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Treacher Collins Prize Essay the Significance of Nystagmus

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Treacher Collins Prize Essay the Significance of Nystagmus Eye (1989) 3, 816--832 Treacher Collins Prize Essay The Significance of Nystagmus NICHOLAS EVANS Norwich Introduction combined. The range of forms it takes, and Ophthalmology found the term v!to"[<xy!too, the circumstances in which it occurs, must be like many others, in classical Greece, where it compared and contrasted in order to under­ described the head-nodding of the wined and stand the relationships between nystagmus of somnolent. It first acquired a neuro-ophthal­ different aetiologies. An approach which is mological sense in 1822, when it was used by synthetic as well as analytic identifies those Goodl to describe 'habitual squinting'. Since features which are common to different types then its meaning has been refined, and much and those that are distinctive, and helps has been learned about the circumstances in describe the relationship between eye move­ which the eye oscillates, the components of ment and vision in nystagmus. nystagmus, and its neurophysiological, Nystagmus is not properly a disorder of eye neuroanatomic and neuropathological corre­ movement, but one of steady fixation, in lates. It occurs physiologically and pathologi­ which the relationship between eye and field cally, alone or in conjunction with visual or is unstable. The essential significance of all central nervous system pathology. It takes a types of nystagmus is the disturbance in this variety of different forms, the eyes moving relationship between the sensory and motor about one or more axis, and may be conjugate ends of the visual-oculomotor axis. Optimal or dysjugate. It can be modified to a variable visual performance requires stability of the degree by external (visual, gravitational and image on the retina, and vision is inevitably rotational) and internal (level of awareness affected by nystagmus. Oscillopsia is often the and visual intention) influences. Physio­ result of eye-field instability in acquired nys­ logically it occurs in response to relative tagmus, the perceptual centres unable to com­ movement between the eyes and their visual pensate for the oscillation in their field,incorporating the vestibular response to reconstruction of the visual world. This con­ head movements, and this response is useful trasts with congenital and physiological forms in investigating the function of both the sen­ of nystagmus, in which oscillopsia does not sory and motor components of the visual normally occur. Pathological nystagmus system. Nystagmus inevitably has an effect on reduces visual function to a level less than the visual function, but though in its pathological retinostriate pathway's capacity, and physio­ forms it impairs vision, physiological nystag­ logical nystagmus is a strategy to improve mus is a strategy designed to optimise vision. vision in the face of movement of fieldrelative It is not surprising that nystagmus can be an to eye. This eye-fieldrelationship is central to intimidating clinical sign, giving rise to diag­ an understanding of the significance of nostic difficulty. nystagmus. The ophthalmologist, neurologist and Nystagmus used to be classified as either laboratory neurophysiologist individually motor or sensory.2 Motor and sensory func­ approach nystagmus analytically, but to grasp tions are mutually dependent and indivisible its wider significance their insights must be in relation to fixation, however, since vision Correspondence to: Mr. N Evans, FRCS, FCOphth. THE SIGNIFICANCE OF NYSTAGMUS 817 provides the major contribution to the control ing head movement), is converted by this of eye position, and eye position determines 'neural integrator' into positional infor­ visual input. Division of nystagmus into sen­ mation, which is fed into the oculomotor sory and motor types is an oversimplification, nucleii to produce eye movements. The inte­ and it is necessary to examine the interface grator is located in the brainstem reticular for­ between the sensory and motor visual mation near to the oculomotor nucleii, and systems, nystagmus having consequences for has been placed in either the prepositus and both. The component eye movements of nys­ vestibular nucleiV or in the burst cells of the tagmus will be described, and the nature of PPRF themselves.4 The output of such an normal steady fixation and its control identi­ integrator intrinsically depends on its func­ fied. The circumstances in which nystagmus tional integrity, and it can be analysed accord­ occurs will then be considered, particularly its ing to theories of system-control. variation in relation to different aetiology. Leak and gain are integrator properties of Information will be drawn from neurophysi­ particular importance in relation to nystag­ ological and neuroanatomic descriptions of mus. Leak denotes a loss of signal in the trans­ normal eye movements and pathways, clinical formation from velocity to position, and the reports of nystagmus and visual disorders, output of a leaky integrator shows decay­ and human and animal laboratory studies. In that is the eyes cannot hold an intended new drawing together information from these eccentric position, and drift back towards the sources to understand nystagmus, the func­ primary position. This decay shows an expo­ tion of the normal visual and visumotor nentially-decreasing time constant, the elastic system is also illuminated. recoil of the orbital tissues reducing progres­ sively as the eyes approach the midline. A Eye Movements and the Components of slow movement whose drift from fixation Nystagmus shows exponentially increasing velocity, on The oculomotor system has two kinds of the other hand, implies integrator instability active movement at its disposal, from which with high gain; that is the SEM generator all reflexand voluntary fixating,repositioning drives the eyes in the direction of the target, and following movements are synthesised: the under positive-feedback, at increasing veloc­ fast (FEM) and the slow (SEM) eye move­ ity, which can exceed that of the target. In ments (together with a distinct variant of the both these cases fixationrequires a correcting SEM system producing vergence move­ movement after the eyes have moved off-tar­ ments). FEMs, exemplified by saccades, are get beyond a critical amount, and this is ballistic, that is the movement is prepro­ accomplished by a saccade. If eccentric drift is grammed and not continuously monitored repeated, refoveating saccades will follow in a and controlled. Its peak velocity is propor­ cyclical pattern and nystagmus ensues. The tional to its amplitude, and vision is sup­ origin of nystagmus of any kind, then, lies pressed while it takes place. The SEM, with the slow eye movement. exemplifiedby the smooth pursuit movement, Saccades are fed into the ocular motor is by contrast designed to maintain constant nucleii from the paramedian pontine reticular foveation of a target, and is under visually­ formation (PPRF) in the pons, whereas SEMs controlled monitoring. Its velocity is related are probably driven from the Purkinje cells in to the target velocity, up to a maximum. This the cerebellar flocculus.5 This difference relation between eye and target velocity reflects the greater coordination demanded describes the 'gain' characteristics of a supra­ by slow eye movements, which must take into nuclear unit which summates or 'integrates' account visual field, head position and move­ all the inputs ·influencing eye movements ment. Since the two kinds of eye movement (frontal cortex, visual cortex and secondary arise in separate anatomic locations, they can visual pathways, vestibular system and pro­ be involved in disease processes independen­ prioceptors). Velocity-coded information tly of one another, and disorder in one which (from retinal image slip, burst cell-generated interferes with accurate foveation can be com­ saccades, and from vestibular neurons signall- pensated by a restoring action by the other. 818 NICHOLAS EVANS Pathological nystagmus arises when these Steady fixationoccurs when eye-target rela­ events alternate in a cycle with appropriate tive movement is zero, and can be regarded as time-characteristics. a zero-velocity SEM, displaying the charac­ It may be stated as a general principle that teristic features of other SEMs. Smooth pur­ nystagmus of any sort arises in three circum­ suit or VOR intervenes without a break in stances: asymmetric input to the integrator, foveation if head or target should move. integrator output which is mismatched with Steady fixation requires that the FEM gener­ target movement by inappropriately high ators on each side are silent, equally sup­ gain, or integrator leak preventing a fixated pressed by pause cell discharge. The target from being held. In addition peripheral flocculus, which maintains the eye-field disorder (of motor nerve or extraocular relationship and continuous foveation, has muscle) gives rise to nystagmus if the output inputs from the visual and prefrontal cortex, of the integrator is mismatched with the res­ superior colliculus, substantia nigra and ves­ ponse of muscle yokes, normal gain being tibular nucleii, and, by intracerebellar con­ insufficient to maintain steady fixation in the nection, somatic and extraocular muscle face of progressive effector decay. These proprioceptors. Of all of these, only the classes of nystagmus are examined below in vision-mediated response is a closed loop relation to their clinical setting, and the pat­ system (in which the sensor and effector are terns of their component movements shown the same), the others being
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