Interocular symmetry in nerve fiber layer thickness of normal eyes as determined by polarimetry

Purpose: The existence of asymmetries between the two eyes in number and di stribution of nerve fibers may provide an early anatomic indication of glau comatous disruption if a symmetrical pattern of nerve fibers can be shown i n normal eyes. Normal eyes were tested to determine whether a high degree o f correspondence of regional nerve fiber layer thickness exists between fel low eyes. Methods: Nerve fiber layer thickness was inferred from retardation shift me asured by a scanning laser polarimeter (Laser Diagnostic Technologies, Inc. ) in 40 volunteers without glaucoma or ocular disease (80 eyes). Total thic kness and the pattern of nerve fiber thickness over 208 regions was compare d between the paired eyes. Results: Inferred nerve fiber layer thickness in companion eyes was highly correlated. Variation of the total nerve fiber thickness was much less betw een companion eyes than between eyes of different individuals. In terms of regional (i.e., local) interocular correspondence, the measurements in the inferior retina were observed to be highly similar in the two eyes, but a p ronounced shift of the location of the superior peak (maximum thickness) in the left eye relative to the right was observed, as were differences betwe en the mio eyes in the thickness measurements at the nasal and temporal min ima. Conclusion: Significant interocular asymmetries were detected in regions ot her than the inferior hemiretina. These interocular asymmetries may reflect previously unrecognized anatomic nerve fiber differences or systematic ang ular bias in the instrument for one eye relative to the other. In contrast to the regional measures, the measures of total thickness were very similar between companion eyes, suggesting that the assessment of interocular asym metries of total thickness determined by polarimetry may offer potential fo r detecting glaucoma.