Abstract

Glaucoma is a progressive neuropathy of the optic nerve. It causes degeneration of ganglion cell axons resulting in defects in the retinal nerve fibre layer (RNFL) and characteristic changes in the optic nerve head (ONH). The Heidelberg Retina Tomograph (HRT) is a confocal scanning laser imaging device, which measures the topography of the ONH and the adjacent RNFL. To quantify the measurements of the ONH topography, various stereometric parameters are calculated.

The change in the stereometric parameters of the HRT was studied in 34 eyes with glaucomatous progression in RNFL photographs and 34 eyes without progression. The change in only one stereometric parameter, the cup shape measure, showed a statistically significant correlation with the progression of the RNFL defect. An optimised change in the best three-parameter combination had 77% sensitivity and 79% specificity for progression.

The change in the stereometric parameters was compared in 51 eyes with glaucomatous progression in stereoscopic ONH photographs and 425 eyes without progression. The parameters having the best correlation with progression include cup:disc area ratio, vertical cup:disc ratio, cup volume and rim area. The parameter with the largest area under the receiver operating characteristics curve (0.726) was the vertical cup:disc ratio. A change of 0.007 in the vertical cup:disc ratio had a sensitivity of 80% and a specificity of 65% for progression.

The factors having the most significant effect on the sensitivity and specificity of the stereometric parameters for progression were the reference height difference and the mean topography standard deviation, indicating image quality. The change in image quality and age also showed a consistent, but variably significant influence on all parameters tested.

Exercise was associated with an increase in variance in 17 of the 18 stereometric parameters.

In conclusion, the change in the stereometric parameters provides useful information on ONH topography, especially when image quality is excellent. However, the evaluation of glaucomatous progression should not rely solely on the stereometric parameters of the HRT.