Monocular fixation with the optic nerve head: a case report.

PURPOSE To document and discuss the case of a patient with left esotropia (ET) who uses the left 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 (+/-1 degrees over 30 s) mostly within the left ONH for all the target sizes and positions of gaze tested. Additional clinical binocular evaluations showed concomitant left ET approximately 28 Delta, 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 an 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 peri-papillary areas to determine the target position (although 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 extraretinal 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 (Swan, 1948). We propose the use of a retinal perimeter for documentation of eccentric fixation in strabismus.