PURPOSE: To compare the effect of elevated intraocular pressure (IOP) on retinal 
capillary filling in elderly vs adult rats using optical coherence tomography 
angiography (OCTA).
METHODS: The IOP of elderly (24-month-old, N=12) and adult (6-8month-old, N=10) 
Brown Norway rats was elevated in 10mmHg increments from 10 to 100mmHg. At each 
IOP level, 3D OCT data were captured using an optical microangiography (OMAG) 
scanning protocol and then post-processed to obtain both structural and vascular 
images. Mean arterial blood pressure (MAP), respiratory rate, pulse and blood 
oxygen saturation were monitored non-invasively throughout each experiment. 
Ocular perfusion pressure (OPP) was calculated as the difference between MAP for 
each animal and IOP at each level. The capillary filling index (CFI), defined as 
the ratio of area occupied by functional capillary vessels to the total scan 
area but excluding relatively large vessels of >30μm, was calculated at each IOP 
level and analyzed using the OCTA angiograms. Relative CFI vs IOP was plotted 
for the group means. CFI vs OPP was plotted for every animal in each group and 
data from all animals were combined in a CFI vs OPP scatter plot comparing the 
two groups.
RESULTS: The MAP in adult animals was 108±5mmHg (mean±SD), whereas this value in 
the elderly was 99±5mmHg. All other physiologic parameters for both age groups 
were uniform and stable. In elderly animals, significant reduction of the CFI 
was first noted at IOP 40mmHg, as opposed to 60mmHg in adult animals. Individual 
assessment of CFI as a function of OPP for adult animals revealed a consistent 
plateau until OPP reached between 40 and 60mmHg. Elderly individuals 
demonstrated greater variability, with many showing a beginning of gradual 
deterioration of CFI at an OPP as high as 80mmHg. Overall comparison of CFI vs 
OPP between the two groups was not statistically significant.
CONCLUSIONS: Compared to adults, some, but not all, elderly animals demonstrate 
a more rapid deterioration of CFI vs OPP. This suggests a reduced autoregulatory 
capacity that may contribute to increased glaucoma susceptibility in some older 
individuals. This variability must be considered when studying the relationship 
between IOP, ocular perfusion and glaucoma in elderly animal models.