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Scotopic sensitivity syndrome, also known as Irlen Syndrome and Meares Irlen syndrome is a visual perceptual disorder. It affects both children and adults. Irlen Syndrome is characterized by symptoms of visual stress and visual perceptual distortions which are alleviated by using individually prescribed colored filters. Activities which are visually demanding, such as reading or writing, can become very difficult for those with Irlen syndrome. Recent research suggests that the expression of Irlen syndrome is a continuum. Some people benefit immensely from treatment with colored overlays and lenses and some experience negligible effects. This research further suggests that a significant percentage of people, as high as 20%, may express some degree of this syndrome and benefit from the use of colored lenses or overlays while reading.

The condition was separately described by two people working individually, each unaware of the other's work. In the early 1980s New Zealand teacher Olive Meares described the visual distortions some individuals reported when reading from white paper, while American therapist Helen Irlen wrote a paper about the use of coloured overlays aiding the reading abilities of some people. Irlen's claims about the effectiveness of her coloured overlays in alleviating the symptoms of the syndrome were initially considered quite controversial due to the lack of peer reviewed research data published. Subsequent research has provided some evidence which supports the existence of the syndrome and the beneficial effects of using colored overlays and lenses. However, many of the symptoms associated with Irlen syndrome can also be caused by binocular and accommodative anomalies. The existence of Irlen syndrome and the effectiveness of colored lenses as a form of treatment is still a controversial issue.

Theory
There are many theories which attempt to explain what causes Irlen syndrome. These include but are not limited to: contrast sensitivity, pattern glare, retinal sensitivity, binocular and accommodative anomalies, a defective magnocellular pathway and cortical hyperexcitability. However, most of these theories have trouble accounting for the individual and sometimes precise nature of the tints required. The theories which have been most successful in describing Irlen syndrome so far are pattern glare and cortical hyperexcitability  The lack of a theory which adequately describes Irlen syndrome is one of the largest factors contributing to the controversy surrounding it.

Scotopic sensitivity syndrome Symptoms
One or more of these symptoms may be related to the condition:
 * Eye-strain
 * Fatigue
 * Headaches (including migraine)
 * Nausea, including visually related motion sickness
 * Problems with depth perception (catching balls, judging distance, etc.)
 * Restricted field of view and span of recognition
 * Discomfort with busy patterns, particularly stripes ("visual stress" and "pattern glare")
 * Discomfort with extreme conditions of bright/dark contrast (i.e. backlighting)
 * Discomfort or difficulty reading (reading involves busy patterns, particularly stripes. People with strong symptoms of the syndrome find it very difficult to read black text on white paper, particularly when the paper is slightly shiny.)
 * Text that appears to move (rise, fall, swirl, shake, etc.)
 * Losing text content and only seeing rivers of white through the text
 * Words moving together becoming one unrecognizable word
 * Attention and concentration difficulties
 * Seeing the part and losing the whole
 * Epileptic seizure related to strobing or pattern glare

Diagnosis
Irlen Syndrome is usually diagnosed by the sustained voluntary use of a colored overlay or an immediate improvement on the Wilkins Rate of Reading Test when using a colored overlay. It is also the case the individuals sometimes self diagnose themselves with Irlen syndrome after noticing that they experience many of its related while reading. There are several problems related to the reliability of such self diagnoses especially when screening for Irlen syndrome in schools. For example, the symptoms of Irlen syndrome are sometimes exaggerated by children who are experiencing some small visual distortions, which maybe related to irlen syndrome or may be related to a variety of other visual problems. In other cases symptoms of Irlen syndrome may go completely unrecognized by children who believe that their vision is "normal", that others experience the same distortions while reading that they do.

Treatment
If children express a preference for a coloured overlay then this preference is tested in one of two ways: voluntary sustained use or an immediate improvement in rate of reading. To meet the criterion of voluntary sustained use, the person is issued with their preferred coloured overlay and invited to use the overlay for reading, if it is found to be helpful. Instructions are given that if the overlay is still being used after a half to one school term then the child should seek further testing. To test for an immediate benefit in rate of reading, the Wilkins Rate of Reading Test is used

A problem with diagnosing MIS (Evans & Joseph, 2002) is that there is likely to be a continuum ranging from people who experience no help from coloured filters, through those who show a mild benefit, to the more severe cases who may experience a marked reduction in symptoms and a very marked improvement in reading performance.

