What, exactly, is dyslexia? What causes it, how should it be diagnosed, and stemming from that, how should it be treated? We can even ask a more fundamental question – does it actually exist as a discrete clinical entity?
These questions have existed since dyslexia was first described and named in 1887, by German Opthalmologist, Rudolf Berlin. Not surprisingly, he thought the decreased reading ability was due to “word blindness” – some people had difficulty visually interpreting and therefore decoding written words.
In 1925 Samuel T. Orton, a neuropathologist from the State University of Iowa, advanced his theory that dyslexia was not due to word blindness but rather to a problem with cortical dominance. His theory is was not correct, but it marked the beginning of a shift, from thinking of dyslexia as a visual problem to thinking of it as a neurological problem. The definition moved from opthalmology, to neuropthalmology, and ultimately to neurology.
As an aside, the common misconception (never really a working theory) that those with dyslexia reverse letters stems from the early visual processing theory. However, people with dyslexia do not reverse letters, either letters themselves (like “b” and “d”) or the order of letters in a word. All children as they are learning to read will reverse letters, and those with dyslexia at no higher rate. But it is amazing how much cultural inertia this misconception has.
While dyslexia was being researched and understood as a cognitive issue, not visual, there was still the matter of exactly how to define and diagnosis it clinically. By the 1960s a consensus was emerging that dyslexia is a “specific” learning disability, and that is how it should be diagnosed. What this means is that children with dyslexia have more of a problem with reading that is expected based upon their overall IQ and their learning level.
This definition is very general, and focuses on the disability itself without consideration of cause. It did not answer the question of why some children have difficulty reading out of proportion to their IQ and education. But research continued to address this question.
Another consequence of this general definition was that it fueled denial that dyslexia even exists. Even into the 2000s, UK’s Labour MP Graham Stringer called the condition a “cruel fiction” and stated, “To label children as dyslexic because they’re confused by poor teaching methods is wicked.” Blaming neurological conditions on bad parenting and poor teaching has a long pedigree from ADHD to autism, so this is not surprising.
By the 1990s a new consensus was emerging – that dyslexia is primarily a problem of “phonological awareness”. They have difficulty decoding words because their brains are not good at understanding the relationship between phonemes and words. They don’t assemble the phonemes into words as naturally as the average person. By 1994 this was codified as, “Dyslexia is a specific learning disability that is neurobiological in origin. It is characterized by difficulties with accurate and/or fluent word recognition and by poor spelling and decoding abilities.” In 2002 this was expanded with more complexity and nuance:
“A deficit in processing the phonological component of language resulted directly in difficulty with decoding, spelling, accuracy, and fluency that, in turn, impacted comprehension and reading experience. Impoverished reading experience further impacted the development of vocabulary and background knowledge, which also had a negative influence on comprehension.”
Then in 2009, partly as a response to continued denial that dyslexia is a real neurological disorder, came the Rose Report. This overview of dyslexia reinforced the phonemic awareness theory, that dyslexia is a specific neurodevelopmental disorder with genetic predisposition, and made a number of specific recommendations for interventions based on that understanding. And that is pretty much where the definition of dyslexia has sat until today (while research was ongoing).
Now there is a new attempt to reach an updated consensus of the definition of dyslexia based on the latest research – Toward a consensus on dyslexia: findings from a Delphi study. The conclude:
There was considerable consensus in our expert panel that dyslexia is a difficulty in reading and spelling, associated with multiple factors, and that it frequently co-occurs with other developmental disorders. It was agreed that difficulties in reading fluency and spelling are key markers of dyslexia across different ages and languages. We conclude with a proposed new definition of dyslexia.
And here is that definition, given as a list of consensus statements:
“Dyslexia is a set of processing difficulties that affect the acquisition of reading and spelling.
In dyslexia, some or all aspects of literacy attainment are weak in relation to age, standard teaching and instruction, and level of other attainments.
Across languages and age groups, difficulties in reading fluency and spelling are a key marker of dyslexia.
Dyslexic difficulties exist on a continuum and can be experienced to various degrees of severity.
The nature and developmental trajectory of dyslexia depend on multiple genetic and environmental influences.
Dyslexia can affect the acquisition of other skills, such as mathematics, reading comprehension, or learning another language.
The most commonly observed cognitive impairment in dyslexia is a difficulty in phonological processing (i.e. in phonological awareness, phonological processing speed or phonological memory). However, phonological difficulties do not fully explain the variability that is observed.
Working memory, processing speed, and orthographic skills can contribute to the impact of dyslexia.”
Like many conditions in neurology, dyslexia is a spectrum, with multiple different interactions with other neurological conditions and environmental contexts. Dyslexia is primarily a problem with phonological awareness, but this does not fully explain what we see and the variability of the condition. The full report goes into greater detail if you are interested.
The definition of dyslexia has evolved over time, tracking with the research, and in general has become more specific, nuanced, and complex. This is great for experts and to help guide further research and interventions. It does make it challenging to communicate what dyslexia is to the public, which is partly why simplistic misconception, and even denial, have such cultural staying power.
