The eVect of scleral search coil lens wear on the eye

Background/aim—Scleral search coils are used to measure eye movements. A recent abstract suggests that the coil can aVect the eye by decreasing visual acuity, increasing intraocular pressure, and damaging the corneal and conjunctival surface. Such findings, if repeated in all subjects, would cast doubt on the credibility of the search coil as a reliable investigative technique. The aim of this study was to reassess the eVect of the scleral search coil on visual function. Methods—Six volunteer subjects were selected to undergo coil wear and baseline measurements were taken of logMAR visual acuity, non-contact tonometry, keratometry, and slit lamp examination. Four drops of 0.4% benoxinate hydrochloride were instilled before insertion of the lens by an experienced clinician. The lens then remained on the eye for 30 minutes. Measurements of the four ocular health parameters were repeated after 15 and 30 minutes of lens wear. The lens was then removed and the health of the eye reassessed. Results—No obvious pattern of change was found in logMAR visual acuity, keratometry, or intraocular pressure. The lens did produce changes to the conjunctival and corneal surfaces, but this was not considered clinically significant. Conclusion—Search coils do not appear to cause any significant eVects on visual function. However, thorough prescreening of subjects and post-wear checks should be carried out on all coil wearers to ensure no adverse eVects have been caused. (Br J Ophthalmol 2001;85:332–335) Accurate measurement of the movements of the eyes is often required not only in studies of oculomotor function, but also in studies of the visual and vestibular systems. Accurate measurement of oculomotor parameters has been suggested to have a clinical role in the diagnosis and monitoring of disease. 2 Various methods have been developed to record movements of the eyes including electrooculography, infrared oculography, video oculography, and scleral search coils. Although the most invasive of the available techniques, the scleral search coil method has gained wide acceptance because it provides excellent spatial and temporal resolution. It has been widely used in both animal and human research studies, in investigations of saccades, smooth pursuit, the vestibulo-ocular reflex, and optokinetic responses, in both normal and clinical subjects. 6 Briefly, the technique involves applying a coil of wire to the eye. In the case of human subjects, a contact lens or scleral annulus, in which the coil is embedded, is acutely placed in a locally anaesthetised eye for the length of an experiment, typically no longer than 45 minutes. In animal subjects, coils can be implanted surgically. Movements of this coil in a magnetic field, generated by a set of field coils, induces an electric current in the eye coil proportional to its position in the field. This current, suitably amplified and filtered, provides, strictly speaking, an eye in space position signal. However, where the head does not move relative to the field coils, this is in eVect converted to an eye in orbit position signal. Given the wide use of this technique, the brief report published by Zacher et al claiming that standard coils could have serious eVects on the ocular surface, intraocular pressure (IOP), and visual acuity (VA) caused concern. While obviously raising concerns over the ethics of the technique, such findings, if substantiated, would have implications for a large body of oculomotor results obtained using this technique. It was claimed that the acuity of their subjects declined by up to six lines during coil wear. As typically visual targets used in oculomotor studies are fractions of a degree in size, large changes in visual acuity would seriously modify subject performance. We therefore decided to explore these issues further. Given the obvious ethical diYculties of embarking on essentially a safety study, we used a small group of carefully screened subjects, and limited coil exposure to only 30 minutes, well within the recommended period of 45 minutes. Materials and methods