Physiologic nystagmus

Physiologic nystagmus a form of involuntary eye movement that is part of the vestibulo-ocular reflex (VOR). It is characterized by alternating smooth pursuit in one direction and saccadic movement in the other direction.

Nystagmus can be caused by subsequent foveation of moving objects, pathology, sustained rotation or substance abuse. For example, if one fixates on a stripe of a rotating drum with alternating black and white. As the drum moves, the gaze retreats to fixated on a new stripe. This is first a rotation with the same angular velocity, then returns in a saccade in the opposite direction. The process proceeds indefinitely. The is optokinetic nystagmus, and is a source for understanding the fixation reflex

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 (saccadic) eye movements, while nystagmus is characterised by the combination of a smooth pursuit, which usually acts to take the eye off the point of regard, interspersed with the saccadic 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.

Types
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.

Postrotatory nystagmus
If one spins in a chair vigorously, the eyes move in the opposite direction but after a certain distance a saccade turns them in the direction of rotation. When motion stops, the eyes again move opposite the rotation. This is the postrotatory nystagmus

Measurement
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 and alcohol
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.

As a rule of thumb, 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%.

The field sobriety test studies published by the National Highway Traffic Safety Administration have never been peer reviewed and attempts to duplicate the study results have been unsuccessful.

The horizontal gaze nystagmus test has been highly criticized and major errors in the testing methodology and analysis found. However, the validity of the horizontal gaze nystagmus test for use as a field sobriety test for persons with a blood alcohol level between 0.04-0.08 is supported by peer reviewed studies and has been found to be a more accurate indication of BAC than other standard field sobriety tests.