DEC 20, 2011
Previous studies examining the intrapapillary region of highly myopic eyes have revealed a markedly thinner lamina cribrosa in addition to, and potentially attributable to, the enlargement of the optic nerve head. In this prospective study, investigators histologically examined and compared the parapapillary region of highly myopic and nonhighly myopic human eyes. They found morphological differences that might explain why highly myopic eyes are more susceptible to glaucoma.
They used 36 human globes with an axial length longer than 26.5 mm (highly myopic group) that showed marked glaucomatous optic nerve damage, and 45 eyes with an axial length 26.5 mm or less (nonhighly myopic group). The nonhighly myopic group included 28 globes that had been enucleated because of painful absolute secondary angle-closure glaucoma and 17 eyes that had been enucleated because of a malignant choroidal melanoma.
Highly myopic eyes with a distance of less than 0.5 mm between the optic nerve border and the beginning of Bruch’s membrane showed a markedly elongated and thinned sclera with some overlying retinal nerve fiber layer, without Bruch’s membrane, choroid, retinal pigment epithelium or retinal photoreceptors interposed between the sclera and the retinal nerve fiber layer. This was in contrast to nonhighly myopic eyes, in which, independent of the presence or absence of glaucomatous optic nerve damage, Bruch’s membrane extended to the optic nerve border and the scleral flange was significantly shorter and thicker. Since the scleral flange formed the anterior border of the retrobulbar cerebrospinal fluid space, the elongation of the scleral flange in highly myopic eyes caused an extension of the retrobulbar cerebrospinal fluid space into the parapapillary region.
The authors conclude that since the thickness of the parapapillary sclera influences the biomechanics of the lamina cribrosa, these findings may partially explain the increased glaucoma susceptibility of highly myopic eyes. The biomechanical implications of the absence of the Bruch’s membrane in highly myopic eyes requires further elucidation.