Many functions of the brain are performed by several areas of the brain working together (networks), not by a single area in the brain. Damage to these networks can cause aphasia, apraxia, agnosia, or amnesia.
Usually, doctors can diagnose the type of dysfunction by examining the person. They ask questions designed to evaluate specific brain functions. Imaging tests, such as computed tomography (CT) and magnetic resonance imaging (MRI), are usually needed to identify the cause of the damage.
Aphasia is partial or complete loss of the ability to express or understand spoken or written language. It results from damage to the areas of the brain that control language.
In most people, part of the left temporal lobe called Wernicke's area and part of the left frontal lobe called Broca's area control language function. Damage to any part of these small areas interferes with at least some aspect of language function. Usually, writing and speech are affected similarly. Aphasia is the most common language disorder among older people.
Aphasia usually results from disorders that do not cause progressive damage, such as a stroke, some tumors, head injury, or brain infection. In such cases, aphasia does not worsen. But if it results from a progressive disorder (such as an enlarging brain tumor), aphasia can progressively worsen.
People with aphasia have difficulty expressing or understanding language. But the nature and degree of the difficulty vary. The variety reflects the complex nature of language function. For example, aphasia may involve loss of only the ability to comprehend written words (alexia) or the ability to recall or say the names of objects (anomia). Some people with anomia cannot remember the right word at all. Others have a word in mind but cannot say it. Most people with aphasia have anomia. Or aphasia may involve only the inability to repeat words, phrases, or sentences (conduction aphasia). People with conduction aphasia understand spoken and written words and can speak fluently.
Most people with aphasia have more than one type of aphasia. One type is often more severe than the others.
Wernicke's (Receptive) Aphasia:
If Wernicke's area is damaged, people have difficulty understanding spoken and written language. They usually speak fluently and with a natural rhythm, but the sentences come out as garbled, confused strings of words (sometimes referred to as word salad). They may not know that they are speaking nonsense.
Broca's (Expressive) Aphasia:
If Broca's area is damaged, people may mostly understand the meaning of words and know how they want to respond. However, they have difficulty finding the words to say. Their words are forced out slowly and with great effort, sometimes interrupted by expletives. Most affected people are also unable to write words.
Complete (Global) Aphasia:
If the left temporal and frontal lobes are damaged, people may be almost entirely unable to understand, speak, or write language. People may be able to utter expletives because the right side of the brain, which is more involved in emotions, is not damaged.
Speech therapists can help people who develop aphasia after brain damage due to disorders that do not cause progressive damage (see Rehabilitation: Aphasia). Therapy is usually started as soon as people are able to participate, but it is helpful even when started much later. Usually, most recovery of language skills occurs during the first 3 months, but it can continue for more than 6 months.
Family members and other people who care for a person with aphasia can become frustrated. Remembering that aphasia is a physical disorder and that a person has little control over it can help.
Dysarthria is loss of the ability to articulate words normally.
Although dysarthria seems to be a language problem, it is really a muscular (motor) problem. It may be caused by damage to the brain stem or to the nerve fibers that connect the outer layer of the cerebrum (cerebral cortex) to the brain stem. The brain stem controls the muscles used in breathing (which help make sounds). The nerve fibers relay information needed to control and coordinate the muscles used to produce speech, including those of the lips, tongue, palate, and vocal cords.
People who have dysarthria produce sounds that approximate what they mean and that are in the correct order. Speech may be jerky, staccato, breathy, irregular, imprecise, or monotonous, depending on where the damage is. Because the ability to understand and use language is not usually affected, most people with dysarthria can read and write normally.
Speech therapy helps some people with dysarthria (see Rehabilitation: Dysarthria).
Apraxia is loss of the ability to do tasks that require remembering patterns or sequences of movements.
Apraxia, an uncommon disability, is usually caused by damage to the parietal or frontal lobes. People with apraxia cannot remember the sequence of movements needed to complete simple skilled or complex tasks. For example, buttoning a button, which consists of a series of steps, may be impossible, even though the hands are physically capable of doing the task. People with verbal (speech) apraxia cannot produce the basic sound units of speech because they cannot initiate, coordinate, or sequence the muscle movements needed to talk.
