Overview

Alexia is a reading disorder typically acquired after a traumatic brain injury. People with alexia may be partially or completely unable to read, depending on the severity of the damage. There are several types of alexia, varying in the brain regions affected.

Pure alexia is characterized by an inability to recognise written words, and sometimes an inability to recognise individual letters. People with this form of alexia retain their ability to write and spell.

Surface alexia is characterized by an overreliance on the pronunciation of words in order to ascertain their meaning. For example, because the words ‘sun’ and ‘son’ are pronounced in the same way, somebody with surface alexia would be unable to distinguish between these words if they were written on the same page. This results in impaired reading comprehension.

Phonological alexia is characterized by an inability to ‘sound out’ words to aid with the reading process. This makes reading unfamiliar words especially difficult.

Deep alexia is characterized by the production of semantic paralexias. A semantic paralexia is a type of reading error where the word that is said is related in terms of meaning to the actual written word. For example, if the word ‘lawyer’ was written on a page, someone with deep alexia might read it as ‘attorney’.

Strokes are the most common cause of alexia, however, other neurological diseases can also cause reading deficits.¹ A stroke is clinically defined as a syndrome of acute, focal neurological deficit caused by vascular injury of the central nervous system.² This means that strokes have a relatively short duration, affect only small parts of the central nervous system at one time, and are caused by damage to blood vessels. 

Strokes are the second leading cause of death and disabilities worldwide.³ meaning that understanding exactly what happens during a stroke and how this leads to certain cognitive impairments like alexia is critical as a matter of promoting public health.

What happens during a stroke?

Blood flow to the brain is governed by two internal carotid arteries and two vertebral arteries which link together to make up the circle of Willis. The circle of Willis is a ring of blood vessels which branches off to supply the brain with oxygenated blood. A stroke occurs when blood flow to the brain is impaired such that the brain doesn’t receive sufficient oxygen. Strokes vary in their causes, and lasting damage varies according to the parts of the brain that are affected.

An Ischemic stroke is caused by an insufficient supply of oxygenated blood to the brain. This may be due to either a thrombotic or embolic event. In a thrombotic event, a blood clot forms and obstructs a blood vessel, whereas in an embolic event debris from elsewhere in the body (which could be a clot) migrates and obstructs a blood vessel supplying oxygenated blood to the brain.⁴ In either case, this obstruction (called an embolism) reduces the volume of oxygenated blood reaching the brain. This results in necrosis in the brain, meaning that brain cells die.

A Hemorrhagic stroke is caused by bleeding or leaky blood vessels and is much less common than an ischemic stroke. During a hemorrhagic stroke, blood vessels rupture, resulting in an abnormal accumulation of blood in the brain.⁵ This is very dangerous because the accumulation of blood can compress and destroy brain cells, while also disrupting normal blood flow to certain parts of the brain.

Strokes are always dangerous, but they do not always result in alexia. The exact effects of a stroke are case-dependent, with outcomes varying according to the type of stroke, the location, and the extent of the brain damage.

How can a stroke lead to alexia?

Cognitive impairments, such as alexia, are common consequences of strokes.⁶ The resulting cognitive impairment depends on which part(s) of the brain is(are) deprived of oxygen due to the stroke. For example, strokes affecting the frontal lobe often cause executive dysfunction whereas strokes affecting the temporal lobes are associated with memory deficits.⁷ In the case of alexia, if the stroke affects any brain regions which are important for reading, this disturbs an individual’s ability to read, leading to the symptoms of alexia.

Stroke and pure alexia

Stroke-induced pure alexia is caused by disruption to visual pathways in the brain. This occurs when a stroke in the posterior cerebral artery - part of the aforementioned circle of Willis - deprives the flow of oxygenated blood to the occipital lobe and the splenium of the corpus callosum.

The splenium of the corpus callosum connects the visual processing areas of the right hemisphere with the language centres of the left hemisphere. This pathway is important for normal reading.⁸ Therefore stroke-induced damage to this neural pathway impairs an individual’s ability to read.⁹

The language areas in the left hemisphere include the angular gyrus and the fusiform gyrus. One consequence of reduced oxygenated blood flow to these areas includes an inability to recognise whole words as discrete units. This explains why people with pure alexia engage in ‘letter by letter’ reading whereby they process each letter individually every time, making reading a much more laborious and slow process for those with pure alexia.¹⁰

Additionally, because only visual pathways are damaged and the auditory pathways remain intact, people with pure alexia can still recognise words that are spelled out loud to them.

Stroke and surface alexia

The middle cerebral artery (MCA) is part of the circle of Willis and it supplies oxygenated blood to several brain regions including the temporoparietal junction and parts of the fusiform gyrus. A stroke in the middle cerebral artery can impair the functioning of these brain regions.¹¹

Ordinarily, the left temporoparietal junction - which includes parts of the angular gyrus and the supramarginal gyrus - processes the visual forms of words and links them to their meanings.

The Left Fusiform Gyrus has a critical role in the rapid recognition of whole words.¹² Therefore, a consequence of impaired functioning in these brain regions is an inability to recognise words as whole units, just like in pure alexia. Due to this inability to visually recognise whole words, people with surface alexia often rely on sounding out words.

This means errors in reading typically arise due to phonetic irregularities, for example, words like ‘yacht’ are not easily identified because sounding it out does not produce the correct pronunciation. Regular words, however, can still be read.¹³

Another consequence of damage to the left temporoparietal junction is that, owing to its role in linking visual input with their meaning, the meaning of words often can’t be ascertained until after it has been pronounced out loud. This also means that people with surface alexia struggle to differentiate between real words and pseudowords.¹⁴

Stroke and phonological alexia

Stroke-induced phonological alexia is associated with damage to the following brain regions:

• Broca’s area, is needed for articulating and processing phonological aspects of language.
• Left temporoparietal junction
• Left superior temporal gyrus: Plays a role in processing auditory stimuli¹⁵

Damage to these parts of the brain typically occurs due to a stroke in the Middle Cerebral Artery, as the middle cerebral artery branches to supply blood to vast portions of the brain.¹⁶ The damage causes issues with auditory processing, meaning that people with phonological alexia are unable to sound out words in the way other types of alexics do.

This makes it difficult to read novel words and causes an overreliance on only encountering words they’ve previously been familiar with. As a result, people with phonological alexia are also unable to read pseudowords due to a lack of familiarity.¹⁷

Stroke and deep alexia

Deep alexia typically results from extensive damage to multiple regions in the left hemisphere that are collectively responsible for both language processing and semantic processing. These brain areas include:

• Broca’s area
• Left temporoparietal junction
• Left superior temporal gyrus (Plays a role in processing phonological aspects of language)
  • Left inferior temporal gyrus
• Fusiform gyrus

Again, the Middle Cerebral Artery is implicated in deep alexia. Strokes in this artery can cause extensive damage to the regions necessary for both phonological and semantic processing, leading to deep alexia.

Due to impaired semantic processing, people with deep alexia make errors based on semantics and produce semantically related words instead of the actual word.¹⁸

Summary

• A stroke is clinically defined as a syndrome of acute, focal neurological deficit caused by vascular injury of the central nervous system such as an obstruction in the blood vessel or rupture
• Alexia is a reading disorder typically acquired after a traumatic brain injury like strokes
• Strokes in the middle cerebral artery or the posterior cerebral artery deprive the areas of the brain, which are crucial for reading, of oxygen. This leads to the reading impairments characteristic of alexia
• Deep alexia is associated with the most extensive damage to the brain