Alexia is an acquired neurological disorder that disrupts the ability to read written language following a brain injury. It represents a loss of a previously learned skill, distinguishing it from developmental reading difficulties. This acquired reading impairment does not stem from problems with vision or intelligence, but rather from damage to the neural pathways that process visual information into recognizable words.
Defining Alexia and Identifying Symptoms
Alexia is defined as a significant limitation or complete loss of reading comprehension due to a focal brain injury. Individuals with this condition, sometimes referred to as acquired dyslexia or word blindness, struggle to recognize or interpret written words and letters. Symptoms manifest as an inability to read or comprehend written text, even though the person retains the physical ability to see the words and the capacity to understand spoken language.
A common symptom is “letter-by-letter reading,” where the individual must identify each letter in a word before sounding it out to recognize the word by its auditory form. This process is slow and effortful, and the time required to read a word increases significantly with the number of letters. Alexia is fundamentally different from developmental dyslexia, which is a lifelong learning disability characterized by difficulties in acquiring reading skills. Alexia, conversely, is an impairment that arises in a person who was already literate.
Neurological Events That Lead to Alexia
Alexia is caused by damage to specific regions of the brain responsible for language and visual processing, most commonly in the dominant hemisphere (the left side for most people). The most frequent cause is a cerebrovascular accident, or stroke, which disrupts blood flow and leads to brain tissue death. Infarctions of the posterior cerebral artery are implicated in certain types of alexia.
Other neurological events that can cause the condition include traumatic brain injury (TBI), brain tumors, and neurodegenerative diseases. TBI results from a sudden external force, while tumors can compress or destroy language-processing tissue. The damage typically affects the neural circuits that connect the visual cortex, where letters are first processed, to the language areas, such as the angular gyrus and the visual word form area.
Clinical Classification of Alexia
The clinical presentation of alexia is variable and is classified into distinct syndromes based on the location of the brain lesion and co-occurring language deficits. The three main categories are Alexia without Agraphia, Alexia with Agraphia, and Aphasic Alexia, each representing a unique disruption in the reading network.
Alexia without Agraphia
Alexia without Agraphia, also known as pure alexia, is characterized by a severe inability to read despite the preserved ability to write. Patients can still write spontaneously or to dictation. They can often read what they have written by tracing the letters with their finger, but they cannot visually recognize the words they just produced. This syndrome is caused by damage to the left occipital lobe and the splenium of the corpus callosum.
Damage to the left occipital lobe damages the visual word form area. Damage to the splenium prevents visual information from the right occipital lobe from crossing over to the language centers in the left hemisphere. This creates a “disconnection syndrome” where the visual input cannot reach the language areas for interpretation. The intact writing ability is due to preserved language and motor pathways for writing, which bypass the damaged visual reading route.
Alexia with Agraphia
Alexia with Agraphia, also termed central or parietal-temporal alexia, is a widespread disorder involving the loss of both reading and writing abilities. This type is more severe because the core language-processing areas themselves are damaged, rather than just the visual input pathway. It involves damage to the inferior parietal lobe of the dominant hemisphere, centering on the angular gyrus.
Because the angular gyrus integrates information between various brain regions, damage here impairs the ability to recognize written words and letters, and to form words into written text. Individuals with this condition are unable to recognize letters, cannot comprehend words spelled out to them, and are unable to write or spell. This form frequently co-occurs with other neurological deficits, such as elements of Gerstmann Syndrome.
Aphasic Alexia
Aphasic Alexia refers to reading difficulties that occur as a component of a broader language disorder known as aphasia. Aphasia is an impairment of language that affects speaking, understanding, reading, and writing, resulting from damage to the language-dominant hemisphere. The specific nature of the reading difficulty is directly related to the type of aphasia present.
For instance, reading difficulties in Broca’s aphasia (sometimes called frontal alexia) often parallel problems in spoken language, showing better comprehension than reading aloud. In Wernicke’s aphasia, written comprehension is severely impaired, mirroring the deficit in auditory comprehension. The reading impairment is not an isolated deficit but part of a global disruption to the brain’s entire language system.