Correlation between optic disc area and retinal nerve fiber layer thickness: a study on scanning laser polarimetry with variable corneal compensation

Purpose To evaluate if a significant increase of the peripapillary retinal nerve fiber layer (RNFL) thickness can be measured in a sample of healthy eyes by means of scanning laser polarimetry with variable corneal compensation (GDx-VCC) as the optic disc (OD) area increases. Methods One eye each of 232 healthy subjects (mean age: 57.8 years; range:40-70) was considered. Temporal-superior-nasal-inferior-temporal average (TSNIT Avg) and OD area (area within the ellipse placed on inner border of peripapillary scleral ring) values were collected. Ellipse horizontal and vertical diameters provided on printout were used to estimate OD area using the equation: OD area = pi x horizontal radius x vertical radius. TSNIT Avg values were plotted against OD area and a multiple linear regression including age calculated. Results Mean OD area was 2.19 mm(2) +/- 0.45 (range: 1.23-3.59) and mean TSNIT Avg was 54.3 mu +/- 3.2 (range: 45.8 +/- 61.8). Multiple linear regression equation was TSNIT Avg--57.7-0.096xOD Area-0.055xAge (Pearson r = -0.146: p=0.086). Conclusion In our sample of healthy eyes, no significant correlation was found between TSNIT Avg and OD area. In spite of a shorter distance between OD and measurement ellipse margins, larger discs did not necessarily show a thicker RNFL. Probably the large inter-subject variability for RNFL thickness, and then for axonal count, was a predominant factor over OD area.