Purpose-To document and discuss the case of a patient with left esotropia (ET) who uses the left eye optic nerve head (ONH) for monocular "fixation".Case Report-The patient was an 80 year-old male with left ET from early childhood. Retinal tracking monocular fixation measurements with a Nidek MP-1 revealed stable fixation within the left ONH area. In an attempt to challenge the initial observation, further assessments of fixation were performed with a smaller target size and requiring various gaze positions. MP-1 fixation data showed remarkably stable monocular fixation (±1deg over 30 sec) mostly within the left ONH for all the target sizes and positions of gaze tested. Additional clinical binocular evaluations showed concomitant left ET~28Δ, no movement with cover test regardless of fixation target, and no significant monocular motility restrictions. Visuoscopy also revealed fixation at the left ONH. There was a strong family history of ET, but none of the other affected descendants tested (n=3) demonstrated the same behaviour.Conclusions-This is the first report documenting abnormally developed monocular ocular motor system with principal visual direction and zero retinomotor value shifted from the fovea to the ONH. We do not believe that there is any direct visual input from the ONH. The patient may use visual information obtained by glancing with peripapillary areas to determine the target position (though this was largely ruled out), or obtain position information from the average luminance produced by scattered light around the ONH margin. The abnormal oculocentric direction might then be combined with extra retinal information (efferent copy or extraocular muscle proprioception) of the eye location in the orbit to stabilize the fixation. This patient does not have the Blind Spot Syndrome as previously described (Swan, 1948). Possible aetiologies are discussed. We propose the use of a retinal perimeter for documentation of eccentric fixation in strabismus.
KeywordsBlind Spot; Optic Nerve Head; Eccentric Fixation; Principal Visual Direction; Retinomotor Value; Esotropia; Retinal PerimeterIn early acquired strabismus the visual system typically develops sensory adaptations to avoid the symptoms of diplopia and visual confusion (Pickwell, 1984), often reported as "double vision". If these adaptations did not develop, the patient would experience diplopia, as the image of an object seen with the fovea of the nondeviated eye would also be seen with a peripheral retinal area of the deviated eye (retinal images would not fall on corresponding points); and visual confusion, as the fovea of the deviated eye would receive an image different to that falling on the fovea of the non-deviated eye (two corresponding points would receive different images which would appear superimposed, in the same direction). These phenomena are usually reported, when they occur in central retina, by patients who develop strabismus as adults (at an age when they typically do not develop adaptations). While diplopia and visual con...
