Irlen syndrome
Irlen syndrome, occasionally referred to as scotopic sensitivity syndrome (SSS) or Meares–Irlen syndrome, is a postulated disorder of vision or image-processing in the brain. Irlen syndrome is also sometimes referred to as asfedia or visual stress. Many mainstream professionals are skeptical of the concept. Research on Irlen syndrome has produced mixed results, finding a possible neurological basis for the condition but little or no evidence supporting the most commonly proposed treatment using tinted eyeglasses or overlays to aid reading.

History
In 1980, New Zealand teacher Olive Meares described the visual distortions some individuals reported when reading from white paper. In 1983, American psychologist Helen Irlen wrote a paper about the use of coloured overlays aiding the reading abilities of some people. Similar symptoms were separately described by Meares and Irlen, each unaware of the others work. Irlen, who was the first to systematically define the condition, named her findings "scotopic sensitivity", though in the discussions and debates over the following years, some referred to it as Meares-Irlen syndrome. The Irlen Institute, founded by Helen Irlen, describes scotopic sensitivity as "a perceptual processing disorder" related to brain interpretations of colours and light.It remains controversial whether non-Irlen-certified Meares-Irlen syndrome and the original Irlen syndrome are the same condition. Irlen syndrome, for example, seems to include a broader array of symptoms, including severe variants of the core condition. Basic testing for scotopic sensitivity was tried by optometrists, opticians and orthoptists in UK hospitals, and by optometrists and opticians in private practice employing a technique that used the Intuitive Colorimeter, developed under Medical Research Council licence. An alternative approach to correct Irlen syndrome was tried by Orthoscopics franchise in the UK, with wide colour coverage and tints manufactured by Hoyato match. Other commercial organisations have produced sets of therapeutic tints, although most have not received scientific evaluation.Studies investigating Irlens syndrome as a treatable condition have been criticised for having a biased and subjective approach to their research.Treatments offered by Irlens practitioners have not been found to be effective in improving reading ability.

Classifications
Irlen divides Irlen syndromes into six types: photophobia, distortion of the fundus, graphical distortions during reading, decreased visual field, difficulty in ocular fixation during reading and change in depth perception.

Research
The disorders have been studied in several institutions, including the Psychology Department at Essex University, the former Applied Psychology Unit, Cambridge University in England, and in the case of Meares-Irlen syndrome, Visual Unit at Glasgow Caledonian University in Scotland. As of 2012 the Visual Stress Unit offered non-commercial diagnostic and therapeutic services to individuals, and provided advice to the Scottish National Health Service.
In Australia, Irlen syndrome was researched by Paul Whiting at the University of Sydney. Whiting set up the first Irlen Dyslexia Centre in Australia, which operated in the Childrens Centre at Sydney University for more than 15 years. Irlen syndrome was also studied in Australia by Greg Robinson (1944–2008) at the University of Newcastle. He was director of the Special Education Centre at the School of Education.In the US, peer-reviewed literature on the topic suggests that much is unknown about the cause of these disorders, ranging from the 2011 study in a journal of the American Academy of Pediatrics, "Irlen Colored Overlays Do not Alleviate Reading Difficulties" and the 2012 study in the journal Brain Topography, "A Functional Neuroimaging Case Study of Meares–Irlen Syndrome". The first, purely in relation to Meares-Irlen syndrome, finds that there is no evidence for one of the fundamental claims of therapeutic benefit. The second, which focused primarily on Irlen syndrome, found compelling evidence of unique brain function linked to the syndrome.

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

Irlen Method
The Irlen Method uses coloured overlays and tinted lenses in the form of glass or contact lenses. The method is intended to reduce or eliminate perceptual processing errors; it is claimed the resultant retiming of visual signals in the brain improves the reading difficulties associated with scotopic sensitivity syndrome.

Scientific repudiation
Skepticism relating to scotopic sensitivity syndromes exact pathology has evolved on several fronts: 

Whether it exists as a distinct, predictably identifiable disease with a reasonable pathophysiological mechanism, or whether a range of symptoms from other conditions are being placed under this convenient heading;
Whether it is causally or incidentally related to dyslexia, autism, or other conditions; and
Whether existing methods of scotopic sensitivity syndrome treatment are appropriate and effective.A 2009 report by the American Academy of Pediatrics (AAP) does not find that there is conclusive scientific evidence for the use of coloured lenses (one treatment used to relieve symptoms of scotopic sensitivity syndrome) although it acknowledges anecdotal evidence in support of customized colored lenses. 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 the optometric and ophthalmologic communities. Furthermore, many special education departments at universities challenge the validity of coloured lenses as an effective treatment for the condition 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 treatment.
Critics claim that the symptoms of those with Scotopic Sensitivity Syndrome are related to already known visual perceptual and neurological disorders. According to a statement released by the American Optometric Association in 2004:
There is evidence that the underlying symptoms associated with specifically Meares Irlen Syndrome, are related to identifiable vision anomalies, e.g., accommodative, binocular, and ocular motor dysfunctions, in many patients seeking help from coloured lenses. Furthermore, such conditions return to normal function when appropriately treated with lenses, prisms, or vision therapy. When patients exhibiting Meares 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 coloured overlays or tinted lenses.
As outlined by Hyatt, Stephenson and Carter (2009):
In 1990, the Journal of Learning Disabilities published a special issue that provided intensive coverage of Irlen lenses. In the preface to the issue, the editor in chief, Wiederholt, noted that each of the studies had serious theoretical, medical/physical, and methodological flaws, but were published along with articles critiquing the studies to provide readers with an overview of the Irlen procedures as well as guidance for conducting quality research studies. He, along with Hoyt (1990), Parker (1990), and Solan (1990), noted that these initial studies by Blaskey et al. (1990), OConnor, Sofo, Kendall, and Olsen (1990), and Robinson and Conway (1990) failed to support the treatment validity of colored overlays.

See also
References
Further reading
Hyatt, Keith J (February 2010). "Irlen Tinted Lenses and Overlay" (PDF). MUSEC Briefings. Macquarie University Special Education Centre (22). Archived from the original (PDF) on 29 August 2017. Retrieved 22 April 2017.
Wilkins, Arnold J. (2003). Reading through color: how colored filters can reduce reading difficulty, eye strain, and headaches. Chichester: John Wiley & Sons. ISBN 0-470-85116-3. OCLC 78883050.