Intracranial pressure modulates aqueous humour dynamics of the eye

Abstract

Key points An elevation in intracranial pressure (ICP) lowers conventional outflow facility (increases aqueous outflow resistance) of rat eyes. The reduction in outflow facility correlates with an increase in intraocular pressure (IOP). The effect of ICP elevation on outflow facility and IOP is blocked by TTX. The results indicate that aqueous humour dynamics is modulated by ICP‐driven neural feedback from the brain. This feedback mechanism may act to stabilize translaminar pressure across the optic nerve head and may provide a new avenue for glaucoma therapy. Abstract While intraocular pressure (IOP) is a well‐known risk factor for glaucoma, intracranial pressure (ICP) is attracting heightened interest because of its influence on optic nerve head biomechanics. Studies have shown that ICP can have marked impacts on posterior eye health by modifying the translaminar pressure gradient across the optic nerve. There is also growing evidence that IOP and ICP may be interconnected, although the mechanism of their putative interaction is unknown. We sought to test the hypothesis that ICP modulates IOP by altering aqueous humour dynamics. The anterior chamber and lateral ventricle of anaesthetized Brown‐Norway rats were cannulated with fine‐gauge needles connected to a programmable pump and saline reservoir, respectively. ICP was manipulated by varying reservoir height, and eye outflow facility (C) was determined from the pump flow rate required to hold IOP at different levels. C was 22 ± 4 nl/min/mmHg at resting ICP and 13 ± 3 nl/min/mmHg when ICP was raised 15 mmHg, a reduction of 41 ± 13% (n = 18). The decrease in outflow facility was independent of blood pressure, reversible, scaled with ICP elevation and correlated with increases in resting IOP. It was physiological in origin because C returned to baseline values after the rats were killed and corneal application of TTX though ICP remained elevated. These results indicate that a neural feedback mechanism driven by ICP regulates conventional outflow facility in rats. The mechanism may protect the eye from translaminar pressure swings and may offer a new target for glaucoma treatment.

The Effects of Acute Intracranial Pressure Changes on the Episcleral Venous Pressure, Retinal Vein Diameter and Intraocular Pressure in a Pig Model

“…46 In a recent study of aqueous humor dynamics in a rat model of raised ICP, there was a reduction in the outflow facility ("C" in the Goldmann equation) following ICP elevation, which the investigators attributed to neural feedback mechanism driven by ICP. 47 The investigators did not collect data for EVP, which is a major limitation. Third, EVP has a smaller population normal range and variability (mean ± SD: 7.6 ± 1.3 mm Hg) 20 compared to IOP (mean ± SD: 15.5 ± 2.6 mm Hg), which also shows a skew at higher IOP levels.…”

The eye in extreme environments

“…A recent study by Ficarotta and Passaglia (2020) has introduced a new theory on the interaction of intracranial pressure with intraocular pressure (IOP), which has implications for our understanding of the potential aetiology of SANS (Jaki Mekjavic, Amoaku, Mlinar & Mekjavic, 2020). In a rodent model, these authors varied the intracranial pressure and monitored aqueous humour outflow.…”

“…In contrast to a younger group (Mlinar T., Jaki Mekjavic P. & Mekjavic I.B., unpublished observations), their exercise‐induced increase in IOP was much greater. It is speculated that the difference in the IOP response to static hand‐grip exercise might be the age‐related change in the structure of the lamina cribrosa (Kotecha, Izadi, & Jeffery, 2006) or an age‐related change in the intracranial pressure‐dependent neural feedback mechanism protecting the optic nerve from damage by maintaining the translaminar pressure gradient (Ficarotta & Passaglia, 2020).…”

Non-invasive detection of intracranial pressure related to the optic nerve

“…IOP and ICP are 2 sets of pressure systems that are interrelated and relatively independent. Through the circulation of the aqueous humor and CSF, ICP and IOP are relatively stable (1).…”