Effects of adrenergic drugs on accommodation and distant refraction in daylight and darkness

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Abstract
Near point accommodation was measured using a RAF near point rule, and distant refraction in daylight and darkness were determined using a laser‐optometer in a double‐blind cross‐over study on 10 healthy test subjects (19–31 years). Determinations were made before and 40 min after topical instillations of 3 times 10 μl 10% phenylephrine, 0.5% thymoxamine, 1% adrenaline, 3% isoproterenol, 0.5% timolol and 0.5% betaxolol. Combinations of 3 times 10 μl 0.5% thymoxamine + 10% phenylephrine, 0.5% timolol + 1% adrenaline and 0.5%. betaxolol + 3% isoproterenol were also administered. The near point of accommodation decreased 0.8 + 0.3 diopter after instillations of the alpha1 adrenoceptor agonist phenylephrine, while an increase of 0.6 ± 0.2 diopter was determined after administration of the alpha adrenoceptor antagonist thymoxamine. Thymoxamine plus phenylephrine did not alter the pretreatment values. Beta adrenoceptor stimulation by isoproterenol or beta adrenoceptor inhibition by timolol or betaxolol had no effect on the near point accommodation. The difference between distant refraction in daylight and darkness, that is low‐luminance myopia, amounted to −1.25 ± 0.1 diopter. Phenylephrine caused a myopic shift in distant refraction in daylight and darkness of 0.32 ± 0.1 and 0.9 ± 0.2 diopter, respectively. This effect was due to the mydriatic action of phenylephrine. While using a 2 mm artificial pupil in four subjects, phenylephrine did not change distant refraction in daylight or darkness. After beta adrenoceptor stimulation by isoproterenol distant refraction in darkness shifted 0.12 ± 0.07 diopter towards hyperopia while beta adrenoceptor inhibition by timolol or betaxolol caused a myopic shift of 0.2 ± 0.1 diopter. These shifts were, however, not statistically significant. We conclude from these data that alpha1 adrenoceptor stimulation decreases while alpha adrenoceptor antagonism increases the near point accommodation. Furthermore, alpha1 adrenoceptor stimulation of the ciliary muscle is blocked by the alpha adrenoceptor antagonist thymoxamine in vivo.

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