INTRODUCTION TO THE ELECTRONEUROGRAPHY (ENG)

In the routine clinical practice one of the most important tasks is the discovery of the disease behind the patient’s complaints and symptoms. Without a correct diagnosis there is little chance to treat the patient well. Electrodiagnostic studies play an important role in the evaluation of patients with neuromuscular disorders and, in each case, the study must be individualized and based on the patient history, neurological examination and differential diagnosis. It is also essential to reinvestigate the patient in case of progression or if new information is gained.

The electrodiagnostic studies such as the electroneurography (ENG) and the electromyography (EMG) are most often used to diagnose disorders of the peripheral nervous system and striated muscles. These include diseases affecting the primary motor neurons (anterior horn cells), sensory neurons (dorsal root ganglia), nerve roots, brachial and lumbosacral plexuses, peripheral nerves, neuromuscular junctions (NMJs), and muscles. After the history of the patient is taken and the physical examination is performed, most of the studies begin with the ENG to assess the nerve conduction velocity (NCV) or the nerve functional status measuring amplitude of the responses recorded. The needle EMG examination is done after the nerve conduction studies are completed, because the findings on the ENG studies are used in the planning and interpretation of the needle EMG.

For each nerve conduction study, recording and stimulating electrodes are used. The study of nerve conduction assumes that when a nerve is electrically stimulated a reaction should occur somewhere along the nerve. The stimulus is must be adequate to evoke a motor or sensory response and, in absolute terms, the electric stimulus used in the ENG studies is defined by duration (measured in millisecond, ms), waveform, and a strength or intensity measured in voltage (measured in millivolt, mV) or current (measured in milliampere, mA). The stimulus may be graded as subthreshold, threshold, submaximal, maximal, or supramaximal. The threshold stimulus is that stimulus sufficient to produce a detectable response. Stimuli less than the threshold stimulus are termed subthreshold. The maximal stimulus is the stimulus intensity after which a further increase of the stimulus intensity causes no increase in the amplitude of the evoked potential. Stimuli of intensity below this level but above threshold are submaximal. Stimuli of intensity greater than the maximal stimulus are termed supramaximal. Ordinarly, supramaximal stimuli are used for nerve conduction studies. By convention, an electric stimulus of approximately 20% grater voltage/current than required for the maximal stimulus may be used for supramaximal stimulation. Bipolar stimulation is performed using probe stimulators or surface electrodes; the stimulating electrode (i.e. the cathode, -) of must be positioned toward the active recording electrode for all ENG studies.

The reaction of the nerve to electrical stimulation can be monitored with appropriate surface recording electrodes (ring electrodes, disk electrodes, bar electrodes) and motor, sensory, or mixed nerve studies can be performed giving important information on the underlying nerve pathology. In some cases, especially when sensory responses are very low in amplitude, needle electrodes can be used for recordings. The reliability of the nerve conduction studies (NCSs) is increased when the technical methods are standardized. All the standardized methods presented in this mobile application represent the consensus option of expert medical clinicians, neurophysiologist, researchers, and health care practitioners who routinely perform NCSs.