A practical guide to...
Vision fitness to drive
Tim Carter, chief medical adviser
to the Department for Transport, outlines the most common primary care eye problems that can impair vision and fitness to drive
When driving, people use foveal (central)
vision as a directed gaze for discrimination of detail, and their outer peripheral vision acts mainly to alert them to their surroundings. The driver identifies potential problems either at a distance, when they are in central vision, or because of peripheral alerting signals. The brain has to prioritise the use of gaze, and many of the skills of reading the road relate to the best use of different parts of the visual field and the correct interpretation and anticipation of events. Innate and learned cognitive processes also play a part, and errors in them are also significant contributors to road accident risk.
Several aspects of eye function are relevant:
• visual acuity (distance and near) with or without correction
• field of vision, including discrete areas of loss of vision (scotomas)
• low-light vision and dark adaptation
• contrast sensitivity
• susceptibility to glare.
Many of the more common causes of visual impairment are associated with ageing, and most lead to stable or slowly progressive functional limitations. More than one visually-impairing condition is often found in the same person. The speed and quality of
information processing also reduce with age. Consequently, the overall ability of
older drivers to see and respond to visual information may be reduced in ways that are complex and hard to predict.
In conditions such as glaucoma, macular degeneration and cataract, several aspects of vision can be affected. Refractive defects, the most common impairment, are associated with loss of visual acuity, but with increasing age both corrected and uncorrected defects may be further impaired by the early stages of other conditions. While visual acuity and fields of vision can readily be tested, it is the subjective recognition of other problems by the driver, (such as glare or poor dark adaptation during night driving) that is the norm.
A number of conditions cause reductions in visual acuity, whereas others predominantly cause a reduction in the fields of
vision. Some present with a mixed picture.
Refractive errors and reduced visual acuity
Visual acuity is most commonly reduced by errors in the refractive functions of the eyes. Reduced visual acuity is also a predominant feature of cataracts, macular degeneration and corneal disease. Diabetic eye disease may present with both refractive errors and field of vision defects.
Near vision is needed for reading maps and in-car instruments, but it is the quality of distant vision that is crucial to driving performance. Distance vision is normally tested in terms of central performance,
under good lighting conditions and using stationary symbols. The ability to detect movement and other visual clues anywhere in the visual fields, to respond to them by co-ordinating eye movements and to analyse their significance is essential for safe
• Most visual defects are reduced by the
assessment of refractive error and the use
of corrective glasses or contact lenses.
• Corrective lenses for large refractive
errors can narrow the field of view, as can thick frames.
• Refractive surgery to the cornea can provide improvements in visual acuity without the need for external visual correction, but increased glare sensitivity and a reduction in low-light contrast sensitivity are not uncommon after treatment.
• The frequency of refractive errors increases with age, as hardening of the lens leads to limited accommodation and presbyopia. Several studies show that unrecognised
visual acuity defects, mainly loss of distance vision, are the most common abnormality found when screening drivers over 50 years of age for health-related impairments to
Most cataracts that interfere with visual function are in older drivers. The progressive opacity of the human lens is usually bilateral. The diffuse scatter of light from the opaque lens reduces acuity and contrast sensitivity; it also increases the susceptibility of the eye to glare from bright lights. All these have the potential to reduce driving performance, but glare at night or from the low sun causing temporary impairment to vision are particularly common problems in the early stages of cataract.
• Surgery provides good restoration of function, but people with early-stage cataracts will have impaired visual function as drivers prior to surgery.
• Five-year follow-up confirms the maintenance of improved visual function.
Age-related macular degeneration selectively impairs central vision and may cause total loss of central vision. Population studies on crashes are equivocal, probably because of other concomitant age-related impairments and because such loss of vision will be apparent to the sufferer and lead to modification or cessation of driving. Macular degeneration is progressive and often not amenable to treatment.
Corneal damage, from any cause, can lead to a distorted or clouded image and increased sensitivity to glare.
Diabetic eye disease
For driving, the usual problem is reduction in acuity either from diabetic retinopathy or cataract. The pace of progression of retinopathy may depend on the underlying disease and the quality of control. Progression may be delayed by laser therapy to the retina, which if given early can enable driving to continue. Progression is variable, with or without laser therapy. Laser therapy can itself lead to scotomas and to reduction in the field of vision; however, the newer laser techniques are far less damaging than those previously used. Old treatments
involving extensive pan-retinal photocoagulation often restricted the field
Reduction in the field of vision is the effect of glaucoma that is most likely to impair driving. The pattern is inconsistent but generally results in a reduction in mid-peripheral vision, sometimes associated with loss of areas in the central fields. As with cataract, the condition is usually found in older drivers likely to have other visual or general impairments. There are particular difficulties in deciding the significance of measured visual field loss in an individual. As the condition is irreversible and damage is progressive, any impairment will almost certainly worsen over time.
