Excerpt from Aphasia

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Background

Aphasia is an acquired disorder of language due to brain damage. Aphasia does not include (1) developmental disorders of language, often called dysphasia in the United States; (2) purely motor speech disorders, limited to articulation of speech via the oral-motor apparatus, referred to as stuttering, dysarthria, and apraxia of speech; or (3) disorders of language that are secondary to primary thought disorders, such as schizophrenia.

Encompassed under the term aphasia are selective, acquired disorders of reading (alexia) or writing (agraphia). Closely related to aphasia are the family of disorders called apraxias (disorders of learned or skilled movements), agnosias (disorders of recognition), acalculias (disorders of calculation ability), and more global neurobehavioral deficits such as dementia and delirium. Such related syndromes may coexist with aphasia or exist independently.

Pathophysiology

Aphasia may occur secondary to brain injury or degeneration and involves the left cerebral hemisphere to a greater extent than the right. Language function lateralizes to the left hemisphere in 96-99% of right-handed people and 60% of left-handed people. Of the remaining left-handed people, about one half have mixed hemisphere language dominance, and about one half have right hemisphere dominance. Left-handed individuals may develop aphasia after a lesion of either hemisphere, but the syndromes from left hemisphere injury may be milder or more selective than those seen in right-handed people. 

Most aphasias and related disorders are due to stroke, head injury, cerebral tumors, or degenerative diseases. The neuroanatomic substrate of language comprehension and production is complex, including auditory input and language decoding in the superior temporal lobe, analysis in the parietal lobe, and expression in the frontal lobe, descending via the corticobulbar tracts to the internal capsule and brainstem, with modulatory effects of the basal ganglia and the cerebellum. Aphasia syndromes have been described, based on patterns of abnormal language expression, repetition, and comprehension. These syndromes have been roughly correlated with specific left hemisphere locations, though there are clearly overlaps and individual differences that make the aphasia syndromes limited in their reliability. Patients may lose the ability to produce speech, to comprehend speech, to repeat, and to hear and read words in many nuanced ways.

Frequency

United States

Data on incidence of aphasia in the United States are limited. Aphasia occurs in a variety of cerebrovascular, traumatic, and degenerative conditions. Stroke is likely the most common cause of aphasia, and it has been estimated that about 20% of stroke patients develop aphasia. More than 700,000 strokes occur in the United States each year, and approximately 170,000 new cases of aphasia every year are related to stroke. The number of patients with language disorders secondary to traumatic brain injury, brain tumors, and other brain lesions such as arteriovenous malformations is not precisely known. Patients with neurodegenerative disorders such as Alzheimer disease and frontotemporal dementia frequently manifest language deficits. The prevalence of Alzheimer disease in the United States is approximately 5 million cases.

Mortality/Morbidity

Aphasia is a condition, not a disease; therefore, it has no attributable mortality rate.

Race

No reliable data exist on the incidence of aphasia in different racial groups. Within disease entities, however, such differences are well known. In stroke, for example, African Americans have almost a 2 times higher incidence as compared with whites. In addition, specific types of stroke, such as cerebral hemorrhage, lacunar infarctions, and intracranial artery stenoses, are known to be more common in blacks than whites. One might therefore surmise that poststroke aphasias would be more common in African Americans.

Sex

Not enough data are available to evaluate differences in the incidence and clinical features of aphasia in men and women. Some studies suggest a lower incidence of aphasia in women because they may have more bilaterality of language function. Differences may also exist in aphasia type, with more women than men developing Wernicke aphasia.

Age

Age may be an important factor in recovery. Some studies suggest that recovery from aphasia due to a stroke is less favorable in patients older than age 70 than in younger patients. However, at any age, recovery of various degrees can occur, even at times remote from the brain injury.

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