The use of tinted lenses in glasses and coloured overlay sheets has been prescribed by many doctors; however, the efficacy of such treatment is questionable. It has been felt to be efficient treatment by some, and inappropriate by others, because more conventional treatments are sometimes more appropriate.

The College of Optometry (UK) has specified guidelines for optometrists who use the colorimeter system. A society for colored lens prescribers has been established to provide a list of eye-care practitioners with expertise in the provision of colored lenses for the treatment of visual stress.

The Promethean Trust, a Norwich-based charity for dyslexic children, has found that the use of a cursor has eliminated the need for colored overlays or lenses. The cursor is simply a piece of card or plastic, approximately the size of a business card, with a notch cut out of one corner. The reader (or the remedial teacher) uses this to track print from left to right, and at the same time the card prevents the eyes from wandering ahead. Although no formal research has been conducted, it is likely that most cases of visual confusion result from the eyes moving in mini-saccades when the reader encounters an unfamiliar word. This occurs as the reader subconsciously tries to scramble letters to achieve a 'fit' with a familiar word. This creates the subjective impression that the letters 'won't stay still'.

Irlen Method
The Irlen Method is a controversial system that is intended to improve reading difficulties associated with scotopic sensitivity syndrome using tinted lenses and overlays.

Irlen Screener
"Irlen Screeners are certified to administer the first testing session and determine whether an individual will benefit from further evaluation for Irlen Spectral Filters."

Irlen Diagnosticians
"Irlen Diagnosticians are certified to administer both testing sessions. During the initial testing session, an individual is screened to determine whether wearing Irlen Spectral Filters will make a difference and the amount of improvement for reading and other academic activities.  In addition, during this session 12 other areas will be evaluated and recommendations made.  Only Irlen Diagnosticians are certified to test and determine your customized spectral filter worn as Irlen glasses or contact lenses and conduct yearly rechecks."

Intuitive Colorimeter
Developed by Arnold Wilkins, Ph.D., University of Essex, England, an alternative system for the identification of tint to reduce symptoms.

Lack of Scientific Proof
Skepticism surrounding scotopic sensitivity syndrome has evolved on several fronts:
 * 1) Whether it exists as a distinct, predictably identifiable disease with a reasonable pathophysiological mechanism;
 * 2) Whether it is causally or incidentally related to dyslexia, autism, or other conditions; and
 * 3) Whether existing methods of scotopic sensitivity syndrome treatment are appropriate and effective.

A 2009 report by The American Academy of Pediatrics (AAP) does not believe that there is any scientific evidence or basis for the use of colored lenses (the treatment used for Scotopic Sensitivity Syndrome). When discussing its scientific basis, the AAP mentions that "[t]he method used to select the lens or filter color has been highly variable,the color selection has also shown considerable variability,and the test-retest consistency has been poor" (p. 843)

The association of scotopic sensitivity syndrome and dyslexia has been challenged by many authors in both the optometric and ophthalmologic communities. Furthermore, many special education departments within universities also challenge the validity of SSS and Irlen lenses. As outlined by the Macquarie University Special Education Centre :
 * In a joint statement, The American Academy of Ophthalmology, American Academy of Pediatrics, American Association for Pediatric Ophthalmology and Strabismus and American Association of Certified Orthoptists firmly repudiated the use of lenses, stating that there was no scientific evidence supporting their use. The expense of such treatment is unwarranted and may divert resources from evidence-based reading interventions.

Critics claim that the symptoms of those with Scotopic Sensitivity Syndrome are related to already known visual disorders. According to a statement released by the American Optometric Association in 2004: "There is evidence that the underlying symptoms associated with the Irlen Syndrome are related to identifiable vision anomalies, e.g., accommodative, binocular, and ocular motor dysfunctions, in many patients seeking help from colored lenses. Furthermore, such conditions return to normal function when appropriately treated with lenses, prisms, or vision therapy. When patients exhibiting the Irlen Syndrome were treated with vision therapy, their symptoms were relieved. These patients were no longer classified as exhibiting this syndrome, and therefore did not demonstrate a need for the colored overlays or tinted lenses."

Terminology
Critics assert that the term "scotopic sensitivity" is a misnomer given that the symptoms of "Scotopic Sensitivity Syndrome" reportedly occur during photopic conditions. The term "Scotopic Sensitivity" seems dubious, given that scotopic vision is the vision of the eye under low light conditions and as such vision is provided by rod cells on the retina, which have little or any role in colour vision, it does not make sense that a coloured lens or coloured overlay would have any impact upon "Scotopic Sensitivity Syndrome".