Cataracts and driving - Part 2

The DVLA uses a limited test 'battery' which consists of visual acuity and visual field examinations.
As discussed in the first article of this series (optician, January 14, 2005) the effects of cataract are not best examined using a Snellen chart due to its poor scoring method and inability to explain or quantify the level of impairment experienced by the driver over various contrast levels. Nor does this test reflect the dynamic environment the driver is naturally in. Another point to note is that the actual requirement for vision is the ability to read a numberplate at 20.5m in good light, instead of the Snellen chart which is often requested by the DVLA.
The results of these two tests have been shown to have poor correlation. The visual field assessment as performed on the binocular Esterman grid is only able to reflect the intraocular scatter of light through recordings of low threshold values and possible scotomata or visual field constriction in cases of mature cataracts.
An important consideration, as well as the extent of visual field, is the extent of visual field the driver is able to concentrate over which is ignored by the current system and has been shown to play an important role in road traffic accidents at traffic junctions. The DVLA, therefore, needs to produce a test battery that is able to describe accurately a patient's visual status, such that a fair decision can be made on the driver's ability to continue driving. Such a test battery could consist of dynamic visual acuity using a contrast sensitivity target, useful field of view (UFOV), disability glare testing, contrast sensitivity, visual field and stereo-acuity measures. These tests would be able to accurately assess a patient's ability to drive safely in various road and lighting conditions.
The reliability, validity and discriminative ability of the test battery described depend upon each individual test, so these parameters must be considered when selecting each constituent of this test battery. After careful analysis of various research papers it may be possible to suggest a method or piece of equipment for assessing each parameter of functional vision (Table 1).

testing the limits
Tests such as driving simulators, dynamic acuity using a target with single contrast letters and static VA were not included in the test battery for various reasons.
The driving simulator to date has been unsuccessful in discriminating patients with a high probability of being involved in a future road traffic accident, despite it being able to detect drivers that have previously been involved in accidents. Dynamic acuity measurements made using letters of a fixed contrast (for example, the Snellen chart) have been found to be inaccurate as well as having poor discriminative ability and repeatability, as cited by Charman.1
An important consideration to make at this point with regards to visual function testing is that the test battery should not be excessively rigorous. The test battery discussed previously has been selected with this in mind, without compromising the safety of other road users and pedestrians.
Poor attention has been associated with an increased risk of elderly drivers being involved in a car crash at traffic junctions. This situation has been attributed to a constricted UFOV.
This test, therefore, has been included in the test battery to identify drivers with UFOV constriction with the intent of helping these drivers to enlarge their UFOV and thus make them safer on the road instead of acting as a test to prevent someone from driving.
Other tests like contrast sensitivity, visual fields and disability glare have been selected in order to identify drivers with an impairment that could seriously affect their safety on the road under certain driving conditions.

driving and Age
A study completed by Owsley2 found that elderly drivers are twice as likely to crash per 100,000 miles driven when compared to younger drivers.
When elderly drivers are involved in crashes they are more likely to be killed as they are frail compared to younger drivers. In the US, these avoidable fatalities attributed to an economic cost of $31 million according to Ship.3 Of this sum, Blincoe and Faigin4 determined that 80 per cent of this cost is due to lost workplace and house-hold productivity. This estimate does not include fatalities of pedestrians, cyclists, drivers and passengers of other cars. Therefore, the economic cost is far greater. By considering the potential saving of preventing fatalities associated with car accidents it is possible to justify the expense to prevent such fatalities.
The DVLA believes the cost of developing new test centres is unjustified. Studies by Burg,5 Hills & Burg6 and Ball et al 7 showed a weak correlation between poor static visual acuity and increased crash rates, which supported the DVLA's claims. However, after reviewing various parameters of visual function, such as contrast sensitivity, the risk of crashing due to defective vision is higher. Studies (eg Owsley et al8) have shown that drivers with a contrast sensitivity deficit worse in one eye were three times more likely to be involved in a crash.
Babizhayev9 also found elderly drivers with cataract were unable to drive safely at night because their contrast sensitivity and response to glare was poor. The examples of visual function testing apparatus described earlier have been shown to be more representative of a driver's visual status - one would expect these tests to show an elevated crash risk to drivers that have performed poorly on the test battery and thus show a more conclusive relationship between defective vision and a significant risk of crashing.
The DVLA to date has been apprehensive in implementing the construction of adequate test centres. However, a possible solution to this problem would be to franchise the right to perform testing on the DVLA's behalf to independent companies. This would help to reduce the cost incurred by the DVLA and such an arrangement enables the DVLA to avoid costs of setting up and purchasing test centres across the UK, as well as allowing the DVLA to share the cost of employing professional optometrists and supporting staff. The independent company benefits from receiving more patients to whom it could possibly dispense spectacles and contact lenses. An added benefit is that the costs of updating testing equipment could be shared or covered by the DVLA according to the agreed contract.

