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Editors Selection IGR 17-4

Tonometry: TonoLab

Comment by John Morrison on:

22726 Assessing intraocular pressure by rebound tonometer in rats with an air-filled anterior chamber, Lee EJ; Park KH; Kim DM et al., Japanese Journal of Ophthalmology, 2008; 52: 500-503

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Accurate measurement of eye pressure is essential for research involving animal models of glaucoma. Because the Tono-Pen tip is difficult to accurately apply to the small, highly curved rodent cornea, the TonoLab, a rebound tonometer, is enjoying increased attention, with early papers showing good calibration in normal and glaucomatous rats and mice. Lee et al. (126) demonstrate, with relatively small numbers of eyes, performance of the TonoLab compared to cannulation pressure in rat eyes, but using intracameral air instead of fluid for altering eye pressure. Although the stated reason for this particular paradigm is to test this instrument under conditions used to evaluate retinal ischemia, while many ischemia models actually use fluid instead of air, these results are still valuable as they test the TonoLab over a wide range, from 20 to 100 mmHg, as opposed to previous reports that are limited to 50 mmHg. The authors report that TonoLab readings are approximately 7% less than cannulation pressures, and this discrepancy increases when pressure exceeds 60 mmHg, reaching nearly 20% at 90 mmHg. Some of this underestimation may be due to effects of measuring pressure in an air-filled anterior chamber, and since animal glaucoma models involve pressures well below 50 mmHg, it may not be considered a significant drawback. However, the TonoLab does appear to have the ability to detect higher pressures than previously appreciated. This is reassuring because pressure fluctuation is common to all glaucoma models, and when high pressures do occur they can, if undetected, produce unexpected and uninterpretable results.

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