A condition in which there is involuntary movement of the eyes. In the most common type, jerky nystagmus, the eyes repeatedly move slowly in one direction and then rapidly in the other. Less commonly, nystagmus is “pendular”, with the eyes moving evenly from side to side. Nystagmus may be congenital, in which case the cause is unknown. It also occurs in albinism and as a result of any very severe defect of vision present at birth, such as congenital cataract. Persistent nystagmus appearing later in life usually indicates a nervous system disorder (such as multiple sclerosis, a brain tumour, or an alcohol-related disorder), or a disorder of the balancing mechanism in the inner ear. Adultonset nystagmus is occasionally seen as an occupational disorder in people who work in poor light. Electronystagmography, a method of recording eye movements, may be used to identify the type of nystagmus.