Three dimensional analysis of the lamina cribrosa in glaucoma

Abstract

Background/aim: Structural changes in the lamina cribrosa have been implicated in the pathogenesis of glaucomatous optic atrophy. The aim of this study was to determine a measure the surface variability of the cup floor in normal subjects and patients with glaucoma.

Methods: A sample of age matched normal subjects (NN), patients with low tension glaucoma (LTG), and primary open angle glaucoma (POAG) were included in the study. The glaucoma groups were matched for the severity of the visual field loss. Mean 10 degree topographic images of normal and glaucomatous eyes from the Heidelberg retina tomograph were imported into ERDAS image processing software where topographic analysis of the cup floor could be assessed. Each image was processed using customised spatial filters that calculated the surface depth variation in localised neighbourhood areas across each image. The local change in depth across the cup floor surface was determined and compared between the three clinical groups.

Results: The depth variation in the cup floor was largest in normal subjects followed by LTG and POAG. Highly statistically significant differences in surface depth variability of the cup floor existed between normal and LTG (p = 0.005), between normal and POAG (p<0.0001), and between LTG and POAG groups (p<0.0001). The variability and skewness of depth difference across the optic cup floor were also significantly different between the three clinical groups.

Conclusion: A new parameter quantifying depth variations in the cup floor significantly discriminated between groups of normal and glaucoma patients. This new parameter may contribute to a better understanding of the pathogenesis of the glaucomatous optic nerve damage in different types of glaucoma.

Figure 1

Mean image imported from the Heidelberg retina tomograph (HRT), (left), and the surface profiling tool (ERDAS) (right). The ordinate in the three dimensional plot of the digitised image is the luminance associated with each pixel and, therefore, represents different depths in the lamina cribrosa surface across the floor of the optic cup.

Figure 2

Example of an optic disc showing the delineated area of interest (AOI) of the cup floor selected to be digitised. The AOI specifically excluded vessels in the optic cup.

Figure 3

Surface plots (ERDAS) showing surface variations of the lamina cribrosa in a normal subject (NN, top), low tension glaucoma patient (LTG, middle), and primary open angle glaucoma patient (POAG, bottom). (Note the different range in the ordinate axes of the three figures.)

Figure 4

The neighbourhood averaging filter. A 6×6 matrix (left) was placed on the floor of the optic cup. Each of the 36 cells corresponded to a single pixel or data point on the optic cup image. For each location of the matrix the average depth of the neighbouring cells was calculated as a reference for the centre cell. The value calculated for the centre cell was the difference between its actual depth value and the average depth of its neighbours. If there was no depth difference between a cell and its neighbors then that part of the surface would be relatively flat. Departure from zero depth difference indicates surface variability.

Figure 5

Optic cup floor variability across the three clinical groups represented as the standard deviation of pixel log luminance difference. (LTG  =  low tension glaucoma, POAG  =  primary open angle glaucoma).

Figure 6

The spatial distribution of pixel luminance difference across the optic cup floor in an eye with primary open angle glaucoma. This shows a clustering of values associated with small areas of greater depth in the superior and inferior regions of the cup floor.

Figure 7

Differences between the three clinical groups in the randomness of small areas of greater depth across the cup floor expressed in terms of the skewness index of pixel log luminance difference distribution. (LTG  =  low tension glaucoma, POAG  =  primary open angle glaucoma.)