Purpose: Used in the diagnosis of Horner's syndrome, apraclonidine 1% dilatates the involved eye due to denervation supersensitivity. Recent literature suggests that in healthy volunteers, apraclonidine provokes a mild miotic effect. Since the comparison of both the pathologic and the non-pathologic eye is important, we wanted to further investigate the effect of apraclonidine on the healthy eye. By measuring the effect on the pupil intermittently over a few hours, we tried to determine the best moment for evaluation after instillation with apraclonidine. Therefore, the effect of apraclonidine on pupillary parameters was investigated in 14 healthy volunteers.
Methods: Infrared pupillography was used to measure the scotopic pupil diameter and the dynamic pupil responses to light. The first measurements were performed prior to instillation of apraclonidine. Measurements were retaken 30, 60, 90, 120, 180, 240, 300 and 360 min after random instillation of one eye with one drop of 1% apraclonidine.
Results: The anisocoria after dark adaptation and at minimum pupil diameter differed significantly for the measurements obtained 30 and 60 min after instillation with apraclonidine. The eye with apraclonidine drops showed relative miosis and an increased amplitude of constriction to light. No significant influence was found on the latency, the constriction velocity and redilation velocity.
Conclusions: Instillation of apraclonidine 1% in healthy subjects causes relative miosis, which is most pronounced after 30-60 min. The amplitude of constriction to light also differs significantly. The relative miotic effect of apraclonidine could be explained by the α-2 receptor agonistic effect which is more pronounced than the α-1 agonistic effect in healthy subjects. In patients with Horner's syndrome, the α-1 agonistic effect will dominate because of the supersensitivity of the α-1 receptors, resulting in relative mydriasis. These findings stress the necessity to instill the unaffected eye in diagnosing a suspected Horner's pupil.