Chiasmal crossing defects in disorders of ocular motor function: Three dimensional eye movement recordings in albinism and non-decussating retinal fugal fibre syndrome

Purpose: To evaluate the relationship between misrouted retinal-fugal projections and ocular motor disarrangements, we examined ocular motor characteristics in 5 observers with chiasmal crossing defects. One observer presented with naturally occurring achiasmatic syndrome in which nasal retinal projections and concomitant visuotopic and retinotopic mapping misroute due to the inborn absence of the optic chiasm. The remaining four observers were albinos (autosomal recessive oculocutaneous or X-linked ocular) in which temporal retinal projections misroute due to erroneous decussation at the optic chiasm. Visual fields for all observers were normal.

Methods: Horizontal, vertical and torsional eye movements were evaluated for each eye with a dual-search coil technique. In vitro and in vivo coil calibration was implemented. Fixation, characteristics under binocular and monocular viewing conditions were evaluated at primary, secondary and tertiary gaze angles.

Results: In the achiasmat and albinos, classic congenital nystagmus (CN) was recorded in the horizontal planes. For the achiasmat, see-saw nystagmus also was recorded in the vertical and torsional planes and followed the profile of upward movement and intorsion of one eye with simultaneous downward movement and extorsion of the fellow eye. Vertical vergence in the achiasmat covaried with cycloversion implicating a common source. In the achiasmat and some albinos, additional vertical and torsional oscillations also were recorded. Vertical oscillations could show apparent disconjugacy; associated torsional components were typically conjugate. All observers showed various forms of interocular misalignments which together with horizontal, vertical and torsional instabilites varied markedly and independently with viewing conditions. In general, mean eye position vectors demonstrated a best fit plane shifted from Listing's classic fronto-parallel plane; the direction of the displacement planes was eye dependent and could alter with left, right or both eye viewing.

Conclusions: Achiasmatic and albino models may afford the opportunity to not only better understand the pathogenesis of related eye movement disarrangements but also to gain insights into visual pathway development and how the brain adapts to and processes sensory events even when left and right eye visual fields are cortically projected in mirror reversed order partially, as in albinism, or completely, as in non-decussating retinal-fugal fibre syndrome.