The factors that affect the accuracy with which eye movements can be recorded using electronystagmography (ENG) have been a cause of concern for vestibular clinicians for many years. This review and quantitative exploration of the issue has been prompted by the increase in the use of computerized systems, which have considerably increased the usefulness of ENG and the ease with which it can be carried out. These systems can control a visual target, record and analyse eye movements and compare the results with stored normative data.
The accuracy of the analyses of vestibular test results obtained from a computerized ENG system depends on the accuracy with which the system has been calibrated to record the patient's eye movements. The variability of the calibration sensitivity was investigated by analysing the records of 50 consecutive patients who attended for vestibular testing and a study was carried out using 16 volunteer subjects to examine the contributions that time, changes in lighting and subject movement made to this variable. The effect of calibration frequency on the results obtained from caloric testing was also examined by analysing 16 patient records.
The mean percentage change in sensitivity between the first two calibrations carried out on patients was found to be -17%, with a standard deviation of 18%. However, there seems to be little baseline drift over time under constant lighting conditions. Changes in lighting have a marked effect, and the adjustment to lighting changes is slow. Gentle movement has little effect. Repeated calibration does not seem to be an effective method to control the calibration-related errors in caloric test results.
Careful control of room illumination and the use of adequate adaptation periods to allow patients to adjust to the lighting should help to improve accuracy but, even in the absence of satisfactory calibration, the use of a computerized ENG system can provide valuable diagnostic information.
Norman M, Brown E. Variation in calibration for computerized electronystagmography. British Journal of Audiology, 1999, 33, 1-7.
Dr. Meg Norman, Sunderland Unit, Sunderland. Tel: +44 (0)191-569-9001.
Regional Medical Physics Department, Freeman Hospital, Newcastle upon Tyne, NE7 7DN, UK.
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