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.
- Because many areas of the brain are involved in memory, damage almost anywhere in the brain can cause amnesia.
- How amnesia is caused is only partly understood.
- How long memory loss lasts depends on the severity of the damage that caused it.
- Doctors evaluate memory loss by asking simple questions and doing formal tests of memory.
- The cause of memory loss is treated if possible.
Memory loss can be classified as follows:
- Retrograde: Amnesia for the events before the cause of the amnesia
- Anterograde: Amnesia for the events after the cause of the amnesia
- Sense-specific: Amnesia for events processed by one sense, such as hearing
Memory loss involves facts more commonly than learned skills.
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 more distant past (remote, or long-term) memories.
Processing of memories involves the following:
- Taking in new information (registration)
- Linking new information with memories already stored in the brain, with mental pictures, or with other things that can help with retrieval (encoding)
- Recalling the memory (retrieval)
The brain’s mechanisms for storing information and recalling it from memory are located primarily in the temporal lobe and frontal lobe, but many areas of the brain are involved in memory. For example, the hippocampus, located deep within the brain, is involved in the formation of new memories and retrieval of stored memories. The hippocampus is part of the limbic system, which controls the experience and expression of emotions. Thus, the hippocampus helps connect memories to the emotions experienced when the memories form.
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, damage almost anywhere in the brain can result in amnesia.
How amnesia is caused is only partly understood. It may result from
- A nutritional disorder, particularly thiamin deficiency
- A head injury that affects the brain
- Disorders that reduce the supply of blood or nutrients to the brain (including strokes and cardiac arrest)
- A brain infection (encephalitis)
- Dementia such as Alzheimer disease
- Chronic alcohol abuse
- A brain tumor
- Severe mental stress (as occurs in dissociative amnesia)
- Use of certain drugs (such as some antidepressants, muscle relaxants, or opioids, as well as amphotericin B or lithium)
Depending on the cause, amnesia may be
- Temporary, as occurs after head injury
- Permanent and unchanging (as occurs after a serious disorder such as encephalitis or a stroke that affects a large part of the brain)
- Progressive (as occurs with disorders that cause progressive degeneration of the brain, such as Alzheimer disease)
Depending on the severity of the damage, amnesias can last for minutes, hours, or longer. Sometimes memory is lost suddenly but temporarily (called transient global amnesia).
Some people recover their memory without treatment. However, if brain damage is severe, the ability to form new memories may be lost. Affected people are more likely to remember things from the distant past. For example, people may remember their spouse from their first marriage but not the current marriage.
- A doctor's evaluation
- Formal tests of memory
Doctors evaluate memory loss by asking simple questions (such as repeating a list of three items) and by doing formal tests of memory. Results of this evaluation and the person's symptoms often suggest a cause and other tests that may need to be done.
- Treatment of the cause if possible
If a cause of amnesia is identified, it is treated if possible. Such treatment may or may not lessen the amnesia.
© 2020 Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc., Kenilworth, NJ, USA. Merck Manual Disclaimer