Cyclic Pattern of Intraocular Pressure (IOP) and Transient IOP Fluctuations in Nonhuman Primates Measured with Continuous Wireless Telemetry

ABSTRACT

Purpose: Most studies on intraocular pressure (IOP) to monitor IOP “fluctuations” in glaucoma patients have been performed with snapshot tonometry techniques that obtain IOP measurements at single time points weeks to months apart. However, IOP telemetry has shown that IOP varies from second-to-second due to blinks, saccades, and systolic vascular filling. The purpose of this study was to characterize the cyclic pattern of baseline IOP and transient IOP fluctuations in 3 nonhuman primates (NHPs).

Methods: Bilateral IOP was measured using a proven implantable telemetry system and recorded 500 times per second, 24 hours a day, up to 451 continuous days in 3 male rhesus macaques aged 4 to 5 years old. The IOP transducers were calibrated every two weeks via anterior chamber cannulation manometry and all data were continuously corrected for signal drift via software, filtered for signal noise and dropout, and peaks and troughs were quantified and counted using a finite impulse response filter; waking hours were defined as 6:00–18:00 hours based on room light cycle.

Results: Fourier transform analyses of baseline IOP and the hourly mean frequency of transient IOP fluctuations > 0.6 mmHg, 0.6–5 mmHg and > 5 mmHg above baseline during waking hours exhibited an approximate 16- to 91-day cyclic pattern in all NHPs. There were no measured environmental or experimental factors associated with this cyclical pattern.

Conclusions: While the importance of the cyclic pattern identified in IOP and its fluctuations is unknown at this time, it is plausible that this pattern is relevant to both homeostasis and pathophysiology of the ONH, corneoscleral shell, and aqueous outflow pathways.

KEYWORDS:

• IOP
• telemetry
• nonhuman primate
• cyclic pattern

Acknowledgments

The authors sincerely thank Dr. Lawrence Sincich for his invaluable assistance with the autocorrelation and FFT analyses. We also thank Lisa Hethcox, LVT for her invaluable help in the both the care of the NHPs and data acquisition, and Chester Calvert for his invaluable assistance in data procurement and processing.

Declaration of Interest

The authors report no conflicts of interest and have no proprietary interest in any of the materials mentioned in this article.

Additional information

Funding

This work was supported by the NIH R01 EY024732 (JCD), EY026035 (JCD) and P30 EY003039 (Pittler; UAB NEI Core Infrastructure Grant); EyeSight Foundation of Alabama (Unrestricted Funds); Research to Prevent Blindness (Unrestricted Funds).