Scanning and transmission electron microscopic studies of two cases of pigment dispersion syndrome

We examined four eyes obtained post mortem from two patients who had increased intraocular pressure, normal visual fields, and pigment dispersion syndrome. Gross examination, light microscopy, scanning electron microscopy, and transmission electron microscopy disclosed elongated regions on the posterior iris surface of all the eyes. The cell membrane of the iris pigment epithelium was disrupted and there was extrusion of pigment granules at these locations. These areas of iris had a radial distribution (paralleling the course of the packets of zonular fibers) and corresponded to the peripheral iris transillumination defects. Pigment was dispersed in the anterior segment and on the pars plana. In the trabecular meshwork the pigment was free, within macrophages, and within endothelial cells. We observed giant vacuoles in the endothelium of Schlemm's canal. These findings suggested that friction between the zonular fibers and peripheral iris led to focal disruption of iris pigment epithelium and release of pigment granules. The increased pigment within the trabecular meshwork and Schlemm's canal may indicate that the glaucoma sometimes associated with pigment dispersion is caused by pigmentary obstruction, although congenital imperfections of the outflow channels may also be a factor.