The significance of nystagmus

Nystagmus occurs in a very wide range of circumstances, each type showing characteristic clinical, pathological and electrophysiological features, and analogies between them can be identified by comparing and contrasting nystagmus of different kinds. The effect of altered visual and vestibular conditions on nystagmus, and the features of its waveform, indicate the relationship between eye movements and vision, and the influence of visual and vestibular input in stabilising steady fixation. Ultimately, the significance of nystagmus is that it indicates the state of the mechanisms underlying this stabilisation: in physiological nystagmus they are operating successfully, and in pathological nystagmus they are disturbed. More than this, investigation of nystagmus has shown that the visual system is not divided in a clear-cut way into sensory and motor poles, but that between them there exists a neural region where a 'copy' of the visual world is matched with a programme of potential eye movements, and where sensorimotor information exists indivisibly. Long feedback loops, involving occipital cortex and extraocular muscle proprioceptors, and short ones within the cerebellum and integrator, emphasise the great precision involved in eye movement control, enabling the visual cortex to make optimal use of the resolution capabilities of the fovea. Nystagmus always reflects an asymmetry in the output of the eye movement generators, and it has been shown that the inappropriate movement which is responsible for pathological nystagmus is the slow movement. This may arise because of an intrinsic defect in that part of the generator called the neural integrator, or because of 'tonic imbalance' in its input. Nystagmus occurring with identifiable acquired central nervous pathology can, to some extent, be understood mechanistically, but idiopathic congenital nystagmus poses greater difficulties. Analysis of its waveform suggests that an intrinsic fault in the integrator can explain the clinical and electrophysiological findings in CN, but the cause of the high gain instability in the integrator remains to be explained. The integrator is adaptable, or self-tuning, adjusting its output by visual feedback. Circumstantial evidence suggests that the original disorder in idiopathic CN may occur higher than the integrator, detuning it by conveying an inappropriately organised visual input. In particular if the organisation of the visual system into fields is defective, the gaze generators, whose output is orientated according to field, will have a less accurate 'copy' of the world from which to formulate their movements.(ABSTRACT TRUNCATED AT 400 WORDS)