Optic Nerve Oxygen Tension: The Effect of Timolol and Dorzolamide

Abstract

Abstract: : Purpose: We have previously reported that carbonic anhydrase inhibitors, such as acetazolamide and dorzolamide, raise optic nerve oxygen tension. We investigated whether two other common glaucoma treatments; timolol used alone and dorzolamide and timolol used in combination, have similar effects. Methods: The oxygen tension over the optic nerve (ONPO₂) was measured with a polarographic oxygen electrode mounted with an internal Ag/AgCl reference electrode inside a 20-gauge needle (model 768-20R, Diamond General Development Corporation, Ann Arbor, MI, USA). The tip of the electrode was placed in the vitreous humour 0.5 mm above the optic disc in anaesthetized pigs; the treatment of the pigs followed the ARVO resolution for the use of animals in ophthalmic and vision research. ONPO₂, arterial blood pressure, heart rate and arterial blood gasses were recorded continuously. In 2 groups of pigs, either 100 mg timolol (Timacar®, MSD Inc.) or a combination of 100 mg timolol and 500 mg dorzolamide HCl (MSD Inc.) was injected intravenously. Results: Baseline ONPO₂ (mean ± SD) was 3.95 ± 1.08 kPa (n=9). Administration of 100 mg timolol decreased ONPO₂ non-significantly by – 0.13 ± 0.16 kPa after 30 min (n=5, p=0.157). Administration of dorzolamide and timolol simultaneously increased ONPO₂ by 0.82 ± 0.21 kPa (n=4, p=0.004). The ONPO₂ increase induced by timolol and dorzolamide in combination is not significantly different from the increase caused by dorzolamide alone. Conclusions: Systemic administration of timolol does not affect the optic nerve oxygen tension. Timolol and dorzolamide given simultaneously increases ONPO₂ as much as dorzolamide when administered alone.

Keywords: animal model • blood supply • optic disc