PURPOSE - To investigate quantitatively the relationships between elevated intraocular pressure (IOP), axonal loss, and corneal thickness in the DBA/2 mouse model of glaucoma, to understand better how these factors contribute to disease progression.
METHODS - IOP was measured with a handheld tonometer (Tono-Pen; Medtronic Solan, Jacksonville, FL) in 195 to 446 eyes of mice 2 to 10 months of age sampled from a colony of 400 DBA/2 mice. From a group of 24 eyes at 4, 9, and 10 months of age, correlations were determined between the density and number of RGC axons, corneal thickness, and IOP.
RESULTS - Mean IOP levels in the colony were 15 to 16 mm Hg at 2 months of age and rose almost linearly at a rate of 0.9 mm Hg/mo before reaching 22 to 23 mm Hg at 10 months. Both the density and number of axons decreased with increasing average lifetime IOP. IOP variation within age groups strongly correlated with density. Age-matched mice with lower mean IOP had greater preservation of axons in the optic nerve. Elevated IOP was accompanied by increased corneal thickness at the limbus. Surprisingly, corneal thickness was a strong predictor of axonal density (r2 = -0.75), regardless of age.
CONCLUSIONS - IOP increased with age in most, but not all, DBA/2 mice. In age-matched mice, differences in IOP corresponded to differences in axonal density and number. In young mice with elevated IOP, the loss of axons resembled that of older animals with similar IOP. Whether corneal thickness is a byproduct of elevated IOP remains unknown, but it may be useful as an index of optic nerve degeneration.