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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5253_Библиотеки_им_академика_М_И_Перельмана.pdf
Figure 9.6 Mean (shown by the points) plus and minus one standard
deviation (shown by the line) for chromaticities selected in the Intuitive
Colorimeter by participants who had previously used coloured overlays.
Reproduced under STM Permissions Guidelines from Lightstone et al.7
The association is so weak that the colour of the optimal overlay
provides little guide to the colour of the optimal lens. This was shown
by Lightstone et al.7 who asked patients to read with the optimal
overlay, with lenses that matched the overlay in colour and with lenses
that matched the colour chosen in the colorimeter. The lenses that
matched the overlay in colour did not increase reading speed, whereas
the optimally coloured overlay (selected with overlays) and lenses that
matched the colorimeter setting both did so, despite the different
colours of the overlay and lenses, see Table 9.1. This confirms that it is
important to choose a filter of a colour that is appropriate for the
context of its use, be that overlay or lenses.
Vision, Reading Difficulties and Visual Stress
250
Table 9.1 Mean number of words correctly read per minute (standard
deviation below). Reproduced under STM Permissions Guidelines from
Lightstone et al.7
Nil
Chosen
overlay
Lens
matching
chosen
overlay
Lens
matching
colorimeter
setting
Mean rate of
reading (wpm)
85.6
94.3
90.7
96.5
Standard deviation
28.5
27.8
23.8
29.3
Use of overlays for screening
The differences in the colour optimal for overlays and lenses do not
prevent overlays from being useful as a screening tool (usually with
children) to identify individuals who might benefit from tinted
spectacles when reading. In Chapter 10 we discuss in more detail the
diagnosis of visual stress, including a diagnostic algorithm that was
produced by a Delphi study of leading prescribers of precision tinted
lenses.2 This study, and our clinical experience, tends to support the
practice of using overlays in the diagnostic process, at least for those
whose symptoms centre on reading. Individuals who find overlays
helpful for reading usually benefit from tinted spectacles. As we will see
in later chapters, coloured lenses are useful not only for reading but
for reducing some forms of photophobia in a range of neurological
disorders. In these circumstances coloured overlays are not likely to be
useful in the diagnostic process. For these cases, the Pattern Glare Test
(Figure 8.5), is more likely to be helpful.
Chapter 9 Management of visual stress
251
Often the benefit from tinted spectacles is greater than with
overlays. Likely explanations for this are that the colour of tinted
spectacles can be selected with greater precision than with overlays,
and spectacles can be used for tasks other than reading, including
writing, distance vision such as board work, and tablets and
computers. Also, spectacle lenses can reduce symptoms attributable
to, glare or flicker from lighting. Just occasionally, however clinicians
encounter patients who prefer to use overlays rather than lenses, and
not simply for cosmetic reasons. One possible reason is that overlays
reduce the contrast of the page they cover in a way that lenses cannot.
The issue is worthy of further study.
Determining whether an overlay helps
An overlay may increase reading speed, but the increase is
sometimes measurable only after prolonged use, when the patient has
begun to tire. As noted in Chapter 6, conventional reading tests are
designed to assess reading skills and are not ideally suited to detecting
the immediate effect of visual interventions. The Wilkins Rate of
Reading Test (Chapter 6) was designed to be visually demanding, to tire
the eyes quickly and to measure any effect of overlays in the short
term. But it is not always successful in doing so; sometimes patients,
usually adults, report that an overlay improves the comfort of viewing
even though there is no discernible improvement in reading speed, at
least in the short term. For some people with visual stress, the main
problem appears to be the comfort of text rather than its legibility. In
these circumstances it is advisable to give an overlay on a trial basis to
see if it is routinely used for reading.
Vision, Reading Difficulties and Visual Stress
252
Selecting the right colour for lenses
Testing for the appropriate colour for lenses involves using
subjective feedback from the wearer, supported by measures of visual
performance such as reading speed or visual search speed. Although
it is possible to undertake an examination using a sufficient number of
tinted trial lenses, successively comparing one combination of lenses
with another, such an examination is necessarily lengthy because of
the precision with which the colour often needs to be selected. Not only
is the examination lengthy, it is also important to control and allow for
the spectral power distribution of the illuminating light. The Intuitive
Colorimeter avoids these drawbacks – the spectral power distribution
of the illuminating light is controlled and can be varied systematically
and intuitively in a manner that can allow for the changes in
chromaticity that occur when tinted trial lenses are used under various
lighting sources.
The examination with the Intuitive Colorimeter is described in detail
in the instrument manual, and so will be described only briefly here.
