Dyslexia is a specific learning disability of neurological origin that affects a person’s ability to read and spell. It is characterized by persistent difficulties with accurate or fluent word recognition and poor decoding abilities. These challenges are not due to a lack of intelligence, as dyslexia occurs across all levels of intellectual ability. The underlying cause typically relates to a deficit in the phonological component of language, which involves the manipulation of sounds in speech. This makes it challenging to connect letters to their corresponding sounds, which is foundational for reading.
The Primary Cognitive Subtypes
The modern understanding of dyslexia recognizes distinct cognitive subtypes, each representing a different breakdown in the reading process. These classifications are based on the specific cognitive skills that are impaired, guiding professionals toward the most appropriate intervention strategy. The three most commonly recognized subtypes relate to difficulties with sound processing, word recognition, and processing speed.
Phonological Dyslexia, sometimes referred to as Dysphonetic or P-type dyslexia, is the most prevalent form and is defined by difficulty with phonological awareness. Individuals with this subtype struggle to break down words into their basic sound units, known as phonemes. This impairment makes sounding out new or unfamiliar words challenging, leading to poor decoding skills and difficulties with non-word reading.
Surface Dyslexia, or Dyseidetic/L-type dyslexia, involves difficulty with the visual recognition of whole words. People with this subtype have trouble building a sight-word vocabulary—words recognized instantly without sounding them out. They often over-rely on phonetic decoding, resulting in errors when reading irregularly spelled words. For example, they might read “yacht” as “yat-chet” because they are applying sound rules too rigidly.
A third major classification focuses on a Rapid Naming Deficit, which affects the speed of retrieval rather than accuracy. This subtype involves difficulty quickly naming familiar sequences of visual stimuli, such as letters, numbers, colors, or objects. The delay in rapid automatized naming (RAN) directly impacts reading fluency, making reading slow and laborious even if word accuracy is high.
Distinguishing Developmental and Acquired Forms
Dyslexia can be categorized based on the timing and cause of its onset: developmental or acquired forms. Developmental Dyslexia is the most common, present from childhood, and often has a genetic component. This form is a neurodevelopmental disorder where the brain is wired differently for language processing, impairing the natural acquisition of reading skills.
Acquired Dyslexia, also known as alexia, occurs later in life after a person has learned to read. This condition results from a neurological insult, such as a stroke, traumatic brain injury, or degenerative disease. Symptoms can mirror developmental deficits, but the underlying cause is the loss of a previously established skill rather than a failure to acquire it.
The distinction affects the overall clinical picture and the approach to support. A person with acquired alexia loses access to established mental representations of words, while someone with developmental dyslexia struggles to form those representations. Symptoms of acquired dyslexia can be highly specific depending on the location of the brain damage, sometimes resulting in “pure alexia” where the ability to read is lost while writing remains intact.
Identifying Specific Reading Deficits
Professionals use comprehensive testing to pinpoint the specific cognitive deficit, which is necessary for effective intervention. This assessment involves a battery of tests that look at specific components of the reading process, extending beyond simple reading scores. The goal is to determine the individual’s profile of strengths and weaknesses across various language skills.
Testing for the phonological subtype involves assessing phonological awareness, including tasks like rhyming, blending sounds, and segmenting words into phonemes. A specialized assessment called Rapid Automatized Naming (RAN) identifies a naming speed deficit, requiring the individual to quickly name a series of familiar visual items. These tests determine whether the primary issue is the accuracy of sound manipulation or the speed of information retrieval.
The results from these tests are used to tailor the instructional approach. For example, a severe phonological deficit requires intensive, structured phonics instruction to build the sound-symbol connection, often called the alphabetic principle. In contrast, a surface deficit may require a program focused on orthographic mapping and sight word recognition to improve visual memory for whole words and irregular spellings.