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Optokinetic nystagmus (OKN) is an oculomotor reflex closely linked to the vestibular system which contributes to the stabilisation of retinal images. During combined vestibular and optokinetic stimulation, which occurs during natural situations of self rotation, the optokinetic input takes over as the vestibular drive declines.1 When a continuously moving stimulus is viewed, a characteristic eye movement pattern consisting of a slow phase in the direction of the stimulus and a fast phase in the opposite direction is elicited. In primates, OKN represents the responses of both the smooth pursuit and optokinetic system. The response to a full field moving visual stimulus has two phases. The first response, reflecting mainly smooth pursuit, promptly generates the nystagmus within 1–2 seconds of stimulus onset. The slow phase velocity approximates stimulus velocity. The second response corresponds to a slower buildup of stored neuronal activity. The neural pathway controlling OKN involves the cortex, brainstem, and cerebellum.2-5 Anatomical pathways of the OKN are known from animal studies, lesions in humans and, recently, by functional imaging. A recent study6 using functional magnetic resonance imaging (fMRI) showed the cortical structures involved in small field OKN. These were the occipitotemporal cortex, posterior parietal cortex, precentral, and posterior median frontal gyrus, the anterior and the posterior insula,the prefrontal cortex, and the medial part of the superior frontal gyrus. Subcortical structures shown to be activated during OKN were the caudate nucleus, the putamen, the globus pallidus, and the paramedian thalamus. Horizontal and vertical small field OKN activated the same cortical and subcortical areas in each hemisphere. Regardless of stimulus direction, a significant right hemispheric predominance was found. Many studies suggest that the final neuronal substrate for the slow and quick phase of the OKN corresponds to that of voluntary saccades and pursuit, respectively, and arise in the same brain stem neurons.
Clinical assessment of eye movement disorders often includes examination of horizontal OKN but not vertical OKN and little is known about development of vertical OKN or about vertical OKN in patients with visual system pathology. This may be because of more difficulties in recording and stimulating vertical OKN than horizontal OKN.
For example, there is an abundant literature about horizontal OKN asymmetries in newborn babies and in patients with abnormal binocular vision.7 8 In the first months of life, while the pathways to the cerebral visual areas remain immature, monocular OKN is more readily elicited by temporal to nasal than by nasal to temporal stimulus motion. This temporal-nasal asymmetry disappears between the second and the sixth month of life. This implies that pathways from the retina to the nucleus of the optic tract and the accessory optic system are functional at birth but, with maturation of the cortical visual pathway, projections of the extrastriatal cortex to the brainstem supersede. When strabismus or amblyopia prevent normal development of binocular vision, asymmetries of the horizontal OKN persist. However, little is known about anomalies of the vertical OKN in patients with disturbed binocular vision. Schor and Levi9 found upward-downward asymmetries in patients with amblyopia, the upward beating OKN being smaller.
In this issue of the BJO (p 451), Garbutt and Harris report, by observation of eye movements, abnormal vertical OKN in infants and children with neurometabolic diseases or with brain abnormalities on MRI. This study is the first to investigate vertical OKN in addition to horizontal OKN in a large group of infants and children with pathology in the visual pathways. It is of special interest because differences between horizontal and vertical OKN in patients with abnormalities of the brainstem and/or cerebellum were found. Vertical OKN is important in the diagnosis of Niemann-Pick disease type C, a neurometabolic disease which begins with loss of vertical saccades. Indeed, in one child, vertical OKN anomaly was the presenting sign of Niemann-Pick disease type C. In Gaucher's disease, investigation of OKN is important because its anomalies indicate the neuropathic form (type II or III). However, horizontal OKN is affected first and vertical OKN anomalies are an indication of progression of the disease. Children with lesions in the cortex were most likely to have horizontal and vertical OKN changes. Nine children in this study had disturbances of vertical OKN but normal horizontal OKN. The majority of these patients had lesions in the rostral midbrain. Interestingly, saccades in only one vertical direction can be abnormal. This suggests a separation between pathways for upward and downward OKN. An important aspect of this study, underlining the importance of the examination of vertical OKN, is that vertical OKN disturbance was always associated with pathology, whereas horizontal OKN abnormalities are frequently found to be idiopathic.
This study clearly indicates the importance of examining OKN, including vertical OKN, in patients with visual pathway disorders. In the future, quantitative recordings may show vertical OKN anomalies in a larger number of patients. It is certainly necessary to study systematically the development of the vertical OKN in normal children. In addition, the influence of the afferent visual system on vertical OKN needs to be investigated. The influence of factors such as visual acuity, amblyopia, or binocular vision needs to be examined. A quantitative comparison of smooth pursuit and saccades on one hand and OKN on the other hand would be interesting, in order to identify potential lesions which specifically affect the OKN. Certainly the study by Garbutt and Harris should stimulate investigators to use OKN examinations more frequently in visual pathway pathology.