(p.171) Appendix: Techniques for treatment
(p.171) Appendix: Techniques for treatment
Various techniques for treating visual discomfort are summarized.
In the preceding chapters, temporal and spatial characteristics of visual stimulation have been identified as responsible for visual discomfort. Pulsation of light that is too rapid to see as flicker can induce eye-strain and headaches, and lower frequency flicker can cause epileptic seizures as well. Spatially repetitive patterns, such as stripes, can have effects similar to those of flicker. In this chapter we review techniques for avoiding these unfortunate effects. The purpose is not to provide a complete review, but to refer the reader to those chapters where the issue is discussed in detail.
A.1 Prevention of photosensitive seizures
Patients with photosensitive epilepsy are at risk of seizures when exposed to flickering light, such as discotheque strobes and sunlight passing through road-side trees. Simply closing the eyes is no protection. The light diffuses through the eyelids, increasing the area of retina exposed, which can exacerbate the effects. When exposed to flicker, patients should not close the eyes but should cover one eye with the palm of a hand (Jeavons and Harding 1975). The effects of flicker are usually greatly reduced when the stimulation is monocular (see Section 2.2.3).
Flicker from television is one of the principle causes of seizures in patients with photosensitive epilepsy. Patients should sit at a distance of at least 2 m from the television (more than three times the width of the screen). If they have to go near the screen they should cover one eye with the palm of a hand. In Chapter 7 we gave details of polarized glasses that provide a selective and cosmetic way of preventing light from the television reaching one eye. Some of the new television technologies (100 Hz television, liquid crystal screens) will be beneficial in reducing seizures.
As we saw in Chapter 8, the design of wallpaper and furnishings can be a problem if it provides a repetitive pattern.
There is the possibility that coloured glasses may help prevent seizures in certain cases, as described in Chapter 9. Although treatment with precision tints is available in certain optometric practices, it is advisable for patients with photosensitive epilepsy to be assessed for precision tints in a hospital, (p.172) if possible with concurrent EEG monitoring. This will necessitate a liaison between optometrists and encephalographers.
• Cover one eye when exposed to flicker.
• Watch television from at least 2 m or obtain a liquid crystal television.
• Avoid striped furnishings and clothing.
A.2 Headaches and eye-strain from lighting
In Chapter 6 various techniques for avoiding the unpleasant effects of fluorescent lighting were described. People who suffer headaches and eye-strain from fluorescent lighting may find the following steps helpful:
(1) try and sit as near a window as possible (avoiding direct sunlight), so that the daylight reduces the depth of modulation of the flicker;
(2) substitute a tungsten – halogen uplighter for the fluorescent lamps in their immediate vicinity; or
(3) substitute warm white lamps which have a lower modulation;
(4) try and arrange for high-frequency electronic circuitry at the next lighting refit;
(5) obtain glasses that have been tinted in such a way that they reduce the amount of pulsating light reaching the eyes.
A tint suitable for reducing pulsation is described in Chapter 6.1 It reduces the pulsation from the widely used halophosphate lamps (white, warm white, cool white) but not from triphosphor lamps (Colour 84, Polylux etc.). The tint has been shown to reduce headaches in school children in a small scale but double-blind study. There are large individual differences in the tolerance to coloured glasses, and some people find the colour unpleasantly strong. It is therefore essential to try out a demonstration pair before purchase.
• Avoid or ‘dilute’ the light from fluorescent lamps.
• Consider glasses with a tint that reduces the flicker.
(p.173) A.3 Headaches, eye-strain, and seizures from reading
Repetitive patterns can also be uncomfortable to look at, inducing eye-strain, headaches, and even seizures. Chapter 5 showed that printed text resembles a pattern of stripes. Eye-strain and headaches and seizures can sometimes be prevented by a simple mask1 that darkens and blurs the unnecessary lines.
In Chapter 9 we described the use of tinted glasses and overlays in reducing perceptual distortion of text. Tinting using Irlen's techniques is available privately from the Irlen Institute2. Tinting using the Intuitive Colorimeter3 is available at certain optometric practices in Britain, and elsewhere4.
