurogenic” damage pattern) from the findings of a muscle disorder (“myogenic” damage pattern). The EMG can therefore be used in the early stages of diagnosis to make a basic distinction between muscle (“myogenic”) and nerve disorders (“neurogenic”).

Neurologists experienced in EMG testing can distinguish, based on the electrical potentials recorded by the EMG, between current (“acute”) muscle damage and damage that has existed for some time (“chronic”). In ALS, both acute and chronic changes are typically detectable in combination, as ALS is a progressive neurological disease. EMG findings vary greatly depending on the course of the disease and the stage of the illness. Depending on the clinical course, the EMG can be an important adjunct in the diagnosis of ALS.