Cataract and driving: Part 1

The DVLA states that a driver's vision is deemed satisfactory for driving when he or she is able to read the characters on a number plate from a distance of 20.5m. This rudimentary form of visual analysis has been found to be unsatisfactory by various research papers which will be discussed later.
Although it provides a quick and easy method of assessing vision it also disadvantages some drivers, due to a lack of uniformity. For example, if a driver with cataract has his vision assessed on a clear day he may be able to read a number plate at the specified distance. However, if the test is performed on a different day and the weather is not as clear he may fail the same test. This article aims to develop a more systematic approach to vision assessment for drivers.
Drivers with a unilateral cataract can suffer from distorted motion perception which could have serious consequences if they are allowed to continue driving. These drivers have been identified as having a greater risk of crashing. Therefore I also aim to justify simultaneous bilateral cataract surgery in patients without any co-pathology as this will prevent elderly drivers suffering from various phenomena as a result of sensory deprivation to one eye.

Measurement of visual acuity
In 1935, government established a standard visual requirement to drive and this formed part of the Road Traffic Act. The current legal requirements for vision/visual acuity are outlined in the following statement by the Driver and Vehicle Licensing Agency (DVLA).
'A licence holder or applicant is suffering a prescribed disability if unable to meet the eyesight requirements, ie to read in good light (with the aid of glasses or contact lenses if worn) a registration mark fixed to a motor vehicle and containing letters and figures 79mm high and 57mm wide (ie pre-September 2001 font) at a distance of 20.5m, or 79mm high and 50mm wide at a distance of 20m (ie post-September 2001 font). If unable to meet this standard, the driver must not drive and the licence must be refused or revoked.'
In order to aid optometric investigations into a patient's ability to drive, Charman1 calculated that the angular subtense of a number plate symbol is approximately 13.3 min of arc, which requires a binocular visual acuity of 6/15. However, Drasdo and Haggarty2,3 determined, using mathematical models, that a visual standard of 6/9-2 (6/10) would yield results consistent with the number plate test. The Royal College of Ophthalmologists recommended that the DVLA impose this as the visual standard (6/10) for driving as studies4 indicate that approximately 85.7 per cent of patients who cannot read number plates at 20.5m, have a static visual acuity of 6/12.
These findings are somewhat surprising as one would expect all drivers with 6/12 vision or better to be able to pass the number plate test after taking into account Charman's calculation. However, this is not the case as only 92.3 per cent of subjects with visual acuities of 6/12 or better were found to pass this test. This can be attributed to various confounding factors including illumination, fonts, cleanliness of number plates and the legibility of the characters used, which can all affect contrast and/or visibility. One would expect the choice of yellow or white backgrounds used on the number plates to have a significant effect on a driver's ability to read them, but this was found not to be the case.
The current trend within the UK is to harmonise with the rules and regulations of Europe. The EC states that a driver's vision/visual acuity must be 0.5 (6/12), which is lower than the UK equivalent of 0.6 (6/10). European standards require monocular drivers to score 0.6 (6/10) on a Snellen chart. In the UK there are no such discriminations made on the basis of vision/visual acuity for monocular and binocular drivers.
The general consensus expressed by the Association of Optometrists (AOP) towards the current visual assessment used for driving tests across the country is a negative one. Arguments include that the number plate test has poor repeatability, reproducibility and is subject to confounding factors, as previously discussed. In the opinion of some examiners the number plate test may be considered a less useful test.
Such assumptions are commonly based on notions that the Snellen test is more predictive of driving safety. However, one may consider the number plate test to simulate driving tasks more effectively and to represent accurately the complexity of demands on the visual system in these driving conditions. It is possible to criticise the Snellen test because it is performed indoors and is less relevant to the tasks performed by a driver. The advantage of the number plate test is that it allows the police and the driver to check their vision.
An interesting study completed by McCaghrey5 showed that 58.8 per cent of drivers had no knowledge of this requirement or were unable to quote it correctly. This study also showed that only 7 per cent of subjects had performed the number plate test to check their vision. Currently, information regarding the number plate test is more accessible since the release of the revised Highway Code which stresses the importance of this test and it is further reinforced by the driving theory test, so one would expect these figures to have improved. A study conducted by North6 showed that 16 per cent of UK drivers were unable to pass the number plate test. An important question to consider at this point is whether the responsibility of assessing vision should lie with drivers who have been poor at adhering to the requirements to date or qualified healthcare professionals such as optometrists. Support for the latter has been aired since 1953 by Duke Elder7 and further supported by the AOP.
At present, the DVLA believes there is no conclusive evidence to suggest that defective vision is a significant cause of road traffic accidents. Studies have shown a weak correlation between poor static visual acuity and increased crash rates, which supports the DVLA's claims.8-10 The AOP strongly advises that routine visual assessments should be enforced by the DVLA to ensure that drivers meet current regulations. However, due to the DVLA's belief of inconclusive evidence on this matter, it is unable to justify the expense of such testing facilities. The view of many insurance firms is similar to that of the AOP in that a regular eye examination should be mandatory for drivers.

