Timolol lowers intraocular pressure but does riot inhibit the development of experimental myopia in chick

Reports of intraocular pressure (IOP) being higher in myopes than emmetrope s and of myopes being over-represented in glaucoma statistics, are consiste nt with a role of IOP in the excessive eye growth typically associated with myopia. We tested the hypothesis, based on these observations, that ocular hypotensive drugs would slow myopia progression using the chick as an anim al model and timolol as an example of such a drug. To induce myopia, chicks (n = 56) were fitted with either monocular translu cent diffusers or -15 D spectacle lenses from day 8. The drug treatment pro tocol comprised topical applications of 0.4% benoxinate, a local anaestheti c (to improve drug absorption), followed either by 0.5 % timolol or distill ed water (control), either daily (1000 hr) or twice daily (1000, 1600 hr), Refractive errors and ocular dimensions were measured on days 12 and 17. We also verified the ocular hypotensive effect of timolol in both normal (n = 8) and myopic (n = 12 diffusers; n = 12-15 D lenses) chicks. Here, we took baseline IOP measurements, instilled timolol and then monitored IOP over a further 5-9 hr. We found no difference in the amount of myopia produced in the timolol and control groups at either measurement time point (e.g. day 17, once per day application, diffusers: -26.9 +/- 3.3 D vs -22.7 +/- 9.1 D; lenses: -14.9 /- 3.8 D vs -14.9 +/- 3.6 D). This was in spite of the fact that timolol di d lower IOP in both normal and myopic chicks (27 and 18% reduction, respect ively) While timolol does have an ocular hypotensive effect in the chick, it does not inhibit the development of myopia in this animal model. (C) 2000 Academ ic Press.