Quantitative analysis of the lamina cribrosa in vivo using a scanning laser opthalmoscope.

PURPOSE Structural changes in the lamina cribrosa have been implicated in the pathogenesis of glaucomatous optic nerve atrophy. The purpose of this study was to develop a method for morphometric analysis of the lamina cribrosa pores in vivo, using a scanning laser ophthalmoscope. METHODS A prototype Zeiss confocal laser scanning ophthalmoscope was used to acquire images of the lamina cribrosa. The images were digitised and aligned to compensate for eye movements. Thirty-two consecutive images were averaged to reduce noise. The images were processed to adjust for luminance gradients prior to segmentation and analysis. Details of the image processing are described. RESULTS The end result of processing the images was a binary (black and white) image that can be used for automated computer assisted measurements. The pores of the lamina cribrosa are well represented and retain their overall shape in the binary image, as judged by superimposing the binary image on the unprocessed image. We also established the repeatability, reproducibility and intercession variability of this technique. Repeated images of the internal lamina cribrosa of 10 patients were acquired by two observers in two separate visits, and the images were processed before automated computer measurements. The parameters evaluated were number of pores, area covered by the pores and area covered by the visible lamina cribrosa. The coefficient of variation for number of pores, pore area and lamina area was 6.9%, 2.1% and 4.3% for observer A and 5.5%, 2.1% and 5.8% for observer B. Pearson product moment correlation coefficient between the two observers was 0.94, 0.99 and 0.97 for the above parameters respectively. There was no significant difference between the measurements on visit 1 and 2 for both observers. CONCLUSIONS The technique described allows, for the first time, in vivo morphometry of the internal lamina cribrosa surface. This method has good reproducibility, suggesting future clinical applications.