Monocular vision
Monocular drivers are unable to appreciate stereopsis and hence rely on monocular cues. In conditions of low illumination (such as driving through a tunnel or in the dark) driving becomes difficult because monocular cues diminish.
Both UK and EC regulations state that a monocular driver needs a normal monocular visual field in the remaining eye which can be up to 160 degrees along the horizontal meridian. Controversy has surrounded arguments suggesting that a monocular field greater than 160 degrees is necessary for safe driving. A study completed by Owsley and McGwin11 showed that head and neck muscle movements allow sufficient compensation for a field less than 160 degrees.
With regards to safe driving, a study by Wood and Troutbeck12 found that monocular vision had little impact on driving performance. However, Liesmaa13 found that roadside vision checks performed by the police on dangerous drivers showed that monocular drivers posed a risk three times greater than the study's control group. This may suggest that once monocular drivers are out of a test situation they may not perform the compensatory head and neck muscle movements that are required. The term 'dangerous driving', however, encompasses a whole host of causative factors that could be more significant (for example, alcohol).
To obtain a Group 2 licence (large goods vehicles, passenger carrying vehicles, medium goods and minibuses), a driver must be able to appreciate stereopsis. This effectively bars monocular drivers from obtaining or holding this licence. An interesting study conducted by North14 found stereopsis to be of little relevance while driving at high speeds on motorways. A possible explanation for this is that as objects move closer to us the ability to appreciate stereopsis becomes more important as monocular cues become more subtle. The seating position in HGVs also adds to this effect as the seat is closer to the vehicle in front and located higher than in other vehicles. This could lead to a situation in traffic where a monocular driver misjudges the distance of the vehicle in front as monocular cues begin to diminish, resulting in a collision. The opportunity for fatalities to occur in these situations is greater as vehicles included within this licensing category are likely to exert a greater force in high-speed impacts than other vehicles (as they are considerably larger).

