Amnesia: Memory loss caused by injury or trauma
Amnesia is the inability to form new memories or, in some cases, to remember existing ones. Amnesia occurs when key structures of the brain—such as the hippocampus, which is essential for encoding memories—don’t function properly. Some types of stroke, concussion, chronic alcoholism, seizures, disruption of oxygen supply, or certain kinds of infections (such as viral encephalitis) can cause amnesia. It is also a common side effect of electroconvulsive therapy used to treat major depression.
People with amnesia don’t forget everything, and they can retain their general level of intelligence. They have a normal attention span and can form short-term memories lasting perhaps a few minutes, if not distracted. Their procedural memory—which covers well-established skills such as driving a car or brushing their teeth—remains intact, because retention of these skills doesn’t depend on the hippocampus and surrounding brain structures. The breakdown occurs with acquiring new, long-term declarative memories, which often depend on the hippocampus.
People with anterograde amnesia are unable to form new long-term memories after an injury or the onset of illness. People with retrograde amnesia have difficulty retrieving previously learned information and memories acquired before the onset of the condition that caused the amnesia.
How long amnesia lasts depends on the cause. If the disruption of brain function is temporary (as in a blow to the head that causes a concussion),
most of the lost memory will be restored, although memories formed just before and soon after the injury occurred may be lost forever.
There is also a less common condition called transient global amnesia (TGA). TGA refers to a brief period of time (usually hours) during which a person is unable to retain new information and tends to ask questions repetitively. After recovering, she or he does not remember the events that occurred during the period. TGA is not related to later development of a more serious memory disorder such as Alzheimer’s disease, although the precise cause of the condition is uncertain. Emotionally or physically stressful events such as intense physical activity, sudden immersion in hot or cold water, sexual encounters, or medical procedures can trigger an episode. TGA tends to be more frequent in people who get migraines.
