Demystifying Dyslexia: Defining Dyslexia

Dyslexia is commonly understood as a type of learning disability.  And it is.  However, it is specific to reading.  Dyslexia is not a math disability; that term is dyscalculia.  Moreover, dyslexia is not a writing disability; that term is dysgraphia.  Dyslexia can be identified in outpatient clinics by psychologists and neuropsychologist or in schools by school psychologists.  The criteria for defining disabilities varies between schools and private practice varies as clinicians use the Diagnostic Statistical Manual, Fifth Edition (DSM-5) in private practice, whereas special education is regulated by the Individuals with Disabilities Education Act (IDEA).  However, both settings typically use the term specific learning disability (or disorder) in reading and dyslexia interchangeably.  According to the DSM-5, practitioners can use the term Specific Learning Disorder (SLD) or dyslexia, and   IDEA’s definition of a specific learning disability is, “…include[s] conditions such as…dyslexia.”  Sec. 300.8 (C) (10). Fortunately, I currently work in both private practice and in the school settings.  As such, I readily understand the similarities and differences between the settings, and can write practical, useful reports that address both settings.  To further define what dyslexia is, the National Association of School Psychologist (NASP), the Learning Disabilities Roundtable (2002), and National Joint Committee on Learning Disabilities (2020) have concluded:

• Dyslexia is endogenous in nature.  This means dyslexia occurs within an individual.  People can have reading problems due to environmental factors (e.g., lack of instruction, economic disadvantage, behavior problems, etc.); however, that is not dyslexia.  Dyslexia is an interindividual condition.
• Dyslexia is characterized by neurologically based deficits in cognitive processes.
• Dyslexia deficits are specific; that is, they impact particular cognitive processes that interfere with the acquisition of academic skills
• Dyslexia is heterogeneous (i.e., diverse) in nature. This means there is no single defining cognitive deficit or characteristic common to all types of specific learning disabilities.
• Dyslexia may coexist with other disabling conditions (e.g., sensory deficits, language impairment, behavior problems), but are not primarily due to these conditions.

Therefore, it is important for an individual (child, adolescent, or adult) to undergo a comprehensive psychoeducational evaluation if there is a suspicion of dyslexia.  The identification of an individual’s cognitive profile is instrumental in being able to recommend/implement targeted interventions. Moreover, there are four subtypes of dyslexia. Dysphonetic Dyslexia – characterized by difficulty with phonemic awareness (e.g., sounding out words).  In terms of the brain, dysphoentic dyslexia typically relates to a lesion in Herschel’s gyrus or the superior temporal gyrus. Surface Dyslexia – characterized by difficulty with the automatic recognition of a word (orthographic processing, e.g., difficulty with recognizing sight words despite good phonemic awareness skills).  In terms of the brain, surface dyslexia typically relates to a lesion in the fusiform gyrus. Mixed Dyslexia – characterized by multiple reading deficits, including phonological and orthographic processing problems.  This is the most severe form of dyslexia.  In terms of the brain, mixed dyslexia typically relates to a lesion in the supramarginal gyrus or angular gyrus. Comprehension Deficits – characterized by difficulty deriving meaning from words in print, despite intact word reading skills.  In term of the brain, comprehension deficits typically relates to a lesion in the inferior frontal gyrus. Given that dyslexia is a disorder characterized by neurocognitive deficits, if you are seeking an evaluation to identify dyslexia, you will likely want to ensure that the examiner have specific training in neuropsychological and educational principles.  This will ensure not only a correct diagnosis, but also appropriate treatment.