Short-term IOP Spikes Constitute Over 11% of the IOP Energy the Eye Must Absorb During Waking Hours

Abstract

 

Purpose

 

To quantify the contribution of short-term IOP spikes to overall IOP energy using continuous, bilateral IOP telemetry.

 

Methods

 

We have developed and validated an implantable telemetry system that wirelessly records 500 measurements of IOP per second for up to 2-1/2 years (IOVS 52(10):7365-75). Using an enhanced version of this system, continuous bilateral IOP, bilateral electro-oculogram (EOG), and aortic blood pressure were recorded for twelve 24-hour periods in a 14-day span in 3 young adult male rhesus macaques (NHP) aged 3-6 years old. The IOP transducers were calibrated directly via anterior chamber manometry, and IOP data were corrected for signal drift. IOP fluctuations >1 mmHg over baseline and lasting <~2 seconds were characterized. The number of IOP spikes, and the percentage of IOP energy due to IOP spikes as a percentage of the total IOP energy were calculated for each hour.

 

Results

 

As seen in the Figure, IOP is incredibly dynamic, with blinks, saccades and other external forces generating short-term IOP spikes of significant magnitude over baseline. IOP spike energy varied widely between NHPs, but was significantly higher during waking hours in all three NHPs. Overall, IOP spikes accounted for 11% of the total IOP energy that the eye must absorb during waking hours. In all three NHPs, over 10,000 IOP spikes per hour >2 mmHg hit the eye during the day, which decreased to <4,000/hr. at night.

 

Conclusions

 

The eye is exposed to over 10,000 short-term IOP spikes per hour during the day, which represents a significant but previously uncharacterized part of IOP energy. This constitutes up to 16% of the total IOP exposure the eye must absorb. The dynamic nature of these spikes represents a different biomechanical insult than longer-term IOP fluctuations, and may be important in IOP-related diseases such as glaucoma.

 

 

(Top) A screenshot of 12 seconds of bilateral IOP telemetry data showing the dynamic nature of IOP. (Middle) IOP spikes are detected, counted, and their energy calculated (orange areas) relative to baseline IOP (2 seconds of data shown). (Bottom) Mean hourly IOP spike energy as a percentage of total hourly IOP energy plotted as hourly means from midnight to midnight, averaged over all twelve 24-hour recording periods for each eye in 3 NHPs. Time is plotted on the 24-hour scale; waking hours are hours 7-18, and lights-out are hours 1-6 and 19-24.

 

Keywords: 568 intraocular pressure