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

PURPOSE. TO determine the diurnal intraocular pressure (IOP) response of Brown Norway rat eyes after sclerosis of the aqueous humor outflow pathways and its relationship to optic nerve damage. METHODS. Hypertonic saline was injected into a single episcleral vein in 17 animals and awake lap measured in both the light and dark phases of the circadian cycle for 34 days. Mean lap for light and dark phases during the experimental period were compared with the respective pressures of the uninjected fellow eyes. Optic nerve cross sections from each nerve were graded for injury by five independent masked observers. RESULTS. For fellow eyes, mean light- and dark-phase lap was 21 ± 1 and 31 ± 1 mm Hg, respectively. For four experimental eyes, mean laps for both phases were not altered. Six eyes demonstrated significant mean lap elevations only during the dark phase. Of these, five showed persistent, large circadian oscillations, and four had partial optic nerve lesions. The remaining seven eyes experienced significant lap elevations during both phases, and all had extensive optic nerve damage. CONCLUSIONS. Episcleral vein injection of hypertonic saline is more likely to increase lap during the dark phase than the light. This is consistent with aqueous outflow obstruction superimposed on a circadian rhythm of aqueous humor production. Because these periodic lap elevations produced optic nerve lesions, both light- and dark-phase lap determinations are necessary for accurate correlation of lap history to optic nerve damage in animals housed in a light-dark environment.