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F. A. Vera-Diaz, E. Peli; The Blind Spot as Monocular Principal Visual Direction: A Rare Case of Motor Adaptation. Invest. Ophthalmol. Vis. Sci. 2007;48(13):937.
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© ARVO (1962-2015); The Authors (2016-present)
To document the use of the left blind spot by a patient with left esotropia (LET) as his principal binocular and monocular visual direction.
The patient is an 80 year-old male with LET developed during early childhood. Retinal tracking fixation measurements with a Nidek MP-1 revealed stable fixation within the left optic nerve head area. Further assessments of fixation were performed with various target sizes and gaze positions in an attempt to challenge the initial observation. Additional clinical binocular evaluations were conducted.
MP-1 fixation data showed remarkably stable monocular fixation (±1deg) for 30 sec. with the left blind spot area regardless of the target size and position of gaze. However, glances to the peripapillary area may not have been tracked by the MP-1. LogMAR VA OD=0.76 (due to AION in 2002), OS=1.90. Concomitant LET of 30Δ measured with Hirschberg, no movement with cover test regardless of fixation target, with no monocular motility restrictions. Visuoscopy OS also revealed fixation at the disc. The past point test was accurate with each eye. There is a strong family history of esotropia.
We documented for the first time a case of abnormally developed monocular ocular motor system wherein its principal visual direction has shifted so that it is now located within the blind spot. We do not believe that there is any direct visual input from the optic nerve head. The patient apparently uses visual information obtained by glancing with peripapillary areas to determine the target position.The abnormal oculocentric direction is then combined with extra retinal information (i.e. proprioceptive) of the eye location in the orbit to stabilize the fixation.Others have reported binocular (presumably diplopia avoidance) phenomena of blind spot-syndromes or -mechanisms. In distinction we report a monocular behavior. Eccentric fixation, a monocular phenomenon, usually maintains the normal foveo-centric localization system, unlike our case.
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