Coloured plastic sheets placed upon a page of text (overlays),5 offer an inexpensive alternative to tinted glasses. Indeed the British College of Optometrists recommend that before children receive tinted lenses they should have used overlays for a trial period. This method of treatment has received little research, but it appears that the colour needs to be selected from a large range of filters, using appropriate methods of presentation. The colours that are found to be beneficial when using overlays are hardly ever the same as those that are beneficial in glasses.
The subject should compare each of the overlays in turn to decide whether the filter results in an improvement, a deterioration, or has no effect. When the filters that improve perception have a colour complementary to the colour of those that make matters worse, one can have greater confidence that colour per se is responsible for the effects. When several overlays improve perception, they should be compared side by side, two at a time, under a variety of lighting conditions. The best one can then be selected by a process of elimination.
If the subject benefits from the use of a coloured overlay, this will be revealed in a change in attitude to reading, a willingness to read for longer, and possibly also an improvement in reading fluency and a reduction in headaches. Subjects who benefit in this way from the use of an overlay, may find appropriately coloured spectacles of greater convenience and sometimes of greater benefit, presumably due to the greater precision of colour choice.
• Choose book editions with large widely spaced text, where possible.
• Try masking the text above and below, leaving three lines visible for reading.
• Try a range of coloured overlays, selecting the most comfortable.
• If the overlays are helpful, try glasses with precision tints.
A.4 Colour in therapy: a caveat
We saw in Chapter 6 that certain people are sensitive to the rapid pulsation of light from fluorescent lamps. The pulsation affects the control of eye movements and increases headaches. It can be removed using new electronic circuitry, with a consequent reduction in headaches among office workers. The pulsation can also be reduced by wearing spectacles that selectively absorb light with wavelengths less than 550 nm, and these have been shown to reduce migraine headaches among school children. The reduction of headaches is easy to understand on a physical basis. The spectacles reduce the depth of modulation of the light by about 30 per cent. This may seem a small reduction, but it is sufficient to change the visibility of lower frequency flicker by a large amount. Moreover modulation from fluorescent lamps is usually reduced by ‘dilution’ with daylight. The 30 per cent contribution from the glasses may therefore be sufficient to remove the physiological consequences of the pulsation.
The colours of tints that reduce pulsation from fluorescent lamps exacerbate pattern glare in some people. The lens that reduces the flicker from fluorescent lamps (see Section 6.10) has a reddish hue and, as we have seen, pattern glare is most commonly reduced by colours complementary to red. In general, tints that absorb wavelengths less than 550 nm have colours that lie in the right-hand half of the UCS diagram. This area is opposite the area most frequently chosen to reduce pattern glare from text (see Fig. 9.5(h)).
If the above analysis is correct, and it is too soon to be sure, the choice of a tint colour that is likely to be beneficial will be determined by the relative contribution of two factors: susceptibility to fluorescent lighting and susceptibility to pattern glare. The choice of examination procedure needs to be determined by the clinical history. If the history implicates an aversion to fluorescent lighting, it is not sufficient to use a colorimeter with stable illumination to measure a comfortable tint. The colorimeter provides an estimate of the role of colour in reducing pattern glare, but if susceptibility to flicker is part of the clinical problem, the setting needs to be confirmed (p.175) using trial lenses under conditions of fluorescent lighting that approximate those the patient normally experiences.
• An individual's sensitivity to flicker and to pattern may require incompatible colours. Both sources of discomfort need to be considered together.
(1) The Comfort 41® tint is manufactured by Cambridge Optical Group, Bar Hill, Cambridge, and is obtainable from high street optometrists.
(1) The Cambridge Easy Reader is manufactured by Engineering and Design Plastics, Cherry Hinton, Cambridge, and is obtainable by post.
(2) The Irlen Institute has many branches in the USA and throughout the world, including one in London.
(3) The Intuitive Colorimeter is manufactured by Cerium Optical Group Ltd, Tenterden, Kent.
(4) For a list of optometrists that have the Intuitive Colorimeter, contact the Institute of Optometry in London.
(5) The Institute of Optometry in London supply ‘Intuitive Overlays’. Ten colours are available in an Assessment Pack, and individual colours can also be obtained separately.