Segmental variability of the trabecular meshwork in normal and glaucomatous eyes

Although numerous studies have examined morphologic changes in the trabecular meshwork of glaucomatous eyes, few have systematically looked for segmental variations around the circumference of the meshwork. If segmental variations occur, studies based on random sections or on single trabeculectomy specimens may not give a complete picture of the histologic changes in glaucoma. The purpose of this study was to determine the extent of segmental variability in human trabecular meshwork.

Eyes from five normal donors and eyes from five donors with primary open-angle glaucoma were obtained at autopsy. Histologic sections of each quadrant from nine of the normal eyes and eight of the glaucomatous eyes were analyzed. Light microscopy was used to measure the length of Schlemm's canal, the thickness of the trabecular lamellae, and the area of empty space in the juxtacanalicular (JCT) region. Transmission electron microscopy was used to examine the thickness of the JCT and the ultrastructural components of the JCT, which were quantitated with a computerized image analysis system. Ultrastructural analysis included JCT area, area of empty space, area of empty space touching Schlemm's canal, area of solid tissue, and area of sheath and tendon material.

Differences among quadrants were qualitative rather than quantitative, because significant variations in structure were not found within single eyes. The variability among quadrants was similar for normal and glaucomatous eyes, with a coefficient of variation of approximately 20% for most tissue components. A greater variability was found for the thickness of the JCT. The largest variability was found for the area of empty space touching Schlemm's canal: 81.4% for normal eyes and 86.5% for glaucomatous eyes. No quadrant had consistently higher or lower values of any parameter. The length of Schlemm's canal did not differ significantly between normal and glaucomatous eyes (normal: 256.6 +/- 66.0 microns, glaucomatous: 298.8 +/- 57.7 microns), nor did the mean thickness of the trabecular lamellae (normal: 4.7 +/- 0.8 microns, glaucomatous: 4.8 +/- 0.5 microns). Light microscopy underestimated the total amount of empty space in the JCT by 20% compared with electron microscopy.

Significant segmental differences were not found within single eyes in either normal or glaucomatous eyes. The magnitude of the variability of most tissue components was similar for normal and glaucomatous eyes. The most variable component was the area of empty space touching Schlemm's canal, which may represent the effective pathway for aqueous flow to enter Schlemm's canal. Future quantitative studies of the JCT should include samples from at least three quadrants per eye. Quantitative analysis of the JCT from a single quadrant, as occurs in the study of trabeculectomy specimens, may overestimate or underestimate the amount of solid tissue or empty space by 200%.