This genetic condition impairs vision, both by constricting the peripheral field of vision and by the loss of the low-light receptors or rods in the retina, leading to night blindness, susceptibility to glare and sometimes to major defects in peripheral and central
Loss of parts of the field of vision can also arise from conditions of the eyes, nerves and brain, such as:
• congenital ocular anomalies
• retinal detachment
• birth trauma to the brain or visual pathways
• stroke, the most common cause of hemianopia
• intracranial tumours, especially of the
• after cranial surgery.
The prognosis will depend on the underlying cause, as will the extent of recovery after sudden loss. This may be complicated by associated cognitive effects from causes such as stroke. The effects on driving performance can be expected to be variable.
Three-dimensional awareness of the surroundings is important for near objects but provides little visual information about the position of distant objects, where the brain relies on other clues, such as parallax, to judge distance, motion and speed. Monocular vision and lack of stereoscopic vision are not uncommon and there is no indication of reduced driving performance in people with only one working eye. However, as there is no second eye to compensate for any visual defect in the working eye, the working eye has to be fully functional and free from impairment.
Diplopia, nystagmus and blepharospasm
All these conditions may impair visual performance by creating difficulties with the perception of the retinal image. The key consideration on diplopia is whether the second image interferes with perception of space, distance and speed. If it is a new phenomenon, the driver may need a period of adaptation before their long-term visual performance can be assessed. Functional defects may be reduced by surgical correction or by patching the less good eye.
Nystagmus is frequently associated with reduced vision from other causes, and the overall capabilities of function will determine any risk. Severe blepharospasm can inhibit vision, and it may be erratic and associated with tension or fatigue. The level of impairment may be reduced by the use of botulinum toxin to paralyse the muscles in spasm.
Clinical assessment and advice
Assessment of visual function is the main
indicator of current legal suitability for
driving. Any other defects in visual–spatial and cognitive processes also need to be considered and the underlying pathology will
determine the likely prognosis.
A person's subjective perception of limitations in function can be very important, and may lead many drivers to alter their pattern of road use. However, defects in the fields of vision, except where there is rapid onset, are not usually perceived, as the brain has mechanisms for filling in the gaps in visual
images using recent memory, rather than incoming visual information.
Because of features such as glare associated with cataract or the marked loss of acuity and central distortion associated with macular degeneration, these conditions often lead to reported symptoms. In contrast, glaucoma, with its mid-peripheral field defects, often does not. As diabetic retinopathy is a complication that requires regular surveillance, it is normally screened for as a routine part of disease management.
Acute visual defects from problems such as retinal detachment, acute glaucoma or corneal ulcer may present in primary care. Where these indicate a potential emergency or a need for referral to an ophthalmologist, the individual should be immediately advised not to drive. Less urgent, and in particular refractive, problems will be referred to an optometrist for investigation and treatment. If the standards for visual acuity (number plate test or Snellen equivalent) are not met, the individual should also be advised not to drive until vision is corrected to comply with the DVLA standards.
The DVLA lists criteria for:
• visual acuity (based on number plate reading at 20.5 m for all drivers, supplemented by formal visual acuity testing and more stringent standards for Group 2)
• cataract (visual acuity and glare)
• monocular vision (Group 2 only)
• visual field defects
• diplopia (Group 2 only)
• night blindness
Drivers should be informed of their duty to report their condition to the DVLA if they do not meet the criteria or if assessment is required. When undertaking a Group 2 driver medical, the examining doctor is asked to provide information about these conditions.
As visual defects are common, increase in frequency with age and are often treatable, screening is justified. Visual defects should be considered in patients who have another illness in which they are a known complication, such as stroke and diabetes.
Tim Carter is the chief medical adviser for transport safety at the Department for Transport
This article is adapted from Fitness to drive: a guide for health professionals (chapter 8) by Tim Carter, ISBN 1-85315-651-5, published by the Royal Society of Medicine Press 2006, priced £19.95. Copies of the book can be ordered online at www.rsmpress.co.uk or by contacting Marston Book Services on 01235 465500
advice to drivers
Advice to drivers will depend on reported symptoms and clinical assessment, but the main aspects are:
• Avoid night driving and driving when tired
• Take a passenger to navigate
• Use correct glasses or contact lenses if needed; have a spare pair in case of loss or damage
• Use sunglasses to reduce glare; be aware of the dangers of use in low or erratic light (window
tinting and visors can also reduce glare)
• Be aware of the effect of medications (eye drops and systemic drugs) on visual performance
• Take time to adapt to any visual changes after eye surgery and acute episodes of visual disturbance (confirmation is still needed that visual standards are met)
• Failure to meet the number plate reading test is an immediate legal bar to driving until visual acuity is restored