Pathology of the glaucomas.

In the normal eye, the main resistance to aqueous outflow is in the outermost region of the trabecular meshwork, which is adjacent to and forms the inner wall of Schlemm's canal. In primary open-angle glaucoma, there is increased resistance to outflow, and we assume that the abnormal additional resistance is in the same location. Examinations of pathological anatomy show scarring and sclerosis in the trabecular meshwork, but the specimens are usually taken from advanced cases and it has not been decided to what degree trabecular changes may be due to damage secondary to long-standing elevated intraocular pressure rather than the primary defect. Electron microscopy, which Dr. Tripathi will cover in detail, shows fewer vacuoles in the endothelium of Schlemm's canal in patients with glaucoma. Again, it is not clear whether the fewer vacuoles account for diminished outflow facility or whether they merely reflect alterations in flow dynamics because of resistance elsewhere in the outflow pathway. For example, chemical changes in the juxtacanalicular connective tissue could result in impaired fluid flow with no apparent morphological alteration, but the altered flow pattern might be reflected in the endothelium. Some evidence seems to show occlusion of collector channels in cases of established glaucoma, but it is again a problem to decide if this is the primary event or if collapse of channels is induced mechanically by elevated intraocular pressure acting on resistance in the trabecular region. Pseudoexfoliative glaucoma and pigmentary glaucoma seem to be related to primary open-angle glaucoma. What is not at all clear is whether the pseudoexfoliative and pigmentary materials contribute to the outflow obstruction or whether the material is simply trapped in the meshwork which has poor outflow facility to begin with.