If you watch the eyes of a person who has been diagnosed as having nystagmus you will see that their eyes are constantly moving (usually side to side, although it may be up and down). This involuntary, continuous, and conjugate (same in both eyes) movement of nystagmus may be present from birth and have no obvious cause (idiopathic) or it can develop later in life due to injury or disease.
The eye movements of a nystagmus patient resemble (although the cause is different) the eye movements of someone looking out of the window of a fast moving train. In the traveller the eyes are constantly moving as they re-fixate on moving objects but the individual is unaware of this. The traveller knows they are stationary (all be it on a moving train) but perceives the outside world to be moving.
Although the eyes of a child with nystagmus are constantly moving, they are not consciously aware of it; they know they are not moving and neither is their world.
With nystagmus vision may be normal, slightly reduced or very impaired. Vision may or may not be improved with glasses. Eye movements may be very slight and easy to ignore or very obvious and may increase if a child is stressed, embarrassed, tired or the level of lighting is too bright or too dim. Nystagmus is usually less when both eyes are used together often increasing if one eye is covered so nystagmus may only be picked up at a first eye test (The optometrist then has to decide if the nystagmus is long standing and idiopathic or whether it is necessary to refer- just in case it is of recent origin and is a cause of concern).
Nystagmus may make it difficult for children to maintain eye-contact. Some children can be very self-conscious of their nystagmus, and this will make them less confident and they may try to avoid making eye contact. Spectacles can sometimes be worn as a shield to hide the nystagmus. The spectacles become the focus of attention rather than the nystagmic eyes.
Some children find adopting a head turn or tilt gives more comfortable vision. Although the child may not be consciously aware of any eye movements they may find that their vision is more comfortable when the eyes look in a certain direction, this results in a head turn or tilt. Any attempt to straighten the head will be resisted as it makes for uncomfortable vision. If this is the case then it is important that the child sits in the correct position in class, so they can adopt a comfortable head position to see the board.
Some types of nystagmus are perfectly normal and need no further investigations, indeed their absence could be a cause for concern. However any persistent jerky eye movements that are seen during the first few year of life (or later in life) should be investigated further as they could be a sign of more serious disease but in the majority of children nystagmus is the only problem. The prevalence of nystagmus is not really known because it is not always very visible but suggestions vary from it being present in 1 in 10,000 or maybe 1 in 1,000 children.