Fundamentals of electronystagmography.

This article outlines my method of performing electronystagmography (ENG) and achieving consistent, good-quality results in an office-based vestibular laboratory.

Background

ENG as a method of measuring eye movement has been in clinical use since the early 1950s. While basic nystagmus can be measured, the conditions for inducing nystagmus are not standardized. In general, all ENG tests usually involve some positional testing and alternate bithermal caloric testing in addition to other procedures. Some laboratories also measure eye movements that do not produce vestibular nystagmus, such as saccades, smooth pursuit, pendular tracking, and optokinetic motion. These eye movements can vary significantly depending on the degree of cooperation by the patient. Therefore, these tests are not routine and should not be performed by technicians.

The simultaneous binaural bithermal caloric stimulus induces a vestibular ocular reflex. In a normal patient with equal bilateral functioning, simultaneously stimulating both ears with the same temperature should cause an equal and opposite response that results in no nystagmus. Nystagmus occurs when there is a mismatch, or an unequal response, caused by the pathology of the vestibular disorder that causes dizziness and disequilibrium.

Some clinics perform the Hallpike maneuver with ENG, although this might not be the best way to evaluate induced eye movements, because those movements can be rotary. It might be better to observe eye movements with Frenzel's lenses or with videonystagmography.

Equipment and technique

Laboratory setup. The details of a laboratory's setup are important to attaining consistent results. There should be a room dedicated to ENG, and it should be well ventilated and well lighted.

The most important piece of furniture is the table upon which the test is performed. You should be able to adjust the plane of this table from 0[degrees] (the patient in the supine position) to 30[degrees] (the patient's head elevated). It is preferable that the table's adjustment mechanism be motorized; this is especially helpful in elevating very heavy patients. I prefer to suspend the calibration equipment from the ceiling, while others prefer to mount it on a wall so they can begin calibration before the patient reclines on the table.

Recorders. Ideally, nystagmus recording equipment should be computer-based. A strip-chart recorder is satisfactory, but when it must be replaced, a computerized ENG is much preferable. By and large, the manufacturers of ENG computer equipment provide the necessary specifications for recording. For fundamental clinical recording, computerized ENG recorders sample between 50 and 75 Hz, which is quite satisfactory. For recording very rapid eye movements such as saccades, the computer should be set to a sampling rate of 100Hz. Older alternating-current (AC) recorders provide reliable tracings, but those that were designed with a short time constant can produce tracings that appear to represent nystagmus, but which are actually artifact. The newer computerized models, with their disposable electrodes, allow for trouble-free, direct-current (DC) operation.

In analyzing the recordings, I prefer to look at the best 10 seconds of tracing--that is, the period of the most active nystagmus--and calculate the average eye movement during that span. Some computer programs automatically choose the best set of beats and calculate the average for you. If such a program selects an eye movement that is not a valid nystagmus beat, it can be deleted from the results.

For recording smooth pursuit, optokinetic, and other types of eye movements, some computer programs allow you to modify the calibration bar to provide the appropriate stimulus.

Caloric stimulator. The preferred medium for caloric stimulation is water. Air caloric irrigators are available, but significant questions exist regarding the accuracy of the stimulus and the consistency of the response, depending on the direction of the airflow. Moreover, air cannot be directed to both ears simultaneously without a significant decline in temperature.

Water stimuli have been available since the turn of the 20th century, and they yield well-accepted parameters of response. In order to perform simultaneous binaural bithermal testing, it is necessary to have a water caloric system with two outputs. Both open- and closed-loop water circulation systems are acceptable.

Electrodes. The new disposable electrodes provide excellent coupling to the skin. They are meant to be discarded after one use.

Filing. If you use strip-chart recording, it is better to mount the best 10-second strip on a report form than to keep the entire tracing rolled up in a file. In any case, a neat, attractive report form, be it a computer printout or a mounted strip chart, allows others to review results easily.

From Neurotologic Associates, PC, New York City.