Reversal of tropicamide mydriasis with single instillations of pilocarpine can induce substantial pseudo-myopia in young adults



Pupillary dilation for diagnostic purposes has become an increasingly common procedure in UK optometry in recent years and the consensus seems currently to militate against the routine use of miotics; an eye that is judged safe to dilate is thought to be at minimal risk during the natural recovery phase to normal pupil size. Nevertheless, the optometrist does, on occasion, need to consider whether there might be some advantage in minimising the sometimes debilitating effects of cycloplegia and mydriasis produced in young adults by tropicamide. In this context we compare the effects of single instillations of two miotic agents: the α-adrenoceptor antagonist thymoxamine HCl (0.5%), and the parasympathomimetic pilocarpine HCl (1 and 2%). Tropicamide was used to induce mydriasis in a group of 12 volunteer student subjects aged 20–26 years (7 male, 5 female; mean 21.67 years) selected to provide low (L; n = 4), medium (M; n = 4) and high (H; n = 4) iris pigment levels. Measurements of pupil diameter (Brocca pupillometer), Snellen visual acuity and accommodative amplitude (near point rule) were made every 3 min over a 90 min recording period for 4 trials: (1) a control condition whereby a miotic was not employed; (2) thymoxamine HCl 0.5% was instilled after 30 min; (3) and (4) pilocarpine 1% and 2% was instilled after 30 min, respectively. Tropicamide induced a mean increase in pupil area from 25 to 50 mm2 after 22 min which was generally sustained over the 90 min period and was enhanced for the lower pigment groups. Although a partial reversal was induced by thymoxamine (i.e. a mean decrease from 50 to 35 mm2 35 min after instillation), was not present with pilocarpine, there was clear evidence of a significant pilocarpine-induced pseudo-myopia which, when corrected with negative spheres, had mean values (in DS for pigment levels L, M, H) as follows: for 1%, 0.56 (L), 0.66 (M), 0.34 (H); for 2%, 1.17 (L), 0.71 (M), 0.44 (H). The pseudo-myopic effects were transitory but, if present, became apparent within 30 min after instillation of pilocarpine and lasted for up to 80 min after completion of the drug trials. The results emphasise the need to monitor closely the potential pseudo-myopic effects of pilocarpine, especially in light coloured irides where an adequate standard of vision for driving might be at risk. Further, reinstatement of an easily accessible formulation of thymoxamine (or dapiprazole HCl) should be encouraged.