Optic disc morphometry with optical coherence tomography: comparison with planimetry of fundus photographs and influence of parapapillary atrophy and pigmentary conus.

PURPOSE: Optical coherence tomography (OCT) is an established method for visualising macular pathology and for measuring the thickness of parapapillary retinal nerve fibre layer. The purpose of the present study was to compare OCT measurements of the optic disc with those obtained by planimetry of fundus photographs and to investigate whether OCT measurements are influenced by the amount of parapapillary atrophy or pigmentary conus at the disc margin. MATERIALS AND METHODS: Eighty-two eyes of 41 randomly selected Asian-Indian subjects were included.The mean age was 44.2+/-14.3 years (mean+/-SD). All eyes underwent optic disc assessment by OCT (fast optical disc protocol) and digital 20 degree disc photos.. Planimetry of the discs were performed by outlining the disc and the cup. The amount of parapapillary atrophy (zone beta) was quantified as the number of clock-hours around the disc with adjacent parapapillary atrophy. The same procedure was performed to quantify the amount of pigmentary conus around the disc. RESULTS: Mean+/-SD disc size and mean+/-SD cup size in OCT measurements (2.37+/-0.51, 1.29+/-0.55) were significantly smaller than in photographic planimetry (2.83+/-0.62, 1.56+/-0.5: P<0.001, P<0.001). Optic discs with parapapillary atrophy (P=0.2) in their relative difference of disc sizes between OCT and planimetry. Similarly, optic discs with pigmentary conus (n=12) and discs without pigmentary conus (n=70) did not differ significantly (P=0.65). The relative difference in disc size between the two measurement modalities did not correlate with the amount of parapapillary atrophy (r=-0.17, P=0.29) or with the amount of pigmentary conus (r=-0.04, P=0.9). CONCLUSION: OCT analysis of the optic disc produces significantly smaller parameters, compared to the established method of optic disc planimetry. Neither presence, nor extent of parapapillary atrophy zone beta and pigmentary conus seems to produce a systematic error in measurements of disc size with the OCT.