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are seen by individual eyes, and a subtle misalignment (less than one
degree of arc) indicates the patient may be struggling to keep the eyes
aligned in everyday life.14 In addition to the questions regarding the
alignment of the green (Nonius) lines whilst the patient fixates the “X”,
a supplementary question “Do one or both lines ever move?” should
be asked.42 In the clinic, any movement of one of the green lines should
be investigated by determining the minimum prisms (base-in for an
exo-slip) that eliminates the instability, starting with 0.5.14
The Mallett fixation disparity test has good sensitivity (75%) and
specificity (78%) for detecting symptomatic heterophoria.43 Although
these values for sensitivity and specificity are good, they are less than
100%, as is the case with most clinical tests. The algorithm in Table 3.2
is designed with this in mind and highlights the importance of
combining tests and of measuring fusional reserves.
Several copies of the original Mallett unit are available, and it is not
known whether they will produce exactly the same result as the
original instrument. However, if they use similar size targets and have
a good foveal and peripheral fusion lock, and use polarisation44 or a 3-
D display,45 they are likely to give similar results to a genuine Mallett
unit.14 Approaches that use red-green (anaglyph) separation of each
eye’s image are more dissociative/less naturalistic and this is probably
44
(One copy of the
.
Mallett unit is manufactured by a company called Evans Instruments,
which is independent of this book’s authors)
why they have been found to give different results.
Chapter 10 Clinical protocol
281
Figure 10.3. Mallett Near Vision Fixation Disparity Test, which is used to
determine the aligning prism (associated heterophoria) or aligning sphere.
Fusional reserves should be assessed at near using a prism bar or
phoropter.14 A prism bar is probably the best method because the
patient’s eyes can be observed. Base-out prism is used to measure the
convergent fusional reserve and base-in prism to measure the
divergent fusional reserve. The fusional reserve that opposes the
heterophoria should be measured first (i.e., convergent reserve for
exophoria).46 The patient is asked to fixate a detailed target while the
clinician increases the prism strength before one eye. The prism power
is gradually increased (approximately 1 per second) with the power
being recorded when the target first becomes blurred (blur point,
which may not exist), becomes double or moves to the side (break
point), and then as the prism power is reduced, when the target
becomes single again (recovery point).14 A limitation of this test is that
the test-retest repeatability is quite poor, with 95% limits of agreement
for near testing break points approximately ±12 for convergent
fusional reserves and ±7.5 for divergent fusional reserves.47
If the amplitude (from divergent to convergent break points) is less
Vision, Reading Difficulties and Visual Stress
282
than 20Δ then the patient may have binocular instability (Chapter 3).
14
If the fusional reserve is less than twice the heterophoria it opposes
then the patient may have decompensated heterophoria (Sheard’s
criterion). The poor test-retest repeatability mean that repeat testing is
recommended and diagnosis of binocular instability by this method
alone is only made when a consistent response is obtained over two
testing sessions.
As explained in Chapter 3, another test that may provide useful
information on the vergence system is to assess the rate at which a
person can change their vergence. This has been found to be a useful
test in children with reading difficulties.
48-50
A monocular flipper or
binocular flippers can be used.14
If binocular instability or decompensated heterophoria appear to be
causing symptoms, they should be treated.14 The main approaches to
treatment (fusional reserve exercises, spheres, prisms) are described
in Chapter 3.14 The Mallett unit is useful not only for diagnosing these
conditions, but also for determining the prism (aligning prism) or
sphere that may alleviate the condition when this mode of correction
is favoured. The decision about whether to correct or treat a patient is
considered in Chapter 3 and again towards the end of this chapter.
Accommodative lag
Accommodation is routinely assessed by measuring the amplitude
of accommodation. The conventional method is to use a detailed target
on a ruler, typically combined in an instrument such as the RAF rule.
This test has several limitations that have been reviewed.
51 52
An additional test of accommodative function, MEM retinoscopy, is
particularly useful in children with reading difficulties because it
provides an objective assessment of accommodative accuracy or lag.
The patient binocularly fixates a detailed target on the retinoscope and
Chapter 10 Clinical protocol
283
is asked to keep this clear. Retinoscopy is carried out along the
horizontal meridian and lenses are very briefly held in front of each eye
to neutralise the retinoscope reflex. Each lens should only be present
monocularly and for a split second so as not to disrupt the status of
the patient's accommodative and binocular response. The
accommodative lag is usually about 0.50D, although this is dependent
on refractive error53 and refractive error stability.54 Values greater than
1.00D may represent accommodative insufficiency.
