|
|
|
When assessing the suitability of any candidate for a visually demanding profession, it is essential that relevant clinical tests are performed. For example, assessment of vision using a standard letter chart is unlikely to predict whether or not a candidate will have difficulties driving at night. The standard Snellen letter chart is the sole method of assessment used by many professionals in the fields of ophthalmology and optometry. Unfortunately it is not particularly sensitive to the main visual side effects that can be induced by refractive surgery.
Visual performance is dependent on a number of factors:
In turn, the quality of the retinal image is dependent on:
Refractive surgery procedures are designed to minimise refractive error but because they achieve this by modifying the cornea or lens in some way, they also tend to alter the optical quality of these structures to some degree. In the majority of post-surgery patients, these changes are clinically insignificant, resulting in no apparent loss of visual performance. A minority of patients may have compromised vision and yet may be unaware of it because they rarely find themselves in visually demanding environments. Others report poor quality vision, particularly under low illumination, despite achieving good visual acuity on the Snellen letter chart. This reduction in visual performance has been attributed to an increase in forward scattered light within the eye and increased aberrations (optical imperfections).
Any reduction in the clarity of the cornea or lens (e.g. corneal haze following photorefractive keratectomy or the development of age-related cataract) will result in a higher proportion of incident light being scattered within the eye. This stray light is superimposed over the retinal image reducing its contrast. A reduction in the contrast of a high contrast object such as a letter on a Snellen chart, will have limited impact – it will still be visible, just slightly fainter. Reducing the contrast of a low contrast object such as those found in "real-world" scenes, is likely to result in the image contrast falling below the threshold for discrimination, i.e. the object will no longer be visible. Scattered light can cause disability glare (image degradation) in all individuals in the presence of significant glare source such as car headlights, but those who have raised levels of intraocular light scatter suffer reduced vision even when no bright light source is present, due to light scattering from one part of the retinal image to another.
The optical quality of the cornea and lens also influence the retinal image. Deviations from the perfect optical system are referred to as aberrations and are found in all eyes to some degree. Refractive errors such as myopia and hyperopia are "first-order" aberrations that can be corrected with lenses, but higher-order aberrations that cannot be corrected with spectacle lenses, also exist. The most significant higher-order aberration is spherical aberration. The cornea is designed to flatten towards the periphery to optimise the optical quality of the eye. An eye with positive spherical aberration has less flattening in the periphery compared to the central cornea and suffers from a reduction in retinal image quality because peripheral light rays are focussed in front of the retina and therefore do not form a sharply focussed image on the retina. As with scattered light, the effect is to reduce the contrast of the retinal image. Most normal eyes have some spherical aberration, which can be either positive or negative depending on the shape of their cornea. Most refractive surgery techniques that involve treatment of the cornea cause some increase in spherical aberration due to the altered corneal profile.
Night vision in the normal population is poor compared to vision under good illumination for a number of reasons. Firstly the dark-adapted retina relies on the rod receptors, which have poor resolution compared to cone receptors and are more sensitive to scattered light within the eye. Secondly, as the pupil dilates, aberrations from the more peripheral parts of the cornea increase by up to 30 times, resulting in reduced image contrast (Oshika et al., 1999a). The peripheral cornea is also known to scatter more light than the central cornea so pupil dilation also increases the stray light within the eye (Edgar et al., 1995). Thirdly, the contrast of an object against the background tends to be much lower at night and so any reduction in image contrast as a result of scatter or aberrations is more likely to render the object invisible. Many studies in the literature in the past, reported a high incidence of night vision problems such as halos, starbursts and poor quality vision, following laser surgery (O'Brart et al., 1994a; Dello Russo, 1993). These problems were associated with high levels of haze causing scattered light, and treatment zones significantly smaller than the average pupil leading to extreme aberrations. Nowadays, haze is much less severe and generally only present for the first 2-3 months post-PRK because high myopes are not treated with PRK. LASIK causes little or no haze in the majority of cases and ablation zone diameters for both PRK and LASIK have increased from around 4 or 5mm up to 6 or 6.5mm, making them larger or the same size as the average pupil under low illumination.
Methods of assessing visual performance
The drawback of high contrast letter charts is their insensitivity to increased scatter and aberrations, a critical factor when assessing post refractive surgery patients. Reducing the contrast of a high contrast letter does not significantly alter its visibility. The Snellen letter chart is the most commonly used chart for assessing vision despite having some significant drawbacks (figure 13).
The level of vision is defined in terms of the familiar Snellen fraction, e.g. 6/12 - letters on the 12-metre line can be resolved at 6 metres (each limb subtends 2 minutes of arc). Average vision is 6/6 - letters on the 6-metre line can be resolved at 6 metres when each limb subtends 1 minute of arc. Disadvantages of the Snellen chart include a non-geometrical progression between letter sizes so that a loss of 2 lines of corrected vision (considered significant following refractive surgery) is much more significant if the change is from 6/6 to 6/12 than from 6/4 to 6/6. In addition, some charts have a lowest line of 6/6 while others continue down to much smaller 6/4 letters. Using a 6/6 chart to record the vision of an individual who can resolve a 6/4 letter will be misleading and could mean that a post-surgical reduction in vision goes unrecorded.
The log MAR chart (log Minimum Angle of Resolution) is an improvement on the Snellen chart because it has equal numbers of letters on each line, a geometrical progression of 1.25x between letter sizes on each row and a more accurate scoring system (figure 14). It is still insensitive to scatter and aberrations in its high contrast format, but is also available as a low contrast chart, improving its sensitivity (figure 15).
Fig. 14: High contrast log MAR chart Fig. 15: Low contrast log MAR chart
The contrast sensitivity function of the eye (figure 16) relates to the smallest target that can be resolved over a range of contrasts. Not surprisingly, it is easier to resolve small objects when they are of high contrast.
Contrast sensitivity tests range from time-consuming computer-based tests capable of plotting the function over a range of spatial frequencies (target sizes), to quick chart-based tests that sample a single point on the function, e.g. the Pelli-Robson chart. Although such tests provide significantly more information that the Snellen letter chart alone, the tests differ in their conditions and criteria leading to poor consistency between tests. One of the reasons for this is that most tests are used under bright illumination when the pupil is small and the influence of scatter and aberrations on vision is minimal.
One test that is quick and simple to use is the Pelli-Robson chart. This consists of letters of a constant size that gradually decrease in contrast as the subject reads down the chart. The chart attempts to sample the peak of the contrast sensitivity function and combined with data from both low and high contrast letter charts, the clinician can gain a rough idea of the contrast sensitivity function, (figure 17).
Figure 16: Contrast sensitivity function of a normal eye In figure 17, the subject with increased scatter and /or aberrations has compromised visual performance due to a depressed contrast sensitivity function. However, high contrast acuity is normal (line meets x-axis at the same point). This abnormality would only be detected if low contrast acuity or contrast sensitivity were assessed.
Figure 17: Using low and high contrast letter charts and the Pelli-Robson chart to estimate the contrast sensitivity function
The consequence of a glare source for any eye is to increase intraocular light scatter and hence reduce contrast sensitivity. Some studies have shown a reduction in visual performance in the presence of glare but glare testing has its limitations - the addition of a point glare source to mimic the presence of car headlights within the field of view, has the effect of reducing pupil size and consequently masking some of the reduction in visual performance due to corneal aberrations, (Boxer-Wachler B.S. et al., 1999). The effect of glare on visual function is increased if the pupils remain dilated due to careful choice of the glare source or viewing conditions.
|
Figure
13: The Snellen letter chart
