Diurnal variations in intraocular pressure in the albino rabbit: relationship to morning intraocular pressure

Intraocular pressure (IOP) measurements were carried out in 88 adult male New Zealand white rabbits. IOP of each rabbit was measured in the morning (8.00-9.00 h), at noon (12.00-13.00 h) and in the afternoon (16.00-17.00 h) on 6 separate days. A linear relationship was found between the noon IOP and the morning IOP: noon IOP = 0.77 morning IOP + 4.37; r = 0.950. This regression line intersects the line of IOP equality (morning IOP = noon IOP) at morning IOP = 19 mm Hg. Thus, in animals with morning IOP < 19 mm Hg, there was an increase in IOP between the morning and noon readings whereas animals with a higher morning IOP (20-30 mm Hg) showed a progressive decrease in the morning-noon IOP. This observed relationship explains the disparity among previously published IOP diurnal curves for rabbits. It is suggested that future studies of IOP changes should present IOP values and not only relative changes in IOP (delta IOP or percent change in IOP).