Pulfrich's phenomenon
Pulfrich's phenomenon can be demonstrated in 66 per cent of unilateral cataract patients This phenomenon has been described as the apparent elliptical motion of an object/pendulum moving side to side and is thought to arise from the delay of visual information from one eye.
The practical implications of such an effect on driving have been studied by Enright10 and Scotcher.11 However, a scenario that could result in a fatality or hospitalisation of a pedestrian or cyclist to date has been overlooked in studies investigating the effects of Pulfrich's phenomenon on driving.
When a pedestrian, for example a child, runs on to a road, their path across the road is straight as shown by the dashed line in Figure 1. A driver with a uniocular cataract perceives the child to be following an arcuate path (as shown by the grey arc). This leads to the driver under-estimating or over-estimating the distance to the child. A study completed by Scotcher et al (1997) found that a neutral density filter placed in front of a subject's left eye produced an anti-clockwise movement of an oscillating pendulum and when the filter was placed over the subject's right eye the pendulum moved clockwise. As the strength of the filter was increased the perceived depth of motion also increased.
An interesting finding by Scotcher et al was that patients with an acuity of 0.48 did not appreciate Pulfrich's phenomenon but those with an acuity of 0.30 did. Therefore, for this phenomenon to occur there needs to be a delay of visual information from one eye, rather than the effective suppression of one image.
A mild cataract can be considered to have the same effect as a neutral density filter as it causes a delay of visual information from one eye. This is achieved through backwards light scatter and possible absorption by a pigmented lens. Therefore, it will produce the same effect as those noted in the Scotcher et al study. However, if the cataract causes a reduction of acuity to 0.48 or worse, the depth of motion of Pulfrich's phenomenon will cease to occur and the phenomenon will no longer be appreciated.
The study failed to identify the types of cataract that produce this effect. The subjects were instructed to comment on the motion of a target. Therefore, it is possible to assume that the subjects maintained foveal fixation when they perceived Pulfrich's phenomenon. Hence, a delay of visual information to the fovea can be considered more important for the perception of this phenomenon.
The majority of visual information that reaches the fovea comes from the nodal point of the lens. This means that the following mild cataracts could result in the Pulfrich effect being perceived:

 Posterior subcapsular cataract
 Anterior subcapsular cataract
 Nuclear cataract
 Cortical cataract
 Lamellar cataract.

These cataracts are more likely to affect the passage of light through the nodal point, which results in a delay of visual information as retinal processing takes longer in one eye due to the poor image quality.
Other forms of cataract, for example a mild coronary cataract, are unlikely to produce the same effect, as pupil constriction can significantly reduce light scatter, thus effectively eliminating the delay.
If the driver has a unilateral cataract and perceives Pulfrich's phenomenon they could be driving down a street and a child could run out straight into the road. If the driver has a cataract in their left eye the child will appear to follow an anti-clockwise or arcuate path away from the driver as shown in Figure 1.
By looking at Figure 1 it is possible to note that the driver's right eye sees the child at point X and his left eye sees the child at point Y. The disparity between the images is interpreted as depth, therefore the patient is perceived as being further away at point Z. Hence, the driver will over-estimate the child's distance from the car and may collide with the child. The effects of the collision on the child will depend on the driver's speed as they may be unable to react in time due to the depth of the Pulfrich's effect.
The depth of motion perceived by the driver depends on the cataract's density, as a denser cataract will produce a greater depth of motion. This means the driver will over-estimate the distance of the child to a greater extent than a driver with a milder cataract. Therefore, the speed on impact will be greater as the driver would have started braking later, than if the cataract was milder and the subsequent depth of motion was less.
An important consideration is the magnitude of this effect as there are approximately 2 million people aged over 70 with a driving licence13 in the UK and according to Gibson et al14 46 per cent of them (aged 60) will have a cataract. Also, two-thirds of them will receive a unilateral cataract extraction, according to Scotcher et al12 and 66 per cent of them will suffer from Pulfrich's phenomenon.
Hence, the magnitude and possible consequences could be significant. There have been studies to show that the presence of a cataract is associated with an elevated risk of crash involvement. Owsley found that a bilateral cataract made a driver six times more likely to crash and the presence of a uniocular cataract resulted in a risk factor three times that of a normal driver.
The population of the UK is ageing and the number of elderly drivers is increasing. It is estimated the number of people aged over 70 with a driving licence will increase to 4.5 million by 2015, according to Noble. As a result of this increase, the number of injuries and fatalities in road traffic accidents will also increase. The number of accidents, therefore, resulting from impairment caused by unilateral cataracts will become more significant, which may require government intervention and the primary mode of action may be to implement the routine practice of simultaneous bilateral cataract surgery.

