Abstract
Purpose :
Intraocular pressure (IOP) and intracranial pressure (ICP) oppose each other at the optic nerve head, which can cause cupping from large pressure imbalances in either direction. Such cupping can lead to retinal ganglion cell damage and vision loss. This study aims to characterize short-term fluctuations of ICP and IOP in anesthetized and conscious rats.
Methods :
In one set of experiments (n = 14), adult Brown-Norway rats were maintained under ketamine anesthesia for several hours while ICP and IOP were concurrently recorded via a 25-gauge needle in a cerebral ventricle and a 33-gauge needle in the anterior chamber of an eye. In another set of experiments (n = 3), ICP was recorded in awake free-moving animals via a 27-guage needle implanted in a cerebral ventricle and connected to a tethered telemetry system. Data were collected at 10 Hz and processed by Fourier analysis in 30-min segments. Three frequency bands were isolated, corresponding to slow (SO: <0.5 Hz), respiratory (RO: 0.8-1.6 Hz), and cardiac oscillations (CO: >3.5 Hz). RMS amplitude was computed for each band, and differences were assessed statistically via t-tests.
Results :
Baseline ICP and IOP were 6.1±1.1 and 11.0±2.0 mmHg, respectively, in anesthetized rats and 5.3±3.9 mmHg in awake rats. Slow aperiodic fluctuations (amplitude: 0.75±0.26 mmHg) were observed under anesthesia in ICP records but not IOP records. The aperiodic fluctuations were also seen in awake animals during idle periods and were larger in size (3.2±2.2 mmHg, p<0.01). Excluding these waves, ICP amplitude in SO, RO, and CO bands was 0.39±0.05, 0.31±0.14, 0.35±0.22 mmHg, respectively, and much larger than IOP amplitude in each band (SO: 0.15±0.07mmHg, RO: 0.05±0.02, CO: 0.07±0.04 mmHg, p<0.01) under anesthesia. Moreover, ICP amplitude was not significantly different from idle rats for any band or from active rats for RO and CO bands. The SO band was markedly greater, though, for active rats (1.3±0.4 mmHg, p<0.05).
Conclusions :
Hence, over the scale of seconds to minutes, ICP variability in anesthetized rats is similar in frequency content but larger in amplitude than IOP variability. The excess variability exacerbates the potential impact of ICP on the optic nerve head. Higher ICP variability in ambulatory rats is likely related to animal movement since RO and CO band amplitudes were comparable in anesthetized, idle, and active rats.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.