Nystagmus in infancy.

Spontaneous nystagmus in infants may be present at birth but more frequently appears in the first 6 months.' We present a classification in which nystagmus with an onset before 6 months is called early onset nystagmus (EON) and is divided into three categories: sensory defect nystagmus (SDN) in which there is a proven sensory impairment, congenital idiopathic nystagmus (CIN) (sometimes called 'motor nystagmus'2) in which no visual or neurological impairment can be found, and neurological nystagmus (NN) which is associated with neurological disease. Though the distinction between SDN and CIN is made by many authors,36 they are sometimes collectively called congenital nystagmus. The presentation ofall types ofnystagmus can be similar, but in infants it is important to differentiate between SDN and CIN because of the different underlying defects, investigation, management, and genetic counselling (Fig 1). Congenital idiopathic nystagmus CIN is diagnosed after exclusion of any underlying defect. It normally affects acuity moderately,78 and estimates of its incidence vary from 1 per 350 to 1 per 20000,9-" reflecting the different definitions of 'congenital nystagmus' and ways of data collection.