bilateral removal
The answer to why we do not perform simultaneous bilateral cataract surgery in the UK may lie in the fact that ophthalmologists fear patients may develop bilateral endopthalmitis. There has been research performed on this subject and the risk of infection associated with such a procedure has been found to range from 0 per cent to 3.3 per cent by various researchers. Our argument for the routine removal of both cataracts on patients simultaneously stems from the disastrous consequences Pulfrich's phenomenon could have on drivers with a unilateral cataract.
Another concern is the possibility for acquired central fusion disruption to develop in patients with mature cataracts having undergone first eye cataract surgery and then going through a long period of sensory deprivation to receive treatment for the second cataract.
This condition results in the patients developing an intractable diplopia which renders them unsuitable for driving.
An interesting statistic cited by Talbot and Perkins16 was that 52 per cent of patients following a unilateral cataract operation were found to be unable to fulfil the DVLA's driving requirements. This would no doubt significantly affect an elderly patient's mobility and hinder such a patient's independence and quality of life. An expense associated with such an impairment by the local authority would be the cost of employing a minibus driver and maintenance of this facility to enable elderly patients to go shopping.
A study completed by Wang and Carr17 noted an increase in depressive symptoms following a patient's licence being revoked. However, a lot of these costs and negative effects on patients' lives can be minimised with the introduction of simultaneous bilateral cataract surgery.
A study conducted by Talbot and Perkins18 found 86 per cent of drivers to be able to continue driving after their second cataract was removed. Studies have also shown that, in the majority of cases, binocular summation is regained and stereoacuity, psychosocial health status, binocular visual acuity and contrast sensitivity improve following the removal of the remaining cataract.
Castell et al19 also found the effects of glare to improve from 41 per cent to 81 per cent in patients having undergone removal of their second cataract. This means drivers will be able to report a significant improvement in their quality of vision and feel more confident driving at night.
To give patients the best quality of vision it would be advisable for ophthalmologists to implant the Technis IOL in to their patients as it has been shown to offer the best recovery of contrast sensitivity.

Driving simulators
The National Advanced Driving Simulator (NADS) is one of the most sophisticated research driving simulators in the world.
It was developed by the National Highway Traffic Safety Administration and offers high-fidelity and real-time driving simulation. The simulator consists of a large dome where images of cars and the cabins of trucks as well as buses are projected. The cabins can be electronically and mechanically coupled to the driving simulator using special instruments which are specifically designed for a particular make and model of vehicle.
A motion system provides 400sq m of horizontal and longitudinal travel as well as 360 of rotation in either direction (The National Advanced Driving Simulator). This enables the driver to feel acceleration, braking and steering cues like those experienced while actually driving a real car, truck or bus.
Driving simulators have been used in various studies and have been found to produce results which correlate with those obtained under real road conditions.
A retrospective study completed by Lee et al21 found that individuals with an increased risk of crashing could be identified using the PC-based interactive driving simulator. This study identified the need for further testing to determine whether the driving simulator can predict future crash events therefore, the expense of such testing facilities would go unjustified until their validity, discriminative ability and reliability has been proven.

References
References are available on request from optician's clinical editor. Contact: william.harvey@rbi.co.uk

 Jaspal Punia is a pre-registration
optometrist working in Felixstowe, Suffolk