The examination procedure involves obtaining a suitable chromaticity
using the colorimeter followed by confirmatory testing undertaken
with trial lenses, starting with those that match the optimal
chromaticity from colorimetry.
Selecting an appropriate chromaticity for lenses
with the Intuitive Colorimeter
The patient views a page of text illuminated with white light. The
examiner obtains a description of any perceptual distortion and/or
discomfort on viewing the text and uses this description subsequently.
For example, if the patient reports that the main problem is text
appearing to move, the optometrist asks them during testing about the
Chapter 9 Management of visual stress
253
effects of different colours on this movement.
Some patients with visually-precipitated migraine may not be aware
of any symptoms on viewing text. These cases can present a challenge
for the clinician. On running through the list of questions in the
colorimeter program, many will report mild symptoms, which can be
referred to during testing in the usual way. For others, the wide range
of spatial frequencies present in the textual target in the colorimeter
means that this is still likely to be a useful target for the patient to view
during the testing. Clinical experience and research with migraine
patients8 indicates that, in the absence of symptoms of perceptual
distortion, it is best to instruct the patient to concentrate on visual
comfort when viewing the text: comfort is usually more important than
clarity. Once a comfortable colour has been identified, a slightly more
stressful stimulus, such as a small striped pattern can be shown so as
to help the patient optimise the shade of colour. (Too large a pattern
may cause the patient to select a darker tint than they require.) If the
Pattern Glare pattern 2 is used, it may be wise to cover part of the
pattern to reduce the glare.
Patients who benefit from certain colours can have adverse
symptoms when exposed to other colours. The Intuitive Colorimeter
examination is designed to minimise the likelihood of adverse
symptoms from exposure to coloured light. In the first stage of the
procedure the patient is exposed briefly to a wide range of moderately
saturated colours under conditions of adaptation to white light.
Aversive colours are subsequently avoided during the second stage
when measurements are made with more strongly saturated colours
under conditions of adaptation to coloured light.
Patients may be particularly sensitive to coloured light. For these
patients, mild saturations (e.g., maximum 10) should be used in the
first stage of colorimetry.
Vision, Reading Difficulties and Visual Stress
254
The saturation control is then operated to increase and then
decrease the saturation of colour at each of 12 hues evenly disposed
around a hue circle. A note is made on a chart (Target chart) of any
hues that reduce the perceptual distortion and any that increase it. In
the latest version of the colorimeter, the Curve, this recording takes
place automatically using the tablet computer that is provided with the
colorimeter. Hues that improve perceptual distortion are short-listed.
Saturation is then adjusted to the optimum at each of the short-listed
hues and the settings compared two at a time, one after the other, in
order to find the best of the short-listed hue-saturation combinations.
At the best of these hue-saturation settings, hue angle is then adjusted
by a small amount to re-optimise the hue at the optimum saturation.
The saturation is then re-adjusted at the revised hue and the minimum
saturation that eliminates perceptual distortion is selected. After
checks have been made on the consistency of these settings, lower
luminance levels are compared by operating the attenuators.
The attenuators do not assess the need for grey lenses. They
have two very different purposes. (1) The first purpose is to provide
a check for residual discomfort. An underlying principle of testing is
to find the chromaticity that best alleviates symptoms, whilst
striving to keep the saturation as light as possible. This improves the
appearance (cosmesis), and also reduces the risk of the tints
affecting colour perception. If the patient prefers an attenuated
setting it may be because the saturation has been reduced too far
and some discomfort remains despite the colour. If this is the case,
the saturation can be increased slightly to see whether the patient
is then comfortable with no attenuation.
(2) The second purpose of the attenuators is to provide a check
on the acceptability of the darkness of the lens. When the
colorimeter is used without the attenuators set, the light level is that
Chapter 9 Management of visual stress
255
which would occur under normal office lighting when wearing a lens
that transmits about 30% of the light. This is not possible with a
strongly saturated turquoise or blue colour, which can only be
provided in a darker lens. If the patient is not comfortable with the
dark setting (when an attenuator is set) it is important to use the
trial lenses to demonstrate the trade-off between darkness and
strength of colour, and to find an acceptable compromise.
When strongly saturated tints are necessary, wearers should be
cautioned that the tint could interfere with colour perception. This
is most likely when the wearer views pastel colours (e.g.,
watercolour painting or colours used on some maps), but only rarely
is there interference with the perception of traffic signals. The
software provided with the Intuitive Colorimeter cautions when this
is the case. Of course, if there is any risk of this the patient should
be advised not to drive, or ride a bicycle, when wearing the tints. The
Highway Code (which produces guidance for drivers in the UK)
states that tinted glasses should not be used at night or in poor
visibility.
