Abstract
Purpose :
Elevated intracranial pressure (ICP) has been shown to decrease aqueous humor outflow facility and increase intraocular pressure (IOP) in rats, but the sensitivity and time course of its effects were not described. The purpose of this study was to characterize the impact of ICP insults of varying magnitude and duration on aqueous humor dynamics.
Methods :
Experiments were done on ketamine-anesthetized adult Brown Norway rats. Ventricular ICP (vICP) was recorded via a 25-gauge needle inserted into the cerebral ventricles, while IOP was recorded via a 33-gauge needle inserted in the anterior chamber of one eye. vICP and/or IOP were elevated by varying amounts and durations by altering the height of a fluid reservoir or infusing fluid with a feedback-controlled pump, while ventricular and ocular infusion rates were monitored with in-line flow meters. Response time-to-peak (TTP) was extracted from IOP and flow rate records, along with peak amplitude changes relative to baseline levels prior to ICP elevation.
Results :
Elevating vICP by 5 mmHg above baseline for 10, 15, and 30 mins caused IOP to increase with a TTP of 14 ± 0.8 (n=2), 21 ± 15 (n=2), and 29 ± 4.8 (n=3) mins, respectively. The TTPs of 10 vs 30 min duration were significantly different (p=0.023). The peak IOP increase did not depend on ICP duration (p=0.97) and averaged 1.9 ± 0.8 mmHg across experiments. Elevating vICP by 9 mmHg for 15 or 25 min caused peak IOP to increase even higher 4.3 ± 0.8 mmHg (n=2, p=0.01). Elevating vICP by 10 mmHg for 10, 15, and 30 mins, while elevating IOP to 30 mmHg, caused ocular flow rate to decrease by 26 ± 6.5 nL/min with a TTP of 20 ± 5.4 min. Both peak flow rate and TTP appeared independent of ICP duration. Elevating vICP by 15 mmHg caused a similar decrease in peak flow rate (27 ± 5.9 µL/min, n=3; p=0.89), while TTP was significantly shorter (6.7 ± 0.8 min, p<0.01).
Conclusions :
Both IOP and aqueous flow are affected by the amplitude and, to a lesser extent, the duration of vICP insults in rats. A flow rate reduction during concurrent ICP and IOP elevation implies that ICP elevation increases eye outflow resistance, and this resistance increase likely underlies IOP changes to ICP elevation alone.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.