The influence of alpha-adrenergic agents on tonic accommodation

This study has examined the effect of an alpha-1 adrenergic agonist (phenylephrine HCl, 2.5%) and a non-selective alpha antagonist (thymoxamine HCl, 0.5%) on tonic accommodation (TA). TA was measured at 1 min intervals throughout a 45 min period using an objective infra-red optometer (Canon Autoref R-1). Additionally pupil diameter and the near-point of accommodation (NPA) were monitored in order to assess the efficacy of the instilled drug. A double-blind protocol was adopted between thymoxamine, phenylephrine and a saline control for 7 emmetropic subjects. There was no significant difference between the values of TA obtained under the three pharmacological conditions. However, both adrenergic agents induced changes in pupil diameter and NPA. Furthermore in all conditions TA increased by approximately 0.25D while the subject sat passively in darkness. The data indicates that alpha-adrenergic innervation alters the closed-loop accommodative response independently of TA.