Nystagmus

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What is nystagmus?
Nystagmus is characterised by an involuntary movement or shake in one or both eyes. When experienced in both eyes, the movement (called 'oscillations') may occur independently or simultaneously. Oscillations may be horizontal, vertical, rotary (circular), or a combination of these. Degree of movement can vary with changes in focal distance, direction of gaze, and depending on whether one (monocular) or both (binocular) eyes are in use.

There are a number of different categories of oscillations, the nomenclature of which is usually based on the direction or type of oscillation.

Jerk nystagmus consists of a slow movement in one direction followed by a very rapid movement in the opposite direction. It is comparable to watching a series of telephone poles from the window of a moving train. As the eyes focus on one pole, they move slowly in one direction. When the pole moves out of the field of view the eyes will snap forward in the opposite direction until they meet the next pole. People with congenital jerk nystagmus usually have reduced visual acuity, which is caused by the nystagmus.

Pendular nystagmus refers to a slow rhythmic movement of the eyes. There is no obvious discrepancy between the speeds of the movement back and forth. Pendular nystagmus is almost always associated with severely reduced visual acuity. It is believed that pendular nystagmus is caused by the visual impairment (the opposite is the case for jerk nystagmus). The visual impairment must develop in early infancy to result in pendular nystagmus. With pendular nystagmus, the degree of movement can vary depending on where the eye is pointing.

Other categories include downbeat nystagmus, upbeat nystagmus, torsional (rotary) nystagmus, horizontal nystagmus, seesaw nystagmus, gaze-evoked nystagmus (which appears when the patient is looking in a particular direction) and periodic alternating nystagmus (which consists of an alteration between different types of oscillations).

All categories of nystagmus can be associated with abnormal head posture. In some cases the head is held at an unusual angle to minimise the movement of the eye(s). In other cases (e.g. Spasmus nutans), the nystagmus is accompanied by a nodding of the head. The nodding is merely associated with the condition and is not believed to be a compensatory measure. Spasmus nutans is only seen in very young children (less than 6 years old).

The degree of vision impairment experienced by different people with nystagmus varies from a slight blurring of vision to being registered blind. With the exception of people with other underlying eye conditions, the majority of people with nystagmus are partially sighted and not completely blind.


Causes
The causes of nystagmus are wide and varied. Nystagmus can be congenital (present at birth) or acquired (as a result of, for example, disease or injury).

Eye movement is controlled by three different mechanisms:

Disruption of any of these three mechanisms can result in nystagmus.

Nystagmus can also be associated with:


Symptoms
Examples of characteristics associated with nystagmus (one or more of these may be present):


Everyday Problems
Examples of problems faced by a person with nystagmus will include those typical of any vision impairment. These may include:

Similarly in cases where the person also has tinnitus or deafness, problems will include those typical of a hearing impairment. These may include:

Additionally, the abnormal head posture associated with nystagmus may pose a difficulty for reading, however people who have nystagmus over a long period of time learn to adapt to the condition. This characteristic may pose a problem in situations where it is required to hold the head still (for example, having a photograph, a facial recognition scan or an iris scan taken).

The characteristic head nodding of, for example, spasmus nutans will pose a problem for reading. It will also hamper situations where it is required to hold the head still. However it should be noted that this characteristic is only seen in very young children (less than six years old).

One type of congenital jerk nystagmus, called latent nystagmus, is apparent only when one eye is covered. With this particular condition, a technology or system that requires the use of one eye (i.e. that with a monocular eye piece) may be problematic.

Similarly, in some cases of nystagmus, the condition is apparent only when the eyes are pointed in a particular direction. Familiarity with their condition may enable the person to compensate for this problem him or herself.

Difficulties arise when a person with nystagmus is required to look in a number of different directions (for example, from a keyboard to a screen) and/or at a number of different distances (for example, from a timetable held in the hand to an information screen located further away). The person may therefore require more time to complete an action. This will be a particular problem when a machine or system has an automatic time-out function.

As with other vision impairments, small text and text comprised of all capitals will pose problems. This also applies to moving text (for example, an information screen with scrolling text). All three of these categories of text are associated with difficulty focussing.

The location of text (for example, instructions) is very important to a person with nystagmus as they often need to bring their face quite close to the text in order to read it.


Further Information


Acknowledgements

The information contained in this section was collected from the following sources:


[1] Nystagmus Network (n.d.) Frequently Asked Questions. [accessed 30/01/08].

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