Cataracts and driving - Part 2

After reviewing various aspects of visual function and its assessment for elderly drivers with cataract it can be concluded that no single test is able to reliably assess a driver's road safety. optician/City University prize winner Jaspal Punia argues that the DVLA should employ a 'battery' of vision tests if driving vision is to be adequately assessed

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).