Purpose: The GDx (Laser Diagnostic Technologies, San Diego, CA) is a scanning laser polarimeter designed to assess the peripapillary nerve fiber layer in vivo. On the GDx, a nerve fiber bundle can appear as a single or as a split bundle. The aim of our study was to determine the prevalence of split nerve fiber layer bundles and to demonstrate their clinical relevance.
Design: Cross-sectional study.
Participants: Two hundred fifty-four healthy volunteers participated.
Methods: We imaged 454 eyes of 254 healthy white persons with the GDx. All eyes had intraocular pressure of 21 mmHg or less, normal appearance of the optic nerve head, and normal visual fields (Humphrey Field Analyzer 24-2 full threshold program). According to our working definition, a bundle appeared 'split' when the color-coded pixels corresponding to areas of higher retardation were clearly divided into two more-or-less symmetrical parts not resembling a wedge defect. The classification was performed by two independent observers who used an identical set of reference examples to standardize the classification.
Main outcome measures: The presence of a split nerve fiber layer bundle.
Results: Interobserver agreement was very good (kappa = 0.83), and a consensus was reached in all cases. In 419 eyes (92.3%) there was no split bundle. A split superior bundle was seen in 29 eyes (6.4%). A split inferior bundle was observed in five eyes (1.1%), and in one eye (0.2%) a split bundle was seen superiorly and inferiorly. When considering subjects, a split superior bundle (either in the right eye, or in the left eye, or in both eyes) occurred in 12.0% of normal subjects. The 'superior maximum' parameter was significantly lower in eyes with a split superior bundle than in eyes with a single superior bundle (67.2 microm vs. 89.9 microm; P<0.001). The same was observed for the 'symmetry' parameter (0.88 microm vs. 0.98 microm; P<0.001).
Conclusions: Split nerve fiber layer bundles are a common finding in healthy eyes when imaged with the GDx. A split superior bundle is the most frequent variation, in which cases an abnormal superior maximum or symmetry parameter, otherwise potential indicators of glaucoma, should not readily be interpreted as abnormal.