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Aphasia is a combination of a speech and language disorder caused by damage to the brain that affects about 1 million individuals within the US. Most often caused by a cerebral vascular accident, which is also known as a stroke, aphasia can cause impairments in speech and language modalities. 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, but does not affect intelligence.〔 This also affects visual language such as sign language.〔 In contrast, the use of formulaic expressions in everyday communication is often preserved. One prevalent deficit in the aphasias is anomia, which is a deficit in word finding ability. The term "aphasia" implies that one or more communication modalities are functioning incorrectly. It is not usually used when the language problem is a result of a more peripheral motor or sensory difficulty, such as paralysis affecting the speech muscles or a general hearing impairment. Aphasia is from Greek ''a-'' ("without") + ''phásis'' (''φάσις'', "speech"). The word ''aphasia'' comes from the word ἀφασία ''aphasia'', in Ancient Greek, which means〔(【引用サイトリンク】title=What is aphasia? What causes aphasia? )〕 "speechlessness",〔(ἀφασία ), Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus.〕 derived from ἄφατος ''aphatos'', "speechless"〔(ἄφατος ), Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus.〕 from ἀ- ''a-'', "not, un" and φημί ''phemi'', "I speak". ==Causes== Aphasia is most often caused by stroke, but any disease or damage to the parts of the brain that control language can cause aphasia. Some of these can include brain tumors, traumatic brain injury, and progressive neurological disorders. In rare cases, aphasia may also result from herpesviral encephalitis. The herpes simplex virus affects the frontal and temporal lobes, subcortical structures, and the hippocampal tissue, which can trigger aphasia. In acute disorders, such as head injury or stroke, aphasia usually develops quickly. While when from brain tumor, infection, or dementia it develops more slowly.〔〔(【引用サイトリンク】 Aphasia )〕 There are two types of strokes. An ischemic stroke and a hemorrhagic stroke. An ischemic stroke happens when a person’s artery which supplies blood to different areas of the brain becomes blocked with a blood clot. This type of stroke happens 80% of the time. The blood clot may form in the blood vessel which is called a thrombus or the blood clot can travel from somewhere else in the blood system that is called an embolus. A hemorrhagic stroke occurs when a blood vessel in the brain ruptures or bursts. Overall, people experience bleeding inside or around brain tissue. This type of stroke happens 20% of the time and is very serious. The most common cause of hemorrhagic stroke is an aneurysm. Although all of the diseases listed above are potential causes, aphasia will generally only result when there is substantial damage to the left hemisphere (responsible for language function) of the brain, either the cortex (outer layer) and/or the underlying white matter. Substantial damage to tissue anywhere within the region shown in blue on the figure below can potentially result in aphasia. Aphasia can also sometimes be caused by damage to subcortical structures deep within the left hemisphere, including the thalamus, the internal and external capsules, and the caudate nucleus of the basal ganglia. The area and extent of brain damage or atrophy will determine the type of aphasia and its symptoms.〔〔 A very small number of people can experience aphasia after damage to the right hemisphere only. It has been suggested that these individuals may have had an unusual brain organization prior to their illness or injury, with perhaps greater overall reliance on the right hemisphere for language skills than in the general population. Primary Progressive Aphasia (PPA), while its name can be misleading, is actually a form of dementia that has some symptoms closely related to several forms of aphasia. It is characterized by a gradual loss in language functioning while other cognitive domains are mostly preserved, such as memory and personality. PPA usually initiates with sudden word-finding difficulties in an individual and progresses to a reduced ability to formulate grammatically correct sentences (syntax) and impaired comprehension. The etiology of PPA is not due to a stroke, traumatic brain injury (TBI), or infectious disease; it is still uncertain what initiates the onset of PPA in those affected by it. Finally, certain chronic neurological disorders, such as epilepsy or migraine, can also include transient aphasia as a prodromal or episodic symptom. Aphasia is also listed as a rare side-effect of the fentanyl patch, an opioid used to control chronic pain.〔(【引用サイトリンク】 FENTANYL TRANSDERMAL SYSTEM patch, extended release )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Aphasia」の詳細全文を読む スポンサード リンク
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