7 - ELECTROMYOGRAPHY—NORMAL

Abstract

This chapter examines priorities and objectives of needle electromyography, the derivation of the motor unit potential (MUP), the types of needle electrodes in common use and the kind of information they can provide, and the associated techniques used to assess nerve and muscle function. Needle electrodes bring the recording surfaces of the electrode into much closer proximity to individual motor units (MUs) and muscle fibers (MFs). Thus, it is much easier to appreciate changes in the distribution patterns of MFs accompanying myopathies and alterations in neuromuscular or intramuscular axonal transmission present in these diseases. Moreover, although subcutaneous electrodes are able to detect the action potentials of single MFs, for example, fibrillation potentials, these are far better recorded by intramuscular electrodes. Because the qualities of the MUP and single MF potentials are dependent to some extent on the type of electrode used, it is important to look at the intramuscular electrodes in common use and discuss their particular advantages and limitations. Monopolar needle electrodes are made from stainless steel wire sharpened at the tip. These needle electrodes work well; however, breakdowns in the insulation are common. Concentric needle consists of an outer needle cannula that serves as the reference electrode and a central core electrode that normally has a diameter of about 100 μ and is insulated from the outer cannula by epoxy or equivalent insulating material.