Confirming the appropriate trial lenses and
spectacle lenses
A set of 41 pairs of trial lenses are provided with the Intuitive
Colorimeter. It is possible to match any Intuitive Colorimeter setting
with a stack of these trial lenses so that the colour appearance is
identical, allowing for differences in brightness. This can be done
using lenses from only two dyes, and the dyes are always
neighbours in the circle of colours shown in Figure 9.7.
Vision, Reading Difficulties and Visual Stress
256
Figure 9.7. A circle of colours.
For example, a yellowy green is produced with a combination of
yellow and green trial lenses, and a red with a combination of rose
and orange lenses. There are five pairs of lenses of each colour,
apart from rose and purple which have six pairs. For each
chromaticity, a pair of lenses is provided for testing both eyes
simultaneously: two identical trial lenses should not be placed in
front of one eye.
For each colour of trial lens, the deposition of dye doubles from
one pair to the next in the set which means that the saturation of
colour can be increased in very small steps by combining the lenses,
placing one lens on top of another. With the exception of Rose and
Purple, there are 32 steps for each colour, one for each combination
of trial lenses. Rose and Purple have six lenses and therefore 64
possible combinations. Since adjacent colours can be combined,
there are therefore more than 6000 possible combinations of trial
Chapter 9 Management of visual stress
257
lenses. This means that any chromaticity can be obtained with
sufficient precision (see Chapter 7).
A computer program assists in finding the appropriate
combination of trial lenses. These can then tried out by the patient
under a range of lighting conditions. This is a useful additional check
and confirmation of the colour determined during colorimetry and
may be useful in patients who have severe symptoms, who are
particularly precise in their selection of colour, or who work under
unusual lighting conditions. The procedure described works best
under the white light generated in the colorimeter (LED in the most
recent model, fluorescent in earlier models). The tint usually has
residual efficacy under daylight and incandescent ligh t, although
purple lenses are an exception, as shown in Chapter 7. For
exceptionally sensitive cases, the computer program that is
provided with the colorimeter allows the practitioner to determine
adjustments that need to be made for different light sources. In rare
cases it may be useful to add a grey dye to the coloured dyes, and
the lens set includes grey lenses for this reason.
Should the glasses be worn outdoors in strong sun, it is advisable to
add an ultraviolet blocking dye when the tint transmits light at the short
wavelength end of the spectrum. The lens material absorbs ultraviolet
radiation below 380nm but the addition of a UV-blocking dye further
limits transmission below 400nm. Rarely, it can therefore change the
colour of the tint in a manner that some patients find unacceptable. A
pair of UV-blocking trial lenses is provided so this can be checked.
The trial lenses that come with the colorimeter are of large diameter
(65mm). This is to minimise the amount of light that can reach the eyes
without passing through the lenses. When spectacle frames are
chosen, it may be helpful to choose frames where the lens size is not
too small, for the same reason. The importance of lens size probably
Vision, Reading Difficulties and Visual Stress
258
depends on the case. If a child’s main problem is the appearance of the
page, then the priority is for their view of the page to be completely
covered by the lens. In contrast, if an adult with migraine is bothered
by fluorescent lighting in the office, it may be more important for the
lens to fit close to the face to minimise light reaching the eye around
the periphery of the frame.
For patients who use several pairs of spectacles costs can be kept
down by using either clip-ons or fit-over frames. The latter are easy to
use but are sold as sunglasses, requiring a change of lens.
When coloured spectacles are worn and most of the visual field is
coloured, the eyes are likely to adapt to this colour as they would adapt
to coloured illumination (although coloured spectacles do not have
quite the same physical effect as coloured illumination). Provided the
tint is not strongly saturated, the observer will see surface colours as
they would normally appear.
One colour for life?
Clinical experience would suggest that sometimes patients wear
their tints for a period and then cease to wear them because the
symptoms remit.9 In some cases, it may be that the coloured tints
alleviate the symptoms of visual stress and, as the child benefits from
reading instruction and their reading improves, they start to enjoy
reading more and find their visual perceptual distortions less
troublesome. The finding that pattern glare is less severe in older
people than younger likely reflects a natural tendency for the visual
cortex to become less hyperexcitable with age,10 which will also help
symptoms to improve over time in many cases. Sometimes, people
stop using coloured filters because they leave educational settings and
adopt work and/or a lifestyle that avoids activities that can trigger their
Chapter 9 Management of visual stress
259
symptoms.