14
If a negative lens
is required to neutralise the reflex this suggests that the patient may
be over-accommodating (accommodative spasm). MEM retinoscopy
obtains similar results to an autorefractor (mean error 0.00, 95% limits
of agreement 0.91D).55
In a slightly different approach (Nott retinoscopy), the clinician
keeps the fixation target in a constant position and moves the
retinoscope towards and away from the patient to obtain reversal.
Typically, this reveals a slightly lower degree of accommodative lag.56
Open field autorefractors have clinical potential for objectively
measuring lag of accommodation,
57 58
although this approach is not
widely used clinically at present.
Accommodative facility
Vision, Reading Difficulties and Visual Stress
284
Figure 10.4. Accommodative facility testing using binocular flippers
The ability to alter accommodation rapidly and accurately is called
accommodative facility and this can be assessed using ±2.00DS
accommodative flippers. The flipper consists of a pair of +2.00DS
lenses mounted on one side of a flipper bar and a pair of -2.00DS
lenses mounted on the opposite side (Figure 10.4). The patient fixates
a near target (at 40cm) while the practitioner alters the accommodative
stimulus by placing either the plus lens pair (stimulus of 0.50D) or the
negative lens pair (stimulus of 4.50D) in front of the patient’s eyes. The
test should always begin with the +2.00DS lenses. The patient reports
when the near target is seen as clear and single after each alteration in
accommodative stimulus, with the optometrist counting the number of
times clarity is obtained in one minute. This number divided by two
gives the accommodative facility rate in cycles per minute. Ideally, a
suppression check should be included for any binocular
measurements.14 The OXO letters and Nonius lines on a near Mallett
Unit can be used in conjunction with the polarising filters for binocular
accommodative facility testing. The vertical OXO target is
recommended because the patient is less likely to be distracted by
movement of the Nonius markers than if the horizontal OXO target is
used.
Normative values for children and young adults are sometimes
given as 11 cycles per minute for monocular facility and 8 cycles per
minute for binocular facility. But these figures are based on the work
of Zellers and colleagues,59 who reported that this was the mean value
in a normative study. Usual practice is to define the normal range of
test results as the range within which 95% of results lie. As a rule of
thumb, this is the mean ±2 standard deviations (SDs). The standard
deviation of Zellers et al.’s data was 5, so the mean ± 2 SDs is a very
wide range. This may be because the test is confounded by many
Chapter 10 Clinical protocol
285
variables other than accommodative facility, such as refractive error,60
verbal response, attention, and interpretation of blur. The limitations
of the test mean that caution is necessary in interpreting the results. If
there are no symptoms, it would seem unwise to base treatment on
poor performance with the accommodative facility test at just one
appointment. However, if a patient reports difficulty copying from the
board and performs poorly at accommodative facility, treatment would
seem reasonable.
A study in student volunteers demonstrated that objective
measures of accommodative function were highly correlated with the
subjective responses of persons undergoing the accommodative
facility test.61 Patients with low accommodative facility rates may
benefit from an accommodative facility training regime14 in order to
improve their dynamic accommodation function.
62-64
Coloured filters
If symptoms persist after the detection and treatment of any
significant refractive and/or ocular motor problems, then practitioners
should investigate the effect of coloured filters. It is best, whenever
possible, to test the patients in lighting conditions that are similar to
those where they typically experience the symptoms. Note that lighting
conditions in schools often are far brighter than the recommended
level of 300-500 lux.65 The lighting is usually fluorescent and 80% of
schools continue to be lit with low frequency fluorescent lighting that
emits 100 Hz flicker .65 As noted in Chapter 8, preliminary evidence may
link such lighting with visual stress.
Vision, Reading Difficulties and Visual Stress
286
Figure 10.5. The Intuitive Overlays in use.
With children, testing usually starts by screening with coloured
overlays, although with adults this stage is sometimes bypassed (see
below). There are several systems of overlays available in the UK. In
chronological order of their introduction these are (1) the Intuitive
Overlays (iOO Sales Ltd, London, UK); (2) the Cerium Overlays (Cerium
Visual Technologies Ltd, Tenterden, UK); (3) Crossbow Overlays and
Reading Rulers (Crossbow Education Ltd, Stafford, UK). (The Irlen
overlays are only available to Irlen licensees).
