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hgnh.com HGN Home: HORIZONTAL GAZE NYSTAGMUS: THE SCIENCE & THE LAW A Resource Guide. Nystagmus is an involuntary jerking or bouncing of the eyeball that occurs when there is a disturbance of the vestibular (inner ear) system or the oculomotor control of the eye. Nystagmus is involuntary eye movement that can be part of the vestibulo-ocular reflex (VOR), with the eyes moving first in the direction of the lesioned side (slow phase) followed by a quick correction (fast phase) to the opposite side or away from the lesioned side. The direction of nystagmus is defined by the direction of its quick phase (e.g. a right-beating nystagmus is characterized by a rightward-moving quick phase). The oscillations may occur in the vertical, horizontal or torsional planes, or in any combination. The resulting nystagmus is often named as a gross description of the movement, e.g. downbeat nystagmus, upbeat nystagmus, seesaw nystagmus, periodic alternating nystagmus. These descriptive names can be misleading however, as many were assigned historically, solely on the basis of subjective clinical examination, which is not sufficient to determine the eyes' true trajectory. Over the past forty years, however, objective eye movement recording techniques have been applied to the study of nystagmus, and the results have led to a greater accuracy and understanding of the condition. Nystagmus is not to be confused with other superficially similar-appearing disorders of eye movements (saccadic oscillations) such as opsoclonus or ocular flutter that are composed purely of fast-phase eye movements, while nystagmus is characterised by the combination of a slow eye movement that usually act to take the eye off the point of regard, interspersed with the fast movement that serves to bring the eye back on target. Without the use of objective recording techniques, it may be very difficult to distinguish between these conditions. In medicine, the presence of nystagmus can be benign, or it can indicate an underlying visual or neurological problem. Over forty types of nystagmus have been classified. In police work, testing for horizontal gaze nystagmus is one of a battery of field sobriety tests used by officers in the field to determine whether a suspect is driving under the influence of alcohol. The test involves observation of the suspect's pupil as it follows a moving object, noting (1) lack of smooth pursuit, (2) distinct and sustained nystagmus at maximum deviation, and (3) the onset of nystagmus prior to 45 degrees. A general rule of thumb is that a person's blood alcohol concentration can be estimated by subtracting the angle of onset from 50 degrees. Therefore, a person with an angle of onset of nystagmus at 35 degrees has a blood alcohol concentration of approximately 0.15%. While there is some scientific basis behind this obervation, in practice the test is fatally flawed, for it relies on subjective examination and interpretation of eye movements, and estimation of gaze angles, which are notoriously inaccurate. see NYSTAGMUS
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