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

The existence of asymmetries between the two eyes in number and distribution of nerve fibers may provide an early anatomic indication of glaucomatous disruption if a symmetrical pattern of nerve fibers can be shown in normal eyes. Normal eyes were tested to determine whether a high degree of correspondence of regional nerve fiber layer thickness exists between fellow eyes.

Nerve fiber layer thickness was inferred from retardation shift measured by a scanning laser polarimeter (Laser Diagnostic Technologies, Inc.) in 40 volunteers without glaucoma or ocular disease (80 eyes). Total thickness and the pattern of nerve fiber thickness over 208 regions was compared between the paired eyes.

Inferred nerve fiber layer thickness in companion eyes was highly correlated. Variation of the total nerve fiber thickness was much less between 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 pronounced shift of the location of the superior peak (maximum thickness) in the left eye relative to the right was observed, as were differences between the two eyes in the thickness measurements at the nasal and temporal minima.

Significant interocular asymmetries were detected in regions other than the interior hemiretina. These interocular asymmetries may reflect previously unrecognized anatomic nerve fiber differences or systematic angular 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 asymmetries of total thickness determined by polarimetry may offer potential for detecting glaucoma.