2010 BC Guide in Determining Fitness to Drive

Chapter 21:Vision ImpairmentBACKGROUND21.1 About vision impairmentVision impairment is defined as a functional limitation of the visual system and can bemanifested as reduced visual acuity, reduced contrast sensitivity, visual field loss, loss of depthperception, diplopia (double-vision), visual perceptual difficulties or any combination of these.This chapter focuses on more common vision impairments and medical conditions that can causevision impairments.Visual acuityVisual acuity is the ability of the eye to perceive details. It can be described as either static ordynamic. Static visual acuity, the common measure of visual acuity, is defined as the smallestdetail that can be distinguished in a stationary, high contrast target (e.g. an eye chart with blackletters on a white background). When tested, it is reported as the ratio between the test subject’svisual acuity and standard “normal” visual acuity. Normal visual acuity is described as 20/20 or6/6 in metric. A person with 20/40 vision (6/12 metric) needs to be 20 feet (6 metres) away todistinguish detail that a person with normal vision can distinguish at 40 feet (12 metres). Thestandard Snellen chart for measuring visual acuity and a table of standard ratings is included in21.16.Dynamic visual acuity is the ability to distinguish detail when there is relative motion betweenthe object and the observer. Given the nature of driving, dynamic visual acuity would seem to bemore relevant to driving fitness than static visual acuity. However, barriers to the use ofdynamic visual acuity for fitness to drive decision-making include the absence of a practicablemethod of testing dynamic visual acuity, limited research on its relevancy for driving, and thelack of established levels of dynamic visual acuity required for driving safely.Visual fieldThe visual field is the extent of the area that a person can see with their eyes held in a fixedposition, usually measured in degrees. The normal binocular (using both eyes) visual field is 135degrees vertically and 180 degrees horizontally from the fixed point.The visual field can be divided into central and peripheral portions. Central vision refers tovision within 30 degrees of the point of fixation or gaze. The macula, a small area in the centreof the retina, is responsible for fine, sharp, straight-ahead central vision. Peripheral vision allowsfor the detection of objects and movement outside the scope of central vision.252