Diurnal variation of corneal thickness in the cat.

Central corneal thickness of both eyes of seven cats was measured hourly for 72 hr using ultrasonic pachometry. The mean corneal thickness was 569 +/- 36 micron (mean +/- SD), and the diurnal variation was 49 +/- 14 micron (8.6% of corneal thickness). In a separate experiment, the corneal thickness of one eye of each of five cats was measured following 2 hr of natural sleep; 2 1/4 hr after eye opening, the corneas had thinned an average of 43 +/- 22 micron. The authors conclude that corneal swelling induced by eye closure during periods of sleep is the prime determinant of the diurnal variation in cat corneal thickness. In studies where corneal thickness is to be monitored over a period of time, it is possible to control against this large diurnal variation by ensuring that the cat is active for a period of two hours prior to pachometry measurements.