Involuntary rhythmic movement of the eyeball, which may be continuous or spasmodic. According to the direction of movement it is designated horizontal, vertical, rotary or pendulum (to and fro). The condition is by no means abnormal; it occurs in anyone looking through the window of a moving train, for example. The same occurs in reading. Eye movements of this kind are abnormal only if they occur when looking at stationary objects. Nystagmus can also be caused by irritation of the organ of balance in the inner ear or by the entry of warm water into one ear. Nystagmus can be caused by an illness; a long period spent in the dark (as with miners, for example) causes temporary nystagmus, and the condition also occurs in disorders of the organ of balance such as Meniere’s disease, and in disorders of the central nervous system, such as multiple sclerosis and cerebral tumours or haemorrhage. Specialist examination is necessary to establish the cause; treatment is not always necessary or possible. Sometimes surgery is needed to remove the source of irritation to the organ of balance.