Purpose: To examine the effects that elevated intraocular pressure (IOP), a glaucoma risk factor, has on the size, density, and number of neurons in the primate lateral geniculate nucleus (LGN).
Methods: The monkey model of experimental glaucoma was combined with standard histologic staining and analysis techniques. Fourteen animals were examined.
Results: Mean IOPs higher than 40 mm Hg for 2.5, 4, 8, and 24 weeks resulted in reductions of 10% to 58% in the cross-sectional areas of LGN neurons receiving input from the glaucomatous eye. Reductions for animals with lower mean IOPs (37 and 28 mm Hg) for 16 and 27 weeks were 16% and 30%, respectively. Neurons receiving input from the normal eye also were reduced in size (4 -26%). No differential effect in cell size was seen for magnocellular versus parvocellular neurons. Elevation of IOP resulted in an increase in cell density in all layers of the LGN. The increase was approximately two times greater in parvocellular (59%) than magnocellular (31%) layers. When corrected for volumetric shrinkage of the LGN, the estimated loss of neurons was approximately four times greater in the magnocellular than parvocellular layers (38% versus 10%).
Conclusions: Elevation of IOP affects the size, density, and number of neurons in the LGN, and the volume of the nucleus itself. Although higher mean pressures (more than 40 mm Hg) reduce the period during which these changes occur, comparable damage can be achieved by even moderate (28 -37 mm Hg) levels of elevated IOP. On the basis of cell loss, elevation of IOP appears to have a more profound degenerative effect on the magnocellular than on the parvocellular regions of the LGN.