Effect of a topical beta-adrenergic receptor antagonist on the dynamics of steady-state accommodation

The ocular accommodation response to a stationary near target exhibits microfluctuations which are characterized by two dominant components of activity: a low-frequency component (LFC: less than 0.5 Hz) and a high-frequency component (HFC: 1.0-2.2 Hz). We have shown recently that there is a significant correlation between the frequency of arterial pulse (AP) and the peak frequency of the HFC. If the mechanisms by which AP induces the HFC are intraocular they are likely to involve both rhythmic variations in choroidal blood flow and intraocular pressure (IOP) pulse: the former inducing changes in ciliary ring diameter from pulsatile blood volume changes in the ciliary body, the latter producing changes in effective lens power following small shifts in lens position or, with each drop in IOP, reduced resistance to inherent lens substance and capsule elasticity. In addition to its well-known ocular hypotensive effect, topical instillation of the beta-adrenergic receptor antagonist timolol maleate has been shown to reduce significantly the magnitude of the IOP pulse, an effect that has been attributed to the vasoconstrictive action of the drug on the choroidal vasculature. Using a modified Canon IR optometer we report on the effect of 0.5% timolol maleate on the accommodative microfluctuations for a group of 10 emmetropic subjects viewing a target monocularly at a distance of 25 cm. Following a double-blind protocol against a saline control, timolol was shown to reduce the rms for both LFCs and HFCs. An experiment to separate local from systemic effects involved four subjects whose treated and untreated eyes were examined to determine the effects of the consensual reduction in IOP: the results on microfluctuations in the untreated eyes were equivocal but the correlation between arterial pulse frequency and location of the peak frequency of HFC was maintained. The results show that factors relating to IOP and vasculature will affect the magnitude of accommodative microfluctuations.