Quantitative study of optic nerve head capillaries in experimental optic disk pallor.

We obtained quantitative measurements of capillary numbers, areas, and diameters in atrophic (pale) and normal primate optic nerve heads. The number of capillaries per square millimeter in pale optic disks was not significantly different from that in normal optic disks. Because the loss of all nerve fibers leads to a 50% decrease in nerve head substance, capillaries must atrophy to maintain a constant relationship between capillary number and tissue volume. The mean size of individual capillaries in atrophic nerve heads was smaller than normal, leading to a decrease of more than 27% in the percentage of tissue volume occupied by capillaries. When this decrease in capillary volume was mimicked in the normal optic disk by reducing the hematocrit value, optic disk pallor did not result. Hence, the development of optic disk pallor appears to be the result of thinning of the neural tissue of the rim of the optic disk and the consequent change in tissue composition and optical transparency, rather than of a loss of optic disk capillaries.