Amnesia

Amnesia is total or partial loss of the ability to recall experiences or events that happened in the preceding few seconds (immediate memory), in the preceding few seconds to few days (intermediate memory), or further back in time (remote or long-term memory).
The causes of amnesia are only partly understood. Damage to the brain can produce memory loss of events that occurred just before (retrograde amnesia) or just after (posttraumatic amnesia) the damage occurred. Depending on the severity of the damage, most amnesias last for only minutes or hours and disappear without treatment. However, with severe brain damage, amnesia can be permanent.
Learning requires memory. The brain's mechanisms for storing information and recalling it from memory are located primarily in the temporal and frontal lobes. Emotions originating from the brain's limbic system can influence both the storing of memories and their retrieval. The limbic system is also closely connected to areas responsible for alertness and awareness. Because memory involves many interwoven brain functions, virtually any type of brain damage can result in memory loss.
Transient global amnesia is a sudden temporary loss of the ability to store new memories, resulting in forgetfulness and confusion 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 in people with atherosclerosis, especially older people. It 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 can last from 30 minutes to about 12 hours. The amnesia may totally disorient a person and block recall of events that happened during the previous few years. After an episode, the confusion usually clears quickly, and total recovery is the rule.
The Wernicke-Korsakoff syndrome, an unusual form of amnesia, may develop in alcoholics and other malnourished people. The syndrome is a combination of two disorders: an acute confusional state (Wernicke's encephalopathy) and an amnesia (Korsakoff's syndrome). Korsakoff's syndrome accompanies Wernicke's encephalopathy in about 80% of people.
Both Wernicke's encephalopathy and Korsakoff's syndrome can be caused by a deficiency of thiamin (vitamin B1), which is necessary for the body to process carbohydrates. Drinking large amounts of alcohol without eating foods that contain thiamin decreases the brain's supply of this vitamin. In malnourished people (who do not consume enough thiamin), Wernicke's encephalopathy can be suddenly triggered by eating a large carbohydrate- rich meal (such as spaghetti), drinking highly sweetened liquids, or receiving a large amount of glucose (a sugar) intravenously to treat dehydration.
The Wernicke-Korsakoff syndrome may also result from damage to the temporal lobe by an injury, a stroke, a tumor, or a brain infection (encephalitis) . In addition to confusion, symptoms of Wernicke's encephalopathy include loss of balance, drowsiness, a tendency to stagger, and eye movement problems, such as paralysis of eye movements, double vision, and a rapid movement of the eyes in one direction followed by a slower drift back to the original position (nystagmus). Memory loss is often severe initially.
Korsakoff's syndrome may be permanent if it follows severe or repeated episodes of encephalopathy or of severe symptoms after alcohol withdrawal (delirium tremens). Severe memory loss is often accompanied by agitation and delirium. In Korsakoff's syndrome, immediate memory is retained, but intermediate memory and memory for relatively remote events (of previous weeks or months) are lost. However, more remote memory sometimes survives.
People with chronic Korsakoff's syndrome 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. Bewildered by the lack of memory, they tend to make things up (confabulate) rather than admit that they cannot remember. Then they cannot distinguish real memories from the ones they have made up. People with Korsakoff's syndrome are highly suggestible; for example, they can be led to say they see things that are not there. They can read the same magazine over and over as if for the first time.
For alcoholics, thiamin is given intravenously as soon as possible after symptoms begin. Such treatment can correct Wernicke's encephalopathy. Untreated, Wernicke's encephalopathy can be fatal. For this reason, alcoholics who go to the hospital for any reason are promptly given thiamin. Prompt treatment with thiamin also prevents Korsakoff's syndrome, which develops if treatment is delayed. Thiamin does not correct Korsakoff's syndrome but is necessary because Korsakoff's syndrome can be fatal unless treated promptly. Sometimes Wernicke-Korsakoff syndrome gradually resolves if alcohol is avoided and a healthy diet is consumed. However, when Wernicke-Korsakoff syndrome is due to temporal lobe damage, recovery is gradual and may be incomplete.
[Merck]