Section 2: Vision
2.8 Macular Degeneration
Recent statistics suggest that more than 13 million people in the United States 40 years of age and older have signs of macular degeneration and more than 1.2 million have the later, vision-threatening stages of the disease. Age-related macular degeneration may account for up to 30 percent (230,000 cases) of all bilateral blindness among Caucasian Americans and is a leading cause of blindness in Americans 55 years of age and older (Steinert, 1987). The greatest risk factor for acquiring macular degeneration is age. Other risk factors include gender (females more at risk than males), race (whites more at risk than blacks), smoking, and family history (NEI, 2000).
Macular degeneration is a progressive and irreversible destruction of receptors in the central portion of the retina. This central portion, known as the macula, is responsible for focusing central vision in the eye. Destruction of the macula affects an individual’s ability to read, to drive a car, to recognize faces or colors, and to see objects in fine detail.
Macular Degeneration and Driving Literature Review
Two recent studies have examined the effects of macular degeneration on driving performance. In the first study, Szlyk, Fishman, Severing, Alexander, and Viana (1993) compared the driving performance of 20 subjects with central vision impairment (juvenile macular dystrophies) to that of 29 individuals with normal vision. Mean age for the visually impaired group was 36.1 years and 38.9 years for the control group. Based on self-report, both groups of subjects had similar driving histories. Assessment of driving performance consisted of driving simulator performance and crash rates based on self-report and state records. Results revealed similar crash rates for the two groups. However, subjects with central vision loss who did not restrict their driving to daylight hours had a higher likelihood of being involved in nighttime crashes compared to the control group. Results from simulator performance revealed longer brake response times and a greater number of lane crossings for the central vision loss group. Visual function measures and simulator performance failed to predict crash performance for the visual function loss group.
The second study on central vision impairment (Szlyk, Pizzimenti, Fishman, Kelsch, Wetzel, and Kagan, 1995) compared the driving performance of 10 older subjects with age-related macular degeneration to that of 11 age-similar subjects with normal vision. Driving performance measures consisted of results from a driving simulator and an on-road driving test. Data also were obtained on crash and conviction rates (based on state- and self-report). Impaired simulator performance was noted for the age-related macular degeneration group as evidenced by delayed braking response times to stop signs, slower speeds, greater lane boundary crossings, and more simulator crashes. The age-related macular degeneration group also exhibited poorer overall on-road performance, with more points deducted for driving too slowly and for not maintaining proper lane position. A comparison of crash rates revealed a significantly higher rate of both state- and self-reported crashes for the control group than for the age-related macular degeneration group. However, differences in exposure may account for these findings. Finally, results of the study indicated that the age-related macular degeneration group compensated for their impairments in four ways: not driving in unfamiliar areas, traveling at slower speeds, self-restricting their night time driving, and taking fewer risks while driving (e.g., not changing lanes). Although often seen as a safety enhancing strategy, one of the compensatory strategies (e.g., driving at slower speeds) may in fact lead to crash involvement if the slower speed significantly differs from the speed of other motorists.
A review of the fitness-to-drive guidelines for medical practitioners from Australia (1998) and Canada (2000) reveals no recommendations for individuals with macular degeneration. Given the importance of central vision for driving, decisions about fitness-to-drive for individuals with macular degeneration should be determined on an individual level, with degree of central vision impairment a determining factor.