Electromyographic (EMG) examination Anterior horn cell disease, Neuropathies, Neuromuscular transmission disease Summary of nerve conduction findings in different disease groups
Station 1- Anterior (Ventral) Horn Cell disease: This results in low CMAP amplitudes in muscle innervated by the dying anterior horn cells whose axons travel in the nerve being stimulated. There are fewer (than normal) axons that are able to "drive" action potentials in the muscle, the end result being a smaller (than normal) CMAP. Since there is still a population of normal axons from other anterior horn cells (non diseased) nerve conduction velocity is normal, i.e. the nerve (for instance the median nerve) has normal axons that camouflage the dying ones. The sensory nerve conduction studies are normal because ventral horn cells give rise to only motor fibers. Cell bodies of sensory fibers lie in dorsal root or cranial nerve ganglia.
REMEMBER: axonal or anterior horn cell diseases do not slow nerve conduction velocities appreciably as the remaining axons conduct at normal speed. There are just too few normal axons and thus, the evoked potentials in the muscle (CMAPs) are small.
Station 2- Peripheral Nerve disease: The findings will depend on whether both the motor and sensory axons are affected. In most peripheral nerve diseases both become affected. If the changes result in damage only to the axis cylinders the nerve conduction velocities are normal (healthy axons mask the defect), but both the CMAP and SNAP amplitudes will be reduced. If the peripheral nerve disease is predominantly demyelinating (i.e. all of the axons have demyelinated areas) the findings are marked slowing in both the motor and sensory nerve conduction velocities and relatively normal CMAP and SNAP amplitudes (the axis cylinders are OK).
REMEMBER: demyelinating nerve diseases slow nerve conduction velocities, but the evoked potentials are of relatively normal amplitudes.Station 3- Neuromuscular Junction disease: Nerve conduction studies (motor and sensory) are normal, but the hallmark of these diseases is a decremental CMAP response with repetitive nerve stimulation.
REMEMBER: neuromuscular transmission defects result in decremental CMAP responses with repetitive nerve stimulation.Station 4- Muscle disease: Nerve conduction studies are normal, but the CMAP amplitudes will be low, as there is loss of muscle fibers.
REMEMBER: primary muscle diseases result in low CMAP amplitudes, similar to the findings in ventral horn and axis cylinder lesions.In addition to the nerve conduction studies the EMG also involves:
The needle examination
An electrode is introduced into the muscle and recordings are made with mild to moderate activation of the muscle. This test is accompanied by some discomfort, but if performed appropriately should not be torture!
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