Aphasia is a loss or impairment of the ability to produce or comprehend language, due to brain damage. It is usually a result of damage to the language centres of the brain (like Broca's area) which are most commonly found in the left hemisphere, and can be caused by a stroke or physical injury. Depending on the area and extent of the damage, someone may be able to speak but not write, or vice versa, or understand more complex sentences than they can produce. The brains of young children with brain damage sometimes restructure themselves to use different areas for speech processing, and regain lost function; adult brains are less "plastic" and lack this ability.

Aphasia can been assessed in a variety of ways, from quick clinical screening at the bedside to several-hour-long batteries of tasks that examine the key components of language and communication.

Any of the following can be considered aphasia:

• inability to comprehend speech
• inability to read (alexia)
• inability to write (agraphia)
• inability to speak, without muscle paralysis
• inability to form words
• inability to name objects (anomia)
• poor enunciation
• excessive creation and use of personal neologisms (jargon aphasia )
• inability to repeat a phrase
• persistent repetition of phrases
• other language impairment

The common types of aphasia are

• Broca's aphasia (expressive aphasia)
• Wernicke's aphasia (receptive aphasia)
• Nominal aphasia (anomic aphasia)
• Global aphasia
• Conduction aphasia

A few less common varieties include

• Transcortical motor aphasia
• Subcortical motor aphasia
• Transcortical sensory aphasia
• Subcortical sensory aphasia

Sources:

• National Institute of Health: MEDLINEplus Medical Encyclopdia entry on Speech Impairment (adult)