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Interocular differences in optic disc topographic parameters in normal subjects

Purpose. To test the interocular differences in optic disc topography in normal subjects by means of confocal scanning laser ophthalmoscopy. Methods. Topographic measurements of the optic disc were evaluated by means of confocal scanning laser ophthalmoscopy (Heidelberg Retina Tomograph) in 314 eyes of 157 healthy volunteers. The examination was started randomly either with the right eye or the left eye. Differences between right and left eyes in disc area, cup area, cup volume, cup/disc area ratio, rim area, rim volume, maximum cup depth, cup shape measure, retinal nerve fiber layer thickness, and retinal nerve fiber cross section area for 360° and for the temporal and nasal regions of the optic nerve head were evaluated by means of Student t-test. The same parameters were assessed in a subgroup of 80 elderly (age > 50 years) healthy subjects. Holm's sequentially rejective method was used for significance correction of multiple comparisons. Results. Significant interocular differences in the average retinal nerve fiber layer thickness (p = 0.0010) and retinal nerve fiber layer cross section area (p = 0.0036) were found, with the right eye showing, on the average, lower values.The left eye showed a larger retinal nerve fiber layer thickness in 94 subjects (59.87%) and a larger retinal nerve fiber cross section area in 101 subjects (64.33%). In the temporal optic disc area there were no statistically significant differences in topometric data (p > 0.05). In the nasal area, significant interocular differences in the retinal nerve fiber layer thickness (p = 0.0002) and retinal nerve fiber layer cross section area (p = 0.0003) were found. Similar results were found when the group of subjects older than 50 years was considered. Conclusions. This study demonstrates systematic interocular differences in optic disc topometric data. Such a finding, be it due to methodological or biological reasons, should be taken in consideration in clinical trials.