Morphological changes in the visual pathway induced by experimental glaucoma in Japanese monkeys.

Glaucoma, an optic neuropathy, is the leading cause of world blindness. In this condition, the damage extends from the retina to the visual center in the brain, although the primary region of damage is thought to be the optic nerve head (ONH), with the lateral geniculate nucleus (LGN) being secondarily affected. We investigated time-dependent alterations in the ONH, the optic nerve (ON), and the LGN after intraocular pressure (IOP) elevation in Japanese monkeys (a species more similar to humans than other macaque species). Nine Japanese monkeys, each with an experimental glaucomatous left eye, and two naive monkeys were studied. Ocular-testing sessions (including IOP measurement and fundus photography) were held weekly. Eyes and brains were enucleated at 2-48 weeks after IOP elevation, and alterations in ONs and LGN were evaluated. The IOP of the treated eyes was monitored periodically and found to be elevated continuously throughout the observation period in each monkey. The ONH of the glaucomatous eyes exhibited time-dependent deep cupping and thinning of the rim area from 2 weeks after the IOP elevation. Loss of axons and a decrease in the area of ON were first observed at 4 and 28 weeks, respectively. Neuronal loss was first observed at 2 weeks in layers 1 and 2 of LGN [magnocellular (M)-layer] and at 12 weeks in layers 3-6 of LGN [parvocellular (P)-layer]. Neuronal shrinkage was first observed at 2 weeks in all layers in LGN. These findings indicate that in Japanese monkeys, damage to neurons in LGN can be detected in the early phase (first few weeks) after an IOP elevation, as can damage to ONH.