Visual stress is a condition characterised by symptoms of asthenopia and visual perceptual distortions when observing striped patterns,1 including lines of text.1–6 Visual stress during reading was first characterised by Meares7 and Irlen8 and has also been termed Meares–Irlen syndrome.3,5,9–13 The aetiology of visual stress has yet to be fully elucidated.

In the United Kingdom, people are usually screened for visual stress only if they report their subjective symptoms to optometrists, teachers or other professionals, who are aware of visual stress. Once referred (normally to an optometrist), patients undergo a thorough assessment of binocular vision which may be followed by an assessment of pattern glare. Patients then choose their preferred coloured overlays, and undergo tests of reading speed with and without the chosen overlay or combination of overlays. A diagnosis of visual stress is made if the coloured overlays immediately improve reading speed and/or if there is voluntary sustained use of the chosen overlays.2,10,11,14,15 This screening method is very subjective and ignores those people who benefit from coloured overlays without reporting symptoms of visual stress. There is no generally accepted criterion for the percentage increase in rate of reading necessary to justify prescribing coloured overlays, although a 5% increase is commonly used.16 Kriss and Evans suggest that a criterion of 10% is most appropriate.10 The appropriateness, however, depends upon the sensitivity and specificity required.16 Some individuals with specific learning difficulties,17 dyslexia,10,18 poor reading ability,19,20 migraine21,22 and autism23 show a benefit from coloured filters. Patients who would benefit from coloured filters but do not report their symptoms are not currently identified, and many individuals benefit from overlays without initially being aware of their symptoms. The present paper aims to investigate whether there are indications of visual stress in the results of the clinical tests used in routine eye examinations.

Blaskey et al.24 investigated the effect of vision therapy compared to Irlen filters, in a sample of children and adults, but only selected participants with vision problems. Evans25 stressed the importance of eye examinations to identify those patients whose symptoms indicative of visual stress are in fact due to other, optometric, issues. Other studies investigating the optometric correlates of visual stress have been undertaken with children.12,26,27 Only one of these studies27 used an unselected sample and the participants in this sample were not refracted. In the following study adult participants were included regardless of existing ocular and ocular-motor conditions and participants wore their optimum refractive correction throughout the experimental procedure. This was the first study to use an unselected sample of adults, who wore their optimal refractive correction throughout the testing.

Fig. 1 shows each participant's average reading speed with and without their individually selected coloured overlays.

Figure 1.

Each participant's average reading speed, in words per minute, with and without their individually selected coloured overlays.

(0.09MB).

Points above the diagonal line show those participants who read more quickly with their chosen overlays. For the group as a whole, reading speed increased significantly with an overlay (mean=167.8, SD=29.9) compared to without (mean=162.0, SD=30.9), t(72)=5.26, p<0.0005. Twenty-six (36%) participants increased their reading speed by more than 5% with their individually selected coloured overlays, and in 10 of these the reading speed increase was greater than 10%. One participant had a reduction in reading speed >5% with their chosen overlay compared to without.

Ten participants with a reading rate increase >5% (38% of the group) had an ocular abnormality as defined in Table 1, compared to 13 participants with ≤5% improvement in reading speed (28% of the group). The cut-off values in Table 1 were chosen based on normal values.28 There was no significant association between the presence of ocular abnormality and participants’ reading rate increase (p=0.43, Fisher's exact test, two-tail).

The refractive corrections were expressed as a dioptric power matrix for data analysis, so the vector components could be treated in a simple algebraic manner.36 The distant visual acuities were converted into logMAR values. The stereoacuity in arc seconds was converted into log form to reduce the skew in the distribution of the data.

Table 2 summarises the findings. There were no statistically significant differences in the test results other than for associated heterophoria.

The above analysis was repeated with a more conservative criterion for the change in reading speed (>10%) and the findings were essentially unchanged.

