Patterns of intraocular pressure elevation after aqueous humor outflow obstruction in rats

PURPOSE. TO determine the diurnal intraocular pressure (IOP) response of Br own Norway rat eyes after sclerosis of the aqueous humor outflow pathways a nd its relationship to optic nerve damage. METHODS. Hypertonic saline was injected into a single episcleral vein in 17 animals and awake IOP measured in both the light and dark phases of the ci rcadian cycle for 34 days. Mean IOP for light and dark phases during the ex perimental period were compared with the respective pressures of the uninje cted fellow eyes. Optic nerve cross sections from each nerve were graded fo r injury by five independent masked observers. RESULTS. For fellow eyes, mean light- and dark-phase IOP was 21 +/- 1 and 3 1 +/- 1 mm Mg, respectively. For four experimental eyes, mean IOPs for both phases were not altered. Six eyes demonstrated significant mean IOP elevat ions only during the dark phase. Of these, five showed persistent, large ci rcadian oscillations, and four had partial optic nerve lesions. The remaini ng seven eyes experienced significant IOP elevations during both phases, an d all had extensive optic nerve damage. CONCLUSIONS. Episcleral vein injection of hypertonic saline is more likely to increase IOP during the dark phase than the light. This is consistent: w ith aqueous outflow obstruction superimposed on a circadian rhythm of aqueo us humor production. Because these periodic IOP elevations produced optic n erve lesions, both light- and dark-phase IOP determinations are necessary f or accurate correlation of IOP history to optic nerve damage in animals hou sed in a light-dark environment.