Physiologic nystagmus is 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.

Horizontal optokinetic nystagmus.

Nystagmus can be caused by subsequent foveation of moving objects, pathology, sustained rotation or substance use. For example, if one fixates on a stripe of a rotating drum with alternating black and white, the gaze retreats to fixate on a new stripe as the drum moves. This is first a rotation with the same angular velocity, then returns in a saccade in the opposite direction. The process proceeds indefinitely. This 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.^[1]

[edit] 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, and a left-beating nystagmus by a leftward-moving quick phase). The oscillations may occur in the vertical,^[2] 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.

[edit] Postrotatory nystagmus

If one spins in a chair continuously and stop suddenly, the fast phase of nystagmus is in the opposite direction of rotation, known as the "post-rotatory nystagmus," while slow phase is in the direction of rotation.

[edit] Opticokinetic nystagmus

A nystagmus induced by looking at moving visual stimuli, such as moving horizontal or vertical lines, and/or stripes.

[edit] 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.

Orthoptists may use an optokinetic drum, or Electro-Oculography to assess their eye movements.

[edit] Nystagmus and alcohol

In police work, testing for horizontal gaze nystagmus is one of a battery of field sobriety tests used by officers 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%.^{[citation needed]}

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.^[3]

The horizontal gaze nystagmus test has been highly criticized and major errors in the testing methodology and analysis found.^[4][5] 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.^[6]

[edit] References

[edit] See also

• Pathologic nystagmus
• Opsoclonus
• Eye care professional
• Orthoptist

[edit] External links

• MeSH Physiologic+Nystagmus