The three individuals who were prescribed coloured filters demonstrated reading speed increases of 6.3%, 7.2% and 15.4% with overlays. All participants had a small refractive error, which conventionally would not be corrected by spectacle or contact lenses. They all had normal visual acuity, stereoacuity, binocular amplitude of accommodation, near point of convergence, ocular motility and colour vision. One participant had one prism dioptre of associated heterophoria at near.

Twenty-six of the 73 students read >5% more quickly with their chosen overlay. The proportion (36%) did not differ significantly from the proportion obtained in previous studies: 38/113 (34%)11 and 15/26 (58%) (non-dyslexic students37).

Tests which are completed in routine eye examinations did not identify those participants who benefitted from coloured overlays with the possible exception of associated heterophoria at near.

Evans et al.26 reported optometric measures of 16 children who reported benefit from coloured overlays, and who used their overlays without prompting for 3 weeks. They compared them with 25 control children. They found that the mean amplitude of accommodation, convergent and divergent reserves and stereoacuity were all reduced compared to the controls, although the differences were not clinically significant. However, children with clinically significant refractive errors or ocular motor anomalies were excluded from both groups. Evans et al.12 analysed the optometric measures of 53 of the 68 children used in the double masked study by Wilkins et al.38 The participants in this study all used overlays. Children with decompensated heterophoria or blurred vision secondary to uncorrected refractive or accommodative anomalies were excluded. They reported that the children who used overlays had normal refractive errors and heterophorias but slightly reduced amplitudes of accommodation and reduced vergence and poor stereoacuity relative to age norms in the literature.

In two studies, Scott et al.27 investigated entire classes of school children aged 10–12 or 7–11, and these studies are therefore free of the sample bias of previous work. The children underwent an orthoptic examination (Study 1) or an optometric examination (Study 2). Each study found differences between children who used overlays and those who did not, but the only findings to occur consistently in both studies were reduced binocular amplitude of accommodation and associated heterophoria in children who used overlays.

In the present study, three of the 26 students who read >5% more quickly with an overlay and one of the three who habitually used overlays exhibited a clinically significant associated heterophoria. Of the five studies that have investigated optometric findings in overlay users (including the present study), all have revealed vergence difficulties (fusional reserves and/or associated heterophoria). Although dissociated tests (for example cover test) are used in clinical practice, associated heterophorias, measured using the Mallett test, are used to prescribe prismatic correction, if required, and provide a more natural measure of ocular-motor status due to the binocular fixation. For this reason, associated heterophorias were measured, although the authors recognise that they are not always included in routine eye examinations. The results on the Mallett Unit test were categorised as abnormal if there was a misalignment of the nonius lines, suggesting a decompensated heterophoria. Although the cover test was not performed, no participants exhibited suppression on the Mallett unit suggesting that no heterotropias were present.

While the association between overlay use and associated heterophoria would appear to be consistent across studies, the association is weak. Many individuals who benefit from overlays have adequate fusional reserves and no associated heterophoria. Although an association seems to exist, albeit weakly, the associated heterophoria may directly contribute to the difficulty in reading, contribute to the visual stress that leads to the reading difficulty or be present but not affect reading at all (being a non-causal correlate of visual stress25). Conversely, associated heterophorias or binocular instability could be secondary to visual stress, causing perceptual instability that impairs the fusion lock.4

Symptom questionnaires and assessments of pattern sensitivity are most commonly used to identify whether symptomatic patients would benefit from coloured filters,39 but are not routinely used in optometric practice as a screening tool, so are not discussed or investigated further in this paper.

In conclusion, this study found that tests used in routine eye examinations did not identify people who benefitted from coloured overlays. A quick and more objective screening procedure should be developed, so those asymptomatic individuals who would benefit from colour are not overlooked.

Arnold Wilkins invented the Intuitive Overlays when he was employed by the Medical Research Council. The Council hold the rights and pay an Award to Inventors based on sales. The remaining authors have no conflicts of interest to declare.