Slope analysis of the optic disc in eyes with ocular hypertension and early normal tension glaucoma by confocal scanning laser ophthalmoscope

Abstract

Aims: To determine whether quantitative differences in sector based slope can differentiate between eyes with ocular hypertension with and without glaucomatous disc changes and eyes with normal tension glaucoma with glaucomatous disc changes.

Methods: Seventy six eyes with ocular hypertension or early glaucomatous disc changes were consecutively categorised into three groups: 22 eyes with ocular hypertension and no glaucomatous disc changes (OHND); 35 with ocular hypertension and glaucomatous disc changes (OHD); and 19 with normal ocular tension and glaucomatous disc changes (NTD). Twenty eyes served as controls. The average total slope angle and sector based slope angle of the cup, total contour area, effective area, neuroretinal rim area, half depth area, cup to disc ratio, contour variation, mean contour depth, average depth, volume below, half depth volume, and contour tilt were evaluated with a confocal scanning laser ophthalmoscope.

Results: The earliest changes in eyes with OHND or OHD started in the slope at the nasal inferior sector (p<0.05), followed by the superior and temporal superior sectors (p<0.05). The mean slopes in eyes with NTD and OHD were steeper than in controls (p<0.05). Statistically significant differences were found between controls and disease groups in the half depth area, mean contour depth, and half depth volume. The cup to disc ratios in eyes with OHD and NTD were greater than in eyes with OHND; the volume below was greater in eyes with NTD than in eyes with OHND and OHD.

Conclusions: The steep slope in the nasal inferior section is the first indicator of glaucomatous nerve defects in many eyes. The half depth parameters, half depth area, and half depth volume may be useful for distinguishing ocular hypertension with and without glaucomatous disc changes.

Figure 1

(A) Schematic drawing of optic disc parameters by the TopSS. (B) Image of the reference cap tilting which is automatically corrected by a reference cap.

Figure 2

Image of calculation of slope for one pixel.

Figure 3

Scheme of the optic disc sectors for right eyes: superior sectors (sectors 1 and 12), nasal superior sector (sector 2), nasal sectors (sectors 3 and 4), nasal inferior sector (sector 5), inferior sectors (sectors 6 and 7), temporal inferior sector (sector 8), temporal sectors (sectors 9 and 10), and temporal superior sector (sector 11). For the purposes of analysis the left eyes were mirror images.