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Hereditary nystagmus in early childhood.

Nystagmus is defined as a rhythmic involuntary movement of the eyes, and as an acquired phenomenon arising in later childhood or in adult life is usually a symptom of serious neurological or labyrinthine disease; in such cases the movements of the eyes commonly produce subjective symptoms of objects moving in the visual panorama (oscillopsia). Nystagmus may also be 'congenital', or, more accurately, may first be observed within a few weeks of birth when the infant begins to attempt to fix and to follow visually stimulating targets by means of conjugate movements of the eyes. In such cases, the nystagmus may persist throughout life, but even at a later stage there is always a complete absence of the symptom of oscillopsia. Nystagmus which presents in early infancy is often hereditarily determined, and its aetiology is variable, for it may be a symptom of a serious disorder affecting the eyes or the visual pathways, or the central nervous system. In particular, such clinically obvious affections as total albinism, aniridia, and dense cataracts induce nystagmus at a very early age. In the majority of cases, however, no gross abnormality of the eyes is observed on routine clinical examination of the affected infant, and no other abnormality of the central nervous system can be discerned. In many such cases the nystagmus is then an isolated phenomenon due to some as yet undetermined developmental anomaly in the central nervous mechanism concerned with 'steady fixation' of the eyes; such nystagmus is said to be 'extra-ocular' or 'idiopathic'. There is, however, a residue of cases in which in early childhood the nystagmus is in fact due to an hereditarily determined disorder of the visual apparatus which is not apparent on cursory ophthalmological examination, and which may follow a progressive course. The long-term visual prognosis in such cases is usually much worse than in cases of idiopathic nystagmus, and it is therefore very important from the point of view of genetic counselling that these specific ocular causes of nystagmus should be diagnosed at an early age. After describing the clinical characteristics of various types of hereditary nystagmus and the techniques which are available to differentiate between 'idiopathic' nystagmus and nystagmus as a symptom of an occult disorder of the visual apparatus in early childhood, some description of the modes of inheritance and of the longterm visual prognosis are given in the various categories of infantile nystagmus which can be so defined.