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Simon Bello, Christopher L Passaglia; Statistical analysis of continuous IOP recordings in awake rats. Invest. Ophthalmol. Vis. Sci. 2017;58(8):5328.
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IOP fluctuations are typically measured via tonometry, so round-the-clock variations are not well characterized. This study analyzes the dynamic behavior of IOP over days to weeks in normal conscious rats.
IOP and temperature of adult Brown-Norway rats were continuously monitored using a custom wireless implantable sensor that we have described previously. Animals were housed in a temperature-controlled (21°C) room under a 12hr/12hr light/dark cycle. Data was collected in 0.4s epochs every 20s. Each epoch contained 20 sample points, and the average was recorded. The statistical properties of IOP and temperature records were quantified with custom MATLAB programs that perform histogram, auto/cross correlation, and power spectral analysis on multiple time scales. Correlation times were measured by fitting an exponential decay function to the correlation records. In some experiments, animals were temporarily anesthetized with isoflurane and eye and body temperature were manipulated while IOP was recorded.
Circadian rhythms of IOP were detectable in most rats, with IOP low during subjective day and high during the subjective night. The average day-night difference in IOP across animals was 5.1 ± 1.4 mmHg (n = 7). IOP variability was often measurably larger at night, presumably because rats are nocturnal animals. IOP data were positively correlated at long and short time scales both with themselves and with temperature data. The long-term correlation had a period of 24.5 hrs and thereby reflected the circadian rhythm in IOP and temperature. The short-term correlation had a duration of 1.5 hrs. Direct manipulation of eye and body temperature further confirmed their strong influence on IOP, while isoflurane anesthesia had negligible effect.
Mean IOP varies a daily basis between a daytime level of ~16 mmHg and nighttime level of ~21 mmHg in rats, consistent with reported tonometry measurements. The variation in IOP correlates closely with daily rhythms in body temperature. When performing tonometry on anesthetized animals, eye temperature must be maintained to ensure IOP measurement accuracy.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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