Visual field assessments
The DVLA has stated that: 'The minimum visual field for safe driving is a field of vision at least 120 degrees on the horizontal measured by Goldmann perimetry using the III4e setting (or equivalent). In addition, there should be no significant field defect in the binocular field which encroaches within 20 degrees of fixation either above or below the horizontal meridian. By these means homonymous or bi-temporal defects which come close to fixation, whether hemianopic or quadrantanopic, are not accepted as safe for driving. Isolated scotomata represented in the binocular field near to the central fixation area may also be inconsistent with safe driving'.
The recommendations do not state guidelines regarding eye and head movements. Also, the criterion outlined does not include regulations regarding the use of spectacles for visual field tests.
It is necessary for the DVLA to consider the effects of spectacle corrections on field plots as they must be worn for driving. Studies show that 13 per cent of the driving population wear spectacles.15,16 Spectacles have been shown to cause visual field defects.17 Steel et al18 showed that a plastic frame with a 3mm-thick rim produced an absolute scotoma within 120x40 degrees of fixation in seven out of 10 subjects which could affect a driver's right to continue driving.
This study has highlighted a few interesting points. For example, a frame that is wider than a patient's pupil size will produce a scotoma in their visual field. Also, by reducing vertex distances the angular distance of a field defect from the line of sight will increase and it is possible for a defect to then fall just outside of the 120x40 degree area. Conversely, if a patient's spectacles are poorly adjusted they could slip down the nose and bring defects closer to fixation. Another factor to consider is the effect of bright light on pupils causing constriction and thus resulting in a driver having an inadequate field of view while driving in daylight.
Despite the obvious concern raised by the points discussed, Davison determined that the spectacle-wearing population is not faced with any greater risk of being involved in road traffic accidents than other road users.15
The DVLA has shown a preference for using an Esterman grid for examining binocular patients. The DVLA occasionally requests monocular field plots for patients suspected of suffering from a progressive disease as these programmes allow diagnostic perimetry values to be obtained which cannot be determined from Esterman field plots. When examination of monocular patients is necessary the Goldman perimeter and Humphrey's 24-2 programme are used for careful assessment of the central and peripheral visual field.
The visual field standard has been a recent addition by the DVLA to the list of requirements. Grundy19 discovered that drivers are able to make three fixations per second which means that a driver's central vision will be unable to scan an entire visual scene and compensate for field defects. A worrying statistic shows that 57.6 per cent of drivers with a significant visual field defect were unaware of it and that 13 per cent of drivers aged over 65 have abnormal fields, which is alarming, considering that the elderly represent a significant proportion of the UK's population.20
A study conducted by Wood and Troutbeck (1992) showed that peripheral awareness of pedestrians and vehicles is reduced by a visual field restriction of 40 degrees or less - especially when a driver is overloaded with visual information. A binocular field restriction was associated with drivers missing pavement markings in this study. A driver's ability to manoeuvre and reverse can also be significantly affected. Study subjects with field defects attempted to compensate for their deficit by increasing their driving time in order to negotiate challenging situations. This was shown to be associated with a reduction in the number of driving errors made.12 Therefore, elderly drivers may slow down traffic flow.
The ability to estimate speed is influenced by the same sensory deficit, according to studies.21,22 However, Groeger and Brown23 showed that different sensory modalities such as hearing are also used in the estimation of speed. Therefore drivers may be able to use the sound of an engine to act as a cue to estimate a vehicle's speed, which could help them to compensate for such a deficit.
Johnson and Keltner24 were able to combine studies of abnormal fields (due to glaucoma and other eye problems) and accident history from the previous three years to show that the accident and conviction rates of drivers with abnormal fields are double compared to normal drivers. The incidence of convictions and accidents increased in cases of abnormal fields in both eyes.
By considering Figures 1 and 2 it is possible to see a significant difference in conviction and accident rates between drivers with abnormal and normal binocular fields. Figures 3 and 4 show comparatively little differences between the conviction and accident rates between drivers with normal and abnormal monocular fields. It is also possible to assume from these graphs that accidents and convictions are subject to other variables, for example alcohol consumption and drug abuse as the control group has also been involved in traffic offences and accidents.
Cataracts cause light to scatter across the retina by altering the path of light entering the eye in a particular location, ie a coronary (supra-nuclear) cataract will scatter peripheral light rays. The sensitivity of the peripheral retina is less than the foveal region, so it is possible for an absolute scotoma to be recorded in this region. This could cause the effective constriction of the visual field in certain cases of dense coronary cataract. Therefore, some drivers with cataracts may be regarded as having an abnormal visual field.
Hence, prosecutors involved in legal proceedings for traffic violations where extenuating circumstances exist, ie the driver was unaware of their defective vision, may be able to secure a conviction against the offender. A conviction can be secured as the responsibility of checking vision lies with the driver. Therefore the driver may receive a greater penalty and/or be barred from driving. Under these circumstances it may be in the public's interest for a mandatory visual assessment to be imposed.

