Physiologic nystagmus is a form of involuntary
eye movementEye movement may refer to:* Eye movement , the voluntary and involuntary movement of the eyes* Eye movement in reading, the role of eye movement in reading linguistic text* Eye movement in music reading, the role of eye movement in reading music...
that is part of the
vestibulo-ocular reflexThe vestibulo-ocular reflex or oculovestibular reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head...
(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 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.
Physiologic nystagmus is a form of involuntary
eye movementEye movement may refer to:* Eye movement , the voluntary and involuntary movement of the eyes* Eye movement in reading, the role of eye movement in reading linguistic text* Eye movement in music reading, the role of eye movement in reading music...
that is part of the
vestibulo-ocular reflexThe vestibulo-ocular reflex or oculovestibular reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head...
(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 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
saccadeA saccade is a fast movement of an eye, head or other part of an animal's body or device. It can also be a fast shift in frequency of an emitted signal or other quick change. Saccades are quick, simultaneous movements of both eyes in the same direction...
in the opposite direction. The process proceeds indefinitely. This is optokinetic nystagmus, and is a source for understanding the
fixation reflexThe fixation reflex is that concerned with attracting the eye on a peripheral object. For example, when a light shines in the periphery, the eyes shift gaze on it...
.
Nystagmus is not to be confused with other superficially similar-appearing disorders of eye movements (
saccadicA saccade is a fast movement of an eye, head or other part of an animal's body or device. It can also be a fast shift in frequency of an emitted signal or other quick change. Saccades are quick, simultaneous movements of both eyes in the same direction...
oscillations) such as
opsoclonusOpsoclonus refers to uncontrolled eye movement. Opsoclonus consists of rapid, involuntary, multivectorial , unpredictable, conjugate fast eye movements without intersaccadic intervals. It is also referred to as "saccadomania"...
or
ocular flutterOcular flutter is an opsoclonic disorder in which the eyes incontinuously saccadically move around the point of fixation in the field of vision.See also nystagmus....
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
medicineMedicine is the art and science of healing. It encompasses a range of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
, the presence of nystagmus can be
benignA benign tumor is a tumor that lacks all three of the malignant properties of a cancer. Thus, by definition, a benign tumor does not grow in an unlimited, aggressive manner, does not invade surrounding tissues, and does not metastasize...
, or it can indicate an underlying visual or
neurological problemNeurology is a medical specialty dealing with disorders of the nervous system. Specifically, it deals with the diagnosis and treatment of all categories of disease involving the central, peripheral, and autonomic nervous systems, including their coverings, blood vessels, and all effector tissue,...
.
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, 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 continuously and stop suddenly, the slow phase of nystagmus is in the direction of rotation and the fast phase is in the opposite direction of rotation. This is a
postrotatory nystagmus.
Opticokinetic nystagmus
A nystagmus induced by looking at moving visual stimuli, such as moving horizontal or vertical lines, and/or stripes.
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.
Nystagmus and alcohol
In
policeA police service is a public force empowered to enforce the law and provide security through the legitimized use of force.The term is most commonly associated with police services of a state that are authorized to exercise the police power of that state within a defined legal or territorial area of...
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 influenceDriving under the influence of alcohol or other drugs, is the act of operating a vehicle after consuming alcohol or other drugs...
of
alcoholIn chemistry, an alcohol is any organic compound in which a hydroxyl group is bound to a carbon atom of an alkyl or substituted alkyl group. An important group of acohols is formed by the simple acyclic alcohols, the general formula for which is C
nH
2n+1OH...
. The test involves observation of the suspect's
pupilThe pupil is an opening located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils...
as it follows a moving object, noting
- lack of smooth pursuit,
- distinct and sustained nystagmus at maximum deviation, and
- 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.