Aphasia is a loss of the ability to produce and/or comprehend language, due to injury to brain areas specialized
for these functions. It is not a result of deficits in sensory, intellect, or psychiatric functioning nor
muscle weakness or a cognitive disorder.
Depending on the area and extent of the damage, someone suffering from aphasia may be able to speak
but not write, or vice versa. Patients may display any of a wide variety of other deficiencies in language
comprehension and production, such as being able to sing but not speak. Aphasia may be accompanied with
speech disorders such as dysarthria or apraxia of speech, which may also result from brain injury.
Usually, aphasias are a result of brain damage (lesions) caused by head injury, to the language centers
of the brain such as the Broca's area. These areas are almost always located in the left hemisphere,
and in most people this is where the ability to produce and comprehend language is found. However, in
a very small number of people, language ability is found in the right hemisphere. In either case, brain
damage to these language areas can be caused by a stroke, traumatic brain injury, or other head injury.
Aphasia may also develop slowly, as in the case of a brain tumor or progressive neurological disease.
It may also be caused by a sudden hemorrhagic event within the brain. Certain chronic neurological disorders,
such as epilepsy or migraine, can also include transient aphasia as a prodromal or episodic symptom.