Diplopia
The DVLA states that a driver suffering from diplopia must cease driving on diagnosis. Driving can only be resumed on confirmation to the licensing authority that the diplopia is controlled by glasses or by a patch which the licence holder undertakes to wear while driving. (If patching, note requirements above for monocularity). Exceptionally a stable uncorrected diplopia of six months' duration or more may be compatible with driving if there is consultant support. A study conducted by Ellingham et al25 showed that 31 per cent of patients suffered from a momentary loss of vision in their uncovered eye when a patch was placed over their non-dominant eye. However, this statistic rose to 47 per cent when the patient's dominant eye was patched. As a result of this study, eye care specialists have been urged to discourage patients from driving even if they are able to pass this requirement. Another consideration is the fact that when a patient wears an eye patch they are considered monocular and are accordingly barred from holding a Group 2 licence. Hence a patient whose occupation requires them to drive an HGV must be advised to stop driving an HGV.

Colour Vision
This type of deficiency poses no restrictions on licences held by Group 1 and Group 2 drivers; all that is required by the DVLA is notification. In the past, concerns have arisen regarding possible links between defective colour vision and driving performance, especially in relation to red, green and amber traffic light signals. However, no statistical evidence has been produced to substantiate these concerns. The general advice given to these drivers is to avoid the use of sunglasses while driving.

Blepharospasm
It is possible to describe this as an uncontrollable blinking which usually presents itself bilaterally. In severe cases a driver can be considered unfit for driving. However, the DVLA may allow a driver to continue driving if the condition is mild, after considering a specialist's report. A patient being treated for a mild blepharospasm with botulinum toxin could possibly be allowed to continue driving, provided that the side effects (for example, an uncontrollable diplopia) are not present. The DVLA urges drivers to keep them informed of any changes or deterioration in their condition. If the driver's condition is severe and it is affecting their vision (even if it is treated) the driver's licence must be revoked or refused.

Effects of monocular cataract operations
Patients suffering from binocular cataracts benefit greatly from monocular cataract extraction. Castells et al26 were able to show that an improvement of 2.8 Snellen lines is demonstrable in patients following monocular cataract extraction. Despite the overall improvement in visual acuity following this procedure, there are various conditions that result from sensory deprivation due to the extraction of a single cataract.
The Pulfrich phenomenon can be demonstrated as the apparent elliptical motion of a swinging pendulum in the fronto-parallel plane under binocular conditions. This phenomenon can be induced with the aid of neutral density filters placed in front of one eye to produce an apparent elliptical motion. The phenomenon is thought to be due to a uniocular delay in retino-cortical transmission.27
A uniocular cataract has much the same effect as a neutral density filter; it delays the transmission of visual information by reducing retinal illuminance as well as reducing visual quality through absorption by a pigmented lens and backwards scatter. Two thirds of unilateral cataract patients are able to perceive Pulfrich's phenomenon.27 The median difference in visual acuity between each eye in patients who could not perceive the Pulfrich's phenomenon was 0.48 (on the log-MAR chart), whereas patients suffering from this phenomenon scored a visual acuity of 0.30. This seems to support the fact that the phenomenon requires a delay of visual information rather than the effective suppression of one piece of information. After second eye cataract surgery the Pulfrich phenomenon is eliminated. The effects of this phenomenon can extend to driving and could be associated with an increased risk of these patients being involved in road traffic accidents.
Acquired central fusion disruption has been known to result from a prolonged period (around 2.5 years) of sensory deprivation in unilateral cataract patients with a mature cataract. Following extraction of the cataract an intractable diplopia develops. The diplopia results from a loss of fusion and an inability to suppress one image. Patients suffering from this condition also complain of one image sliding off the other.
In patients with normal binocular function, where both eyes have equal sensitivity, the patient is said to have binocular summation. However, in patients with uniocular cataract, binocular inhibition is present because the sensitivity of both eyes differs. Also, the binocular sensitivity will be lower than the monocular sensitivity. The patient, in other words, will report an improvement in vision when the cataractous eye is closed. Taylor et al28 found that approximately 40 per cent of uniocular cataract patients showed binocular inhibition of visual acuity and contrast sensitivity. Therefore, patients with binocular inhibition may fail the number plate test despite having a good monocular acuity.
In Part 2, I will suggest a more appropriate assessment of vision as predicting the impact of cataract upon driving vision.

References
A list of references will be included in the second part of this article which will be published on February 18.

 Jaspal Punia is half way through his pre-registration year