Dyslexia comes from the Greek language meaning 'difficulty with words'. It's a symptom of a number of different information processing disorders in the brain.
Because there are so many different possible underlying problems (many of which have yet to be understood fully) dyslexia is hard to closely define because it affects children in many different ways. However, the basic problem is a difficulty learning to read, spell and write, despite adequate intellect and teaching.
What causes it?
Dyslexia is caused by differences in the areas of the brain that deal with language, which aren't yet fully understood.
Several areas in the brain interact in a complex way to coordinate the manipulation of words needed for reading, writing and spelling, so the features of any one person's dyslexia will depend on which areas are affected and how.
There may be problems, for example, receiving sensory information through vision or hearing, holding it or structuring it in the brain, or retrieving it later, or there may be problems with the speed of processing information.
Brain-imaging scans show that when dyslexic people try to process information their brains work differently to those without dyslexia. This has nothing to do with intellect - people with dyslexia show a normal range of intelligence.
About four per cent of the population have severe dyslexia, while a further six per cent experience mild to moderate problems.
What are the symptoms?
Dyslexia may become apparent in early childhood, with difficulty putting together sequences (for example, coloured beads, days of week, numbers) and a family history of dyslexia or reading difficulties.
Toddlers may jumble words and phrases, forget the names of common objects, have problems with rhyming or show slightly delayed speech development. They may have never crawled (even if walking early) and have problems getting dressed, putting shoes on the right feet and clapping rhythms.
At school, children may lack interest in letters and words, have problems with reading and spelling, put letters and figures the wrong way round, be slow at written work and have poor concentration.
These problems persist as the child grows up, with poor reading, writing and spelling skills, which can erode their self-esteem.