Long-term non-invasive measurement of intraocular pressure in the rat eye

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Abstract
To study the optic neuropathy associated with glaucoma, a system for accurate, reliable, and non-invasive monitoring of intraocular pressure (IOP) is required. Of particular interest is the effect of sampling frequency on IOP. To address this issue, ten adult male brown Norway rats (group 1) were acclimatized to a 12-h/12-h light/dark cycle. On 20 days over a 30-day period, rats were anesthetized with short-acting isoflurane (Forane) inhalant anesthesia and IOP for each eye was determined by averaging 15 valid individual readings obtained with a TonoPen 2 tonometer. The last 12 measurement sessions were performed on a daily basis. To determine the minimum tolerable interval between IOP measurement sessions, a second group of 10 animals (group 2) was acclimatized in the same manner as group 1, and IOP was measured every 4 days over a period of 80 days. Next, IOP was measured every 3 days over a period of 28 days, and finally, every 2 days over a period of 19 days. For all group 1 measurements, there was no statistically significant difference between the right and left eye IOP, 14.75 ± 1.08 (SEM) and 14.90 ± 1.09 mm Hg, respectively. However, daily measurements produced a steady decrease in IOP and gradual weight loss. For group 2, overall mean right and left eye IOPs were 15.24 ± 1.28 (SEM) and 15.12 ± 1.26, respectively and were not significantly different. Over time, all pressures were stable, giving a linear regression equation of y = −0.008x + 15.34 (r = 0.187) when measured every fourth day, y = 0.12x + 13.95 (r = 0.788) every third day and y = 0.02x + 14.71 (r = 0.202) when measured every other day. Animals continued to gain weight satisfactorily, even with every other day measurements. Rat IOP can be measured reliably and repeatedly with the TonoPen 2 using isoflurane anesthesia.