Aphasia is an inability to comprehend or formulate language because of damage to specific brain regions. This damage is typically caused by a cerebral vascular accident (stroke), or head trauma; however, these are not the only possible causes. Aphasia can also be caused by brain tumors, brain infections, and neurodegenerative diseases such as dementia, however these are far less common and thus not as often mentioned when discussing aphasia. To be diagnosed with aphasia, a person's speech or language must be significantly impaired in one (or several) of the four communication modalities following acquired brain injury or have significant decline over a short time period (progressive aphasia). The four communication modalities are auditory comprehension, verbal expression, reading and writing, and functional communication.

The difficulties of people with aphasia can range from occasional trouble finding words, to losing the ability to speak, read, or write; intelligence, however, is unaffected. Expressive language and receptive language can both be affected as well. Aphasia also affects visual language such as sign language. In contrast, the use of formulaic expressions in everyday communication is often preserved. 

 With aphasia, one or more communication modalities in the brain have been damaged and are therefore functioning incorrectly. Aphasia is not caused by damage to the brain that results in motor or sensory deficits, which produces abnormal speech; that is, aphasia is not related to the mechanics of speech but rather the individual's language cognition (although a person can have both problems, particularly if they suffered a hemorrhage that damaged a large area of the brain). An individual's "language" is the socially shared set of rules as well as the thought processes that go behind verbalized speech. It is not a result of a more peripheral motor or sensory difficulty, such as paralysis affecting the speech muscles or a general hearing impairment.

Aphasia affects about 2 million people in the US and 250,000 people in Great Britain. Nearly 180,000 people acquire the disorder every year in the US alone.Any person of any age can develop aphasia, given that it is often caused by a traumatic injury. However, people who are middle aged and older are the most likely to experience aphasia. Older individuals are at the highest risk for aphasia because risk for stroke increases with age: approximately 75% of all strokes occur in individuals over the age of 65. Strokes account for the most documented cases of aphasia : 25% - 40% of people who survive a stroke develop aphasia as a result of damage to the language processing regions of the brain.

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