Electromyography

Abstract

The most useful investigations for differentiating a myopathy from a neurogenic disease are the electrophysiological techniques. Needle EMG is mandatory to recognize the changes of motor unit (MU) structure and recruitment that characterize myopathic processes. Since in myopathies there is a high variability of morphological abnormalities, the motor unit potentials (MUPs) are extremely variable, and it is possible to find MUPs with typical short duration, low amplitude, and polyphasic shape, together with MUPs which look like the neurogenic ones. Fibrillation potentials and complex repetitive discharges (CRD) can also be present particularly in some metabolic myopathies and in sporadic inclusion body myopathy. Myotonic discharges are present in myotonic dystrophies and in congenital myotonias, where the myotonic phenomenon is also clinically present, but they can also be found in some diseases of ionic channels where the clinical myotonia is lacking. Nerve conduction studies are generally normal in myopathies. The SFEMG and repetitive nerve stimulation (RNS) are necessary when a disorder of the neuromuscular transmission is suspected. For the therapeutic implications, it is very important to distinguish between a presynaptic and a postsynaptic disorder. The former is characterized by a low amplitude of baseline compound muscle action potential (CMAP), which has a further decrement with low-frequency RNS. Only a high-frequency stimulation or a full prolonged maximum voluntary effort will reveal a great amplitude increase.