Intraocular pressure (IOP) has been shown to be higher in recumbent positions than the seated position, and in the lateral decubitus position the dependent (lower) eye has a higher IOP than the non-dependent (higher) for both normal subjects and glaucoma patients.1,2 However, the clinical significance of this elevation in IOP has been unclear, particularly the inter-eye differences in the lateral decubitus position. In this study, Al-Nosairy et al. used pattern electroretinogram (PERG) to assess retinal ganglion cell function in the lateral decubitus position compared with the seated position. A particularly novel aspect of this study is that one of the cohorts had previously placed implantable pressure sensors (Eyemate-IO, Implandata Ophthalmic Products GmbH, Hannover, Germany) in one eye, allowing continuous measurement of IOP during PERG testing. A secondary aim of the study was to determine if the presence of the IOP sensors affected collection of PERG signals.
The decrease in PERG amplitudes with elevated IOP in the lateral decubitus position indicates that there was a detectable change in RGC function in this position
The authors included 15 healthy controls and 15 treated glaucoma patients, among whom eight patients had the Eyemate-IO implanted in the right eye. PERG amplitudes and IOP were compared in the sitting, right and left lateral decubitus positions. IOP was measured using rebound tonometry in eyes without the IOP sensor. As expected, they found that IOP was higher in the lateral decubitus position than the seated position, and the dependent eyes had higher IOPs than the non-dependent eyes. Interestingly, the dependent eyes had lower PERG amplitudes compared with the seated position, but no difference was found between the non-dependent eyes and the seated position. As well, there was no significant effect due to potential electromagnetic interference from the IOP sensor.
The decrease in PERG amplitudes with elevated IOP in the lateral decubitus position indicates that there was a detectable change in RGC function in this position. However, the long-term clinical consequence of this is unclear. Also, it is possible that other physiologic changes beyond IOP (e.g., blood flow, CSF pressure) may be influencing this change as there was no detectable change in the non-dependent eyes even when a significant increase in IOP was recorded compared with the seated position. One potential issue is that the authors did not change the orientation of the monitor used for stimulus when assessing PERG in the lateral decubitus position, as this introduces a potential stimulus difference between sitting and LDP positions. Nevertheless, this novel work appears to clear the way for future research concerning the effect of IOP fluctuations on PERG amplitudes in patients with implanted IOP sensors.