Effects of mydriatics and a miotic on ocular discomfort and pupil responses

Mydriatics produced two different effects on the discomfort threshold following administration of the drugs depending upon whether the mydriasis was produced by paralysis of the sphincter muscle or by activation of the dilator. When the sphincter was paralyzed by tropicamide, the discomfort threshold was elevated during the period of time that the pupillary light reflex was significantly reduced to the flashing stimulus. Initially, phenylephrine-induced mydriasis had no effect on the discomfort threshold, but as the pupil responses returned to normal the threshold rapidly increased and then gradually returned to a pre-drug level. The return of the threshold back to normal seemed to parallel the decline of action of phenylphrine on the dilator muscle. Significant impairment of the light reflex by pilocarpine-produced miosis was also related to an increase of the discomfort threshold.