What causes dyslexia?

There are a great many possible reasons why children find it difficult to learn to read. For many, these difficulties can be explained by disadvantage - poverty, low family support, poor teaching or low ability. Other children however, may have been well taught and are perfectly bright in other ways, yet mysteriously they cannot learn to read. They are 'developmental dyslexics'.

Dyslexia runs strongly in families suggesting a genetic basis. Genetic studies suggest around 60% is inherited, prevalence across cultures is similar (and independent of socio-economic status and IQ), and more males than females are affected. Nine gene variants associated with dyslexia have so far been identified and several more suggested. Many of these genes control the development of the brain, leading to differences in  brain structure, so that some nerve cells fetch up in the wrong places during development. This leads to microscopic differences in connectivity between neurones, and an unusual symmetry of the language areas.

These abnormalities may particularly affect 'magnocellular' neurones which are specialised for timing, sequencing and rapid information processing.

Dyslexics commonly have problems with visually sequencing the letters in a word rapidly and accurately - 'an orthographic deficit' and also with rapidly and accurately ordering the sounds in a word - a 'phonological' deficit. Such impaired sequencing of letters visually and sounds auditorily may result from abnormal magnocellular development.

The gene variants may also make magnocells more vulnerable to immune attack during development of the brain, and to nutritional deficiencies, e.g of the essential 'omega-3' fatty acids that are found in oily fish.