COMPUTER-QUANTIFIED POSITIONAL NYSTAGMUS IN NORMALS

Introduction: Most current authorities agree that, in the absence of visual fixation, low intensity nystagmus can be recorded by electronystagmography (ENG) in clinically normal people. However, there are some notable dissenters, and the reported incidence, intensity, and direction of "normal" spontaneous/positional ( S P) nystagmus vary widely across laboratories. The highly subjective element introduced by manual interpretation of position test records compounds the problem of characterizing "normal" S P nystagmus. Materlals and Methods: A computerized ENG analysis system which removes this element of subjectivity is described, and results of a study validating its results against those of experienced ENG interpreters are reported. Results: The computer found an S P nystagmus behind closed eyelids in at least one of five test positions in 42 of 51 clinically normal subjects (82.3%). This "normal" S P nystagmus had a statistically significant overall directional bias toward left beating, but was affected by position-tending toward right beating in the left lateral position. The upper 95.4% confidence limits for computer-measured slow phase velocities of "normal" S P nystagmus were -5.5°/sec (left beating) and +4.4°/sec (right beating). Conclusion: Possible explanations for "normal" rmS P nystagmus include clinically undertected pathology, normal asymmetries in the vestibulo-ocular reflex, and an imprinted oculomotor pattern such as might be created by reading. © 1993.