Abstract
Purpose :
While it has been observed that gravity inversion increases intraocular pressure (IOP), the relative changes in ocular perfusion pressure (OPP) and intracranial pressure (ICP) are less well documented. In this interventional study we sought to estimate the change in translaminar pressure difference (TLPD) using changes in retinal venous pulsation (RVP). We also aimed to measure which ocular structures changed with the change in IOP, and changes in ganglion cell stress with electrophysiology, while sitting and inverted.
Methods :
We recorded from 15 healthy volunteers, and 5 patients with glaucoma and conditions of interest. The IOP was collected with rebound tonometry (iCare), BP was measured manually with the arm raised to the ear, so the measurement was at the level of the eye. Infrared videos of the optic disc, and spectral domain optical coherence tomography (OCT) of the macula and optic disc, and macula retinal OCT angiography (OCTA) were obtained with Heidelberg Spectralis OCT2. In a subset of volunteers we measured photopic negative response (PhNR) amplitudes using a Diagnosys Espion system. All measurements were taken in both a sitting and inverted position (typically -45 degrees below supine).
Results :
The IOP increased immediately and predictably with inversion, behaving like a physical fluid column of around 0.3 m (average 16 mmHg sitting, 30 mmHg inverted). Mean arterial BP measured at the level of the eye increased from 71 to 98 mmHg with inversion, resulting in a small increase in OPP (from 56 to 68 mmHg) and a corresponding autoregulation vasoconstriction response evident on OCTA. There was no change in retinal venous pulsation (RVP) amplitude for the majority of participants, although some individuals had increased or decreased RVP with inversion. On OCT imaging, the clearest change with inversion was an increase in choroidal thickness. Inversion for 15 minutes was not associated with a statistically significant change in PhNR signal.
Conclusions :
Driven by increased venous pressures, inversion caused an immediate and similar increase in IOP and ICP, and venous pulsations remained unchanged in most participants suggesting little change in TLPD. The BP increased more than IOP with inversion, so OPP increased and vasoconstriction was evident. These findings have implications for advice given to patients with glaucoma or intracranial hypertension.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.