[Basic neurography and its diagnostic importance]

Nerve conduction studies are fundamental elements of the neurophysiological investigation of neuromuscular diseases. They provide information on peripheral nerve function. Knowledge of the biological and technical basis of the method is essential for the clinician to understand the place of nerve conduction testing in the diagnostic process. A characteristic feature of the nerve fibers is their ability to conduct electrical potentials. This conductivity changes in pathologic circumstances; therefore, the patient's nerve conduction data may be important if a neuromuscular disorder is suspected. The electrical activity spreading along the nerve fibres can be detected with special techniques and instruments. To perform an examination, a stimulator, a high quality amplifier and a computer with various accessories are necessary. The examination is usually carried out by surface stimulation and recording electrodes and requires some cooperation. By supramaximal stimuli all nerve fibers in the peripheral nerve are activated, and their summated activity is recorded bipolarly. For technical reasons the procedures for the motor and the sensory nerve conduction measurements are somewhat different, but their principles are similar. A number of parameters, such as the latency, the amplitude, the area and the shape of the evoked potentials are analyzed. These parameters are influenced by many biological (age, gender, body height, etc.), physical (such as limb temperature) and technical factors. The results are compared with the reference data. Nerve conduction studies may help distinguish between normal and diseased nerve function. The latter has two main categories; axonal lesion and demyelinisation. Axonal lesion is characterized by relatively normal conduction velocity and lower than normal amplitude of the potentials. Demyelinisation is almost the opposite with long latencies, slow conduction velocity and relatively spared potential amplitudes. Nerve conduction studies help differentiate between these two forms. Abnormalities found by nerve conduction measurement may reflect the severity of the disease. Repeated studies are suitable for quantitative follow-up. The anatomical, physiological, pathophysiological and technical details are discussed below. The characteristic neurographic findings of various diseases are also summarized.