Some forms of apraxia affect only particular tasks. For example, people may lose the ability to do any one of the following: draw a picture, write a note, button a jacket, tie a shoelace, pick up a telephone receiver, or play a musical instrument.
Occupational therapy (see Rehabilitation: Verbal apraxia) may help some people with apraxia learn to compensate for their losses.
Agnosia is loss of the ability to associate objects with their usual role or function.
Agnosia is relatively rare. Agnosia is caused by dysfunction in the parietal, temporal, or occipital lobes of the brain, where memories of the uses and importance of familiar objects, sights, and sounds are stored. Agnosia often develops suddenly after a head injury or stroke.
Symptoms vary depending on the lobe that is damaged:
Some people with agnosia improve or recover spontaneously. Others must learn to cope with their strange disability. No specific treatment exists.
Amnesia is total or partial loss of the ability to recall experiences or events that happened in the preceding few seconds, in the preceding few days, or further back in time.
Memory loss may involve events that occurred just before the cause of the amnesia (retrograde amnesia) or just after (anterograde amnesia). How far back in time memories are lost varies from a few seconds before the amnesia occurred to a few days, to further back in time, affecting remote (long-term) memories.
The brain's mechanisms for storing information and recalling it from memory are located primarily in the temporal and frontal lobes, but many areas of the brain are involved in memory. Emotions originating from the limbic system can influence the storing of memories and their retrieval. The limbic system includes part of the cerebrum and some structures deep within the brain. Areas that are responsible for alertness and awareness in the brain stem also contribute to memory. Because memory involves many interwoven brain functions, virtually any type of brain damage can result in amnesia.
How amnesia is caused is only partly understood. It may result from a head injury, disorders that reduce the supply of blood or nutrients to the brain (including strokes, seizures, and migraines), brain infection (encephalitis), brain tumors, alcoholism, severe mental stress, or use of certain drugs (such as amphotericin B or lithium).
Depending on the severity of the damage, most amnesias last for only minutes or hours, and most people recover their memory without treatment. However, if brain damage is severe, remote memories can be lost forever. A few people are never able form new memories.
Transient Global Amnesia:
People suddenly but temporarily lose the ability to store new memories and to recall events that happened during the previous few hours to the previous few years. As a result, people become forgetful and confused about time, place, and sometimes the identity of other people.
This type of amnesia may be caused by temporary blockage of the arteries that supply blood to the temporal lobe. Such blockages usually result from atherosclerosis, especially in older people. Transient global amnesia may also be caused by a seizure originating in the temporal lobe. Often, the cause is unknown. In young adults, migraine headaches, which temporarily reduce blood flow to the brain, may cause transient global amnesia.
Most people with transient global amnesia have only one episode in a lifetime. About 10% have repeated episodes. Episodes usually last from 30 minutes to about 12 hours. After an episode, the confusion usually clears quickly, and total recovery is the rule, although people may not remember what happened during the episode.
Treatment depends on the cause.
This unusual form of amnesia may develop in alcoholics and other malnourished people, usually because of a deficiency of thiamin (vitamin B1). The syndrome combines two disorders: an acute confusional state (Wernicke's encephalopathy—see Drug Use and Abuse: Wernicke's Encephalopathy) and an amnesia (Korsakoff's syndrome—see Drug Use and Abuse: Korsakoff's Syndrome). Korsakoff's syndrome develops in about 80% of people with untreated Wernicke's encephalopathy.
Wernicke's encephalopathy causes loss of balance, drowsiness, a tendency to stagger, and eye movement problems in addition to confusion.
Korsakoff's syndrome may initially cause severe memory loss for recent events. More remote memory seems to be less impaired. Thus, people may be able to interact socially and converse coherently even though they cannot remember anything that happened in the preceding few days, months, or years or even in the preceding few minutes. They tend to make things up (confabulate) rather than admit that they cannot remember.
Treatment consists of thiamin and fluids given intravenously. Such treatment can correct Wernicke's encephalopathy, although recovery is usually incomplete. If untreated, Wernicke's encephalopathy can be fatal, but death rarely results in developed countries.
Last full review/revision March 2008 by Juebin Huang, MD, PhD