Longitudinal evaluation of retinal ganglion cell function and IOP in the DBA/2J mouse model of glaucoma

Abstract

Purpose: To characterize progressive changes of retinal ganglion cell (RGC) function and intraocular pressure (IOP) in the DBA/2J mouse model of spontaneous glaucoma.

Methods: Serial pattern electroretinograms (PERGs) and IOPs measures were obtained from both eyes of 32 anesthetized DBA/2J mice over an age range of 2 to 12 months at 1-month intervals. Cone-driven flash-ERGs (FERGs) were also recorded. The endpoint was defined as the age at which the PERG amplitude reached the noise level in at least one eye. At that point, both eyes were histologically processed to evaluate the thickness of the retinal fiber layer (RNFL).

Results: IOP increased moderately between 2 and 6 months ( approximately 14-17 mm Hg) and then more steeply, until it leveled off at approximately 28 mm Hg by 9 to 11 months. The mean PERG amplitude decreased progressively after 3 months of age to reach the noise level (85% reduction of normal amplitude) at approximately 9 to 12 months in different animals. When the PERG was at noise level, the RNFL showed a relatively smaller reduction (40%) in normal thickness. The FERG displayed minor changes throughout the observation period. IOP and PERG changes were highly correlated (r(2) = 0.51, P < 0.001).

Conclusions: Results indicate that inner retina function in DBA/2J mice progressively decreases after 3 months of age, and it is nearly abolished by 10 to 11 months, whereas outer retina function shows little change and the RNFL thickness is relatively spared. This result suggests that surviving RGCs may not be functional. Progression of inner retinal dysfunction is strongly associated with increased IOP.

Figure 1

Representative examples of PERG (A) and FERG (B) recorded in DBA/2J mice aged 3 months. For both PERG and FERG, the amplitude was measured from the positive peak to the negative trough. The latency was evaluated from the stimulus onset (contrast reversal for the PERG, strobe flash for the FERG) to the peak of the positive wave. Note that the PERG had a lower amplitude and longer latency compared to the FERG. (C) Axial section through the optic nerve head of a DBA/2J mouse aged 10 months and stained with hematoxylin–eosin. The thickness of the RNFL was measured at point of curvature of axons entering the optic nerve head.

Figure 2

Progressive changes of PERG amplitude (A), PERG latency (B), FERG amplitude (C), and FERG latency (D) with age. Thick symbols connected by thick lines: mean ± SEM. Thin lines: amplitudes longitudinally measured in individual eyes. Dashed lines: upper limit of the noise range. Note that the PERG amplitude (A), but not FERG amplitude (C) progressively decreased with age and reached the noise level by approximately 11 months. Interrupted thin lines at intermediate ages represent eyes for which follow-up could not be completed due to death or development of cataract.

Figure 3

Residual RNFL thickness at the PERG endpoint. By the time the PERG was at noise level in at least one eye (age range, 9–12 months) the RNFL had lost ~40% of normal thickness (2-month-old mice) but was still much thicker (~135%) than that in 16-month-old mice with advanced glaucoma. RNFL data have been analyzed separately for eyes with worse PERG amplitude (first eye) and better PERG amplitude (second eye) at the endpoint.

Figure 4

Average PERG amplitude, RNFL thickness for both eyes as a function of age. Data are plotted on a normalized scale. Error bars, SD. Axon counts at different ages obtained from previous studies are also plotted for comparison. Note that progressive loss of RGC axons lags behind progressive loss of PERG amplitude by approximately 3 months. Also note that relative changes in RNFL thickness are in agreement with corresponding changes in RGC axons.

Figure 5

Progressive changes of IOP as a function of age. Thick symbols connected by thick lines: mean ± SEM. Thin lines: longitudinally measured IOP in individual eyes. Interrupted thin lines at intermediate ages represent eyes for which follow-up could not be completed due to death or cataract development. Average PERG amplitude (±SEM) data shown in Figure 2A (bottom trace) are also displayed for comparison. For IOP data, the y-axis corresponds to mm Hg; for PERG data, the y-axis corresponds to μV. Note that the close association between IOP increase and PERG amplitude decrease.

Figure 6

PERG amplitude as a function of IOP in individual eyes. (A) Linear PERG amplitude scale; (B) log PERG amplitude scale; solid lines: linear regression of data and ± 95% CI. Dashed lines: the upper limit of the noise range. Note the good correlation between PERG amplitude and IOP.