When should a visually impaired patient stop driving? With clinical measures uncertain, driving tests may be the best way to tell

Howard Larkin

Because driving is so dependent on out for restriction, Dr Peli points out.vision, common sense dictates that driving must be unsafe beyond a certain threshold of visual impairment. This idea is so compelling that traffic authorities worldwide restrict or prohibit driving by persons with defects in visual acuity and often in visual field. Problem is, the scientific evidence linking these clinical measures of vision to unsafe driving performance is weak to non-existent, says Eli Peli, OD, professor of ophthalmology at Harvard Medical School in Boston, US. The correlation with visual acuity is especially tenuous. “There are no firm, evidence-based standards, so people make decisions based on weak reasoning,” Dr Peli says. “Legislators look at the state next door and they adopt similar standards because that is what people are used to and what they will accept.”

The 2005 International Council of Ophthalmology report, “Vision Requirements for Driving Safety,” recommends that countries worldwide adopt 0.5 visual acuity and 120o horizontal visual field as a screening point for an unrestricted licence. The report recommends such requirements “not because one becomes

A person spotting through a monocular bioptic telescope. Shown is a 3.0X Keplerian telescope (Mini, manufactured by Ocutech, inc). This telescope was used by some of the subjects in the Dutch study unsafe at 0.4 but it includes a safety margin for adverse conditions.” In other words, anyone who has 0.5 visual acuity is likely to retain enough vision to drive safely in the dark, rain, fog or other poor conditions.

However, the report further recommends that individual consideration be given to those in the 0.5-0.1 range, with additional vision, cognitive, and functional tests, including a road test, if there is any doubt. Restricted licences also should be available for the purpose of “improv[ing] the safety margin (inherent in the standard requirements) through avoidance of hazardous conditions, especially for those who have prior experience and a good driving record.”

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