Intraocular Pressure, Exophthalmia, and the Effect of an Ophthalmic Carbonic Anhydrase Inhibitor in Fish Eyes
Ocular disease is common in aquarium fish.3 Fish have fixed (in most species), protruding eyes with spherical lenses, an ocular rete mirabile (choroid gland), vascular retrobulbar adnexa, no eyelids, and shallow sockets. As a result, they are vulnerable to trauma, secondary infection and exophthalmia. Exophthalmia is a common indication of systemic or primary ocular disease in fish, and may result from gas emboli (idiopathic gaseous exophthalmia or environmentally-induced gas bubble disease), trauma, viral or bacterial infection or neoplasia. Intraocular pressure (IOP) can be used as a general indicator of ocular disease, with increased IOP indicating glaucoma and decreased IOP associated with uveitis or other ocular inflammation. Carbonic anhydrase inhibitors have been proposed as a potential treatment for gaseous exophthalmia in fish by modifying gas exchange between the pseudobranch and the eye.3 Peribulbar injection of acetazolamide at 6 mg/kg has produced temporary resolution of exophthalmia in some cases,3 and inracoelomic injection of acetazolamide at 10 or 50 mg/kg causes decreased IOP in alewife fish (Alosa pseudoharengus).1,8 However, carbonic anhydrase inhibitors may cause buoyancy disorders through their effects on swim bladder function,3 and there are some anecdotal reports of fatal treatment outcomes, though fish may have been compromised by systemic disease prior to treatment. One mechanism by which carbonic anhydrase inhibitors could cause adverse effects in fish is by inducing plasma electrolyte imbalances, as has been reported in alewife.2 We hypothesized that 1) IOPs would differ between fish species, 2) the ophthalmic carbonic anhydrase inhibitor dorzolamide (Trusopt®) would reduce intraocular pressure in normal eyes, and 3) IOPs of exophthalmic eyes would differ from normal eyes. We measured IOPs on normal eyes of red pacu (Piaractus brachypomus) before and after treatment with the ophthalmic carbonic anhydrase inhibitor dorzolamide, normal eyes of koi (Cyprinus carpio), and exophthalmic and normal eyes of centrarchids (largemouth bass [Micropterus salmoides], bluegill [Lepomis macrochirus], and pumpkinseed sunfish [Lepomis gibbosus]) using applanation tonometry. The instrument we used was the MENTOR Tono-Pen XL. Each fish was placed under general anesthesia, each eye was topically anesthetized with proparacaine, and IOP values were recorded. Each pacu was then given dorzolamide or saline into one eye. IOP values were retested at either four or twenty-four hours following dorzolamide administration. In addition, plasma osmolality and electrolytes of treated and control pacu were measured.
Intraocular pressures differed significantly between normal koi and pacu, with a median (lower quartile, upper quartile) koi IOP of 11 (10, 18) mmHg and a median pacu value of 18 (15, 22) mmHg (12 koi and 21 pacu, Wilcoxon rank sum test, p =0.015). A single application of the carbonic anhydrase inhibitor did not reduce IOP at either 4 h or 24 h, but neither were any adverse effects of treatment noted by visual examination or plasma electrolytes. Exophthalmic and normal eyes of centrarchids did not differ significantly (Wilcoxon rank sum test, p = 0.0726) though exophthalmic eyes tended to have greater IOP and variance was significantly greater for exophthalmic eyes (O'Brien's homogeneity of variance test, p = 0.0032). Small sample sizes and multiple centrarchid speces complicate interpretation of exophthalmic eye IOP.
Funds for this project were provided by the NCSU College of Veterinary Medicine Summer Research Internship Program.
1. Edelhauser HF, Macdonald JM, Edelhauser SM. 1985. Effect of acetazolamide on intraocular pressure in alewife (Alosa pseudoharengus). Bull Mt Desert Isl Biol Lab 25:12-13.
2. Edelhauser HF, Ubels JL, Kormanik GA. 1988. Effect of acetazolamide on intraocular pressure and aqueous humor composition in alewives (Alosa pseudoharengus). Bull Mt Desert Isl Biol Lab 27 96-97.
3. Williams CS, Whitaker BR. 1997. The evaluation and treatment of common ocular diseases in teleosts. Sem Avian Exotic Pet Med 6:160-169.