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Dilatation of the pupil (mydriasis) is a core component of a comprehensive ophthalmic examination and is becoming part of the eye examination routine for optometrists in the United Kingdom.1 As many patients drive to attend their examinations, concerns have been raised regarding the effects of pupillary dilatation on driving and whether or not the visual standard for driving is met after dilatation.2–4 Therefore, we investigated the effect of mydriasis on the visual standard for driving a private vehicle in the United Kingdom.
Twenty adult subjects (mean age 24 years) with normal or corrected to normal visual acuity participated in the study. A selection of six different pairs of number plates (black on white and black on yellow), which conformed to the current UK visual standard for driving (that is, after 1 September 2001),5 was constructed for the study. Monocular and binocular distance logMAR visual acuity and binocular number plate readings were made before and after mydriasis (of at least 6 mm diameter) with 0.5% tropicamide. Each subject performed the number plate test under standard Driving Standards Agency conditions (outside) at a fixed test distance of 20 metres.6 A pass at the number plate test required all numbers and letters to be read correctly. Letter by letter scoring was used to record visual acuity. Data were analysed using paired t tests (one tailed) as appropriate. Subjects were also asked for their views on the effects of mydriasis using a questionnaire.3 The study followed the tenets of the Declaration of Helsinki and approval of the protocol was obtained from the institutional human research ethics committee.
All subjects met the legal requirement for driving both before and after dilatation, irrespective of the number plate used. Visual acuity decreased (average of three letters worse) following dilatation (p<0.05 for right eye, left eye and binocular visual acuity results) (table 1).4,7
Following mydriasis, 13 subjects reported an increase in glare, 15 felt more sensitive to light, 14 reported blurry distance vision, five reported blurry near vision, and one subject reported no effect of the drops. Most subjects reported a slight (nine) or moderate (10) effect of the drops; however, the majority (19) felt confident enough to safely drive home after dilatation.
Despite a loss in visual acuity after dilatation, all subjects passed the number plate test both before and after dilatation and irrespective of the background colour of the number plate. Recently, Goel et al found a significant number of patients (four of 28) did not pass the number plate test after mydriasis.8 However, subjects in our study were relatively young, wore the appropriate refraction and were free from signs of ocular disease. Given the small effect of mydriasis on visual acuity,4,7,9 it is not surprising that, for our subjects, mydriasis did not affect the UK visual standard for driving. Notwithstanding the relatively small loss in visual acuity, most subjects in our study reported a slight or moderate effect of mydriasis on their vision. Nevertheless, all subjects but one felt confident enough to drive home.
Wood et al have shown that mydriasis has a measurable decrease on real life driving performance, specifically on the detection and recognition of low contrast road obstacles and the ability to navigate around them.9 These findings, together with our results, suggest that satisfying the UK visual standard for driving does not preclude a decrease in driving performance. Therefore, all patients should be advised that although the visual standard for driving may be met following mydriasis, caution is advised when driving after pupillary dilatation.
Competing interests: none declared