Aphasia is a loss of the ability to use and understand language. It is not a disease, but a symptom of brain damage. Aphasia can result from any type of injury to the part of the brain associated with language. For most people, this involves damage to the left side of the brain.

Some individuals with aphasia have problems understanding the language of other people. This is called receptive aphasia. Others have problems communicating with people. This is called expressive aphasia. It is also common for people to have both receptive and expressive aphasia. Most people with aphasia also have problems reading and writing.

There is no one language center in the brain. Instead, several large networks of neurons (nerve cells) connect the parts of the brain that are associated with hearing, understanding and producing language. Damage to any part of the neuron networks or the areas of the brain they connect may result in varying degrees and types of aphasia.

An estimated 1 million Americans currently have aphasia, according to the National Institute of Neurological Disorders and Stroke (NINDS). The National Institute on Deafness and Other Communication Disorders estimates that an additional 80,000 people in the United States develop aphasia each year.

The extent of aphasia ranges from mild to severe, depending on the location of the brain injury and the education and health status of the patient. People with mild aphasia may be able to communicate with others or carry on a conversation, but may have trouble following complicated discourse. People with severe aphasia may be unable to say or understand anything. Aphasia does not cause intellectual deficiencies. People with aphasia can think and reason to the same capacity as before the aphasia occurred.

Aphasia rarely affects infants or young children who have not yet developed language skills. This is because aphasia is the result of damage to areas of the brain associated with previously learned language. A rare epilepsy syndrome called Landau-Kleffner syndrome may cause aphasia in children who have already developed language skills. Parents of children who experience problems understanding or using language should consult a pediatrician, who can test for signs of other potential language disorders.

Aphasia may sometimes be confused with dysarthria (muscle weakness that impacts ability to speak). People with dysarthria have trouble articulating words because of damage to parts of the nervous system associated with the mechanics of speech. Their speech may be slurred or they may have consistent problems with pronunciation. However, unlike aphasia, people with dysarthria have no problem understanding language.

Aphasia may also sometimes be confused with verbal apraxia (speech impairment due to a loss of motor function). However, verbal apraxia involves impaired motor functioning (e.g., inability to properly move the tongue to form words) rather than brain damage.

Most cases of aphasia occur suddenly. However, in some people, aphasia develops gradually over time. People who experience aphasia should consult a physician as soon as possible as it may be an indication of a serious medical condition (e.g., stroke, brain tumor, Alzheimer’s disease).