As explained in Chapter 9, the Intuitive Overlays (Figure 10.5)
comprise a set of coloured overlays that have been designed to sample
colours systematically66 and which are of an adequate size to be
effective67 and have a sufficient range of colours.68 The set consists of
twenty A5 sized overlays (two sets of ten different coloured overlays).
One surface of the overlay has a matte finish whereas the other side is
Chapter 10 Clinical protocol
287
gloss. Chapter 9 provides details of the merits and demerits of each
system. The Intuitive Overlays have been used in many research
studies, many of which were included in a review by Evans and Allen.69
Procedure for overlay assessment
The procedure for assessment with Intuitive Overlays is described
in the test instructions. It is always worth confirming the consistency of
the chosen overlay or overlay combination. If the patient consistently
chooses a particular overlay or combination of overlays, it is possible
to measure the effects of the overlays on reading speed (see below) –
this can be a helpful indicator of whether the overlay is likely to be
used.
A computerised version of the coloured overlay test is also available
(Thomson Software Solutions, Hatfield, UK), and this includes the
Wilkins Rate of Reading Test (see below). Whether this provides a
colour suitable for overlays has not been evaluated in published
research.
Coloured overlays are a rapid and easy method of screening for a
benefit from colour, but they have some limitations. If a person
benefits from a coloured overlay, they are likely to find precision tinted
lenses more helpful. This is because precision tinted lenses can be
individually prescribed with greater precision and, in addition to
reading, also help for writing, computer use, whiteboards, and possibly
light from LED/fluorescent lighting that is reflected from surfaces.
Owing to colour adaptation, a person’s optimal colour of precision tint
is likely to be different to the optimal colour of overlay (Chapter 9).70
Therefore, the colour for lenses should not be selected as matching
that for overlays. Precision tinted lenses are typically prescribed on the
basis of testing with the Intuitive Colorimeter.71
Vision, Reading Difficulties and Visual Stress
288
As illustrated in Figure 10.2, children are generally only tested with
the Intuitive Colorimeter if they pass the Delphi criteria (described
below), which usually means they have shown a significant benefit
from a coloured overlay. There are two ways of determining if a person
is benefiting from an overlay. The first approach (‘sustained use’) is to
dispense the child an overlay to use for a few weeks and invite them to
return for testing with the colorimeter if the child, parent, and/or
teacher feel that the overlay is helping. An alternative approach is to
test the immediate effect of the coloured overlay on the child’s
performance. The most common method of doing this is to use the
Wilkins Rate of Reading Test (WRRT), which is described below.
Practitioners need to be flexible and to be prepared to use each of
these methods of determining whether to progress to testing with the
colorimeter. For some children the main benefit from the overlay is in
a reduction of perceptual distortions, so the WRRT may be the best
approach. For others, the main benefit from the overlay is in visual
comfort, so the sustained use approach will work best.
An objection is sometimes raised about using overlays to determine
who will benefit from precision tinted lenses. The objection is that both
the investigative tool that is being used to determine suitability for an
intervention and the interventions are very similar – in essence,
different forms of coloured filters. It should be noted that in other
conditions a treatment is sometimes used in diagnosis (e.g., in
decompression sickness).
72-74
Similarly, within optometry Elliott
advocated that “You can view prescribing glasses as a diagnostic
tool”.75 It is important to be mindful of placebo effects, but in visual
stress, it is easier to be assured of a genuine response. This is because,
a child might expect that the colour lens that should help them most
would match the coloured overlay they have chosen, which would in
fact arouse suspicions in the clinician (see Chapter 9).70 Nonetheless, it
Chapter 10 Clinical protocol
289
Test (PGT), which is described later in this chapter.
Procedure for the Wilkins Rate of Reading Test
(WRRT)
The WRRT76 consists of 10 simple words arranged in random order
and in small closely spaced font (Figure 10.6). The patient reads the text
as quickly as possible, whilst trying to avoid making mistakes. The test
method is described in the test instructions. In summary, the patient
reads four versions of the test in the following order: with the
intervention (typically, coloured filter), without the intervention, again
without the intervention, then finally once more with the intervention.
Each time, the practitioner scores the numbers of words correctly read
in one minute, calculating the mean performance with the intervention
and the mean without the intervention. From this, the effect of the
intervention can be calculated in absolute (number of words per
minute faster or slower with the intervention) or relative terms
(percentage increase or decrease in speed with the intervention). The
cut-off for a clinically significant result is discussed later in this chapter.
is reassuring that an additional diagnostic test exists, the Pattern Glare
Vision, Reading Difficulties and Visual Stress
290