Amnesia refers to the loss of memory. Memory loss may result from two-sided (bilateral) damage to parts of the brain vital for memory storage, processing, or recall (the limbic system, including the hippocampus in the medial temporal lobe).
Amnesia can be a symptom of several neurodegenerative diseases; however, people whose primary symptom is memory loss (amnesiacs), typically remain lucid and retain their sense of self. They may even be aware that they suffer from a memory disorder.
People who experience amnesia have been instrumental in helping brain researchers determine how the brain processes memory. Until the early 1970s, researchers viewed memory as a single entity. Memory of new experiences, motor skills, past events, and previous conditioning were grouped together in one system that relied on a specific area of the brain.
If all memory were stored in the same way, it would be reasonable to deduce that damage to the specific brain area would cause complete memory loss. However, studies of amnesiacs counter that theory. Such research demonstrates that the brain has multiple systems for processing, storing, and drawing on memory.
Causes and symptoms
Amnesia has several root causes. Most are traceable to brain injury related to physical trauma, disease, infection, drug and alcohol abuse
, or reduced blood flow to the brain (vascular insufficiency). In Wernicke-Korsakoff syndrome, for example, damage to the memory centers of the brain results from the use of alcohol or malnutrition
. Infections that damage brain tissue, including encephalitis
and herpes, can also cause amnesia. If the amnesia is thought to be of psychological origin, it is termed psychogenic.
Memory loss may result from bilateral damage to the limbic system of the brain responsible for memory storage, processing, and recall.
(Illustration by Electronic Illustrators Group).
There are at least three general types of amnesia:
- Anterograde. This form of amnesia follows brain trauma and is characterized by the inability to remember new information. Recent experiences and short-term memory disappear, but victims can recall events prior to the trauma with clarity.
- Retrograde. In some ways, this form of amnesia is the opposite of anterograde amnesia: the victim can recall events that occurred after a trauma, but cannot remember previously familiar information or the events preceding the trauma.
- Transient global amnesia. This type of amnesia has no consistently identifiable cause, but researchers have suggested that migraines or transient ischemic attacks may be the trigger. (A transient ischemic attack, sometimes called "a small stroke," occurs when a blockage in an artery temporarily blocks off blood supply to part of the brain.) A victim experiences sudden confusion and forgetfulness. Attacks can be as brief as 30-60 minutes or can last up to 24 hours. In severe attacks, a person is completely disoriented and may experience retrograde amnesia that extends back several years. While very frightening for the patient, transient global amnesia generally has an excellent prognosis for recovery.
In diagnosing amnesia and its cause, doctors look at several factors. During a physical examination
, the doctor inquires about recent traumas or illnesses, drug and medication history, and checks the patient's general health. Psychological exams may be ordered to determine the extent of amnesia and the memory system affected. The doctor may also order imaging tests such as magnetic resonance imaging
(MRI) to reveal whether the brain has been damaged, and blood work to exclude treatable metabolic causes or chemical imbalances.
Treatment depends on the root cause of amnesia and is handled on an individual basis. Regardless of cause, cognitive rehabilitation
may be helpful in learning strategies to cope with memory impairment.
Some types of amnesia, such as transient global amnesia, are completely resolved and there is no permanent loss of memory. Others, such as Korsakoff syndrome, associated with prolonged alcohol abuse or amnesias caused by severe brain injury, may be permanent. Depending on the degree of amnesia and its cause, victims may be able to lead relatively normal lives. Amnesiacs can learn through therapy to rely on other memory systems to compensate for what is lost.
Amnesia is only preventable in so far as brain injury can be prevented or minimized. Common sense approaches include wearing a helmet when bicycling or participating in potentially dangerous sports, using automobile seat belts, and avoiding excessive alcohol or drug use. Brain infections should be treated swiftly and aggressively to minimize the damage due to swelling. Victims of strokes, brain aneurysms, and transient ischemic attacks should seek immediate medical treatment.
Squire, Larry R., and Stuart M. Zola. "Amnesia, Memory and Brain Systems." Philosophical Transactions of the Royal Society of London, Series B 352 (1997): 1663.
— The memory system that links perceptual information to the proper motor response. For example, Ivan Pavlov conditioned a dog to salivate when a bell was rung.
— The memory system that links perceptual information to an emotional response. For example, spotting a friend in a crowd causes a person to feel happy.
— Conscious recall of facts and events that is classified into episodic memory (involves time and place) and semantic memory (does not involve time and place). For example, an amnesiac may remember he has a wife (semantic memory), but cannot recall his last conversation with her (episodic memory).
Magnetic resonance imaging (MRI)
— MRI uses a large circular magnet and radio waves to generate signals from atoms in the body. These signals are used to construct images of internal structures.
Motor skill learning
— This memory system is associated with physical movement and activity. For example, learning to swim is initially difficult, but once an efficient stroke is learned, it requires little conscious effort.
— The memory system that joins perceptual and conceptual representations.
— The memory system that relates to the task at hand and coordinates recall of memories necessary to complete it.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.
pathologic impairment of memory. Amnesia is usually the result of physical damage to areas of the brain from injury, disease, or alcoholism. Psychologic factors may also cause amnesia; a shocking or unacceptable situation may be too painful to remember, and the situation is then retained only in the subconscious mind. The technical term for this is repression
. (See also dissociative disorders
Rarely is the memory completely obliterated. When amnesia results from a single physical or psychologic incident, such as a concussion suffered in an accident or a severe emotional shock, the victim may forget only the incident itself; the victim may be unable to recall events occurring before or after the incident or the order of events may be confused, with recent events imputed to the past and past events to recent times. In another form, only certain isolated events are lost to memory.
Amnesia victims usually have a good chance of recovery if there is no irreparable brain damage. The recovery is often gradual, the memory slowly reclaiming isolated events while others are still missing. Psychotherapy may be necessary when the amnesia is due to a psychologic reaction.
impairment of memory for events occurring after the onset of amnesia. Unlike retrograde amnesia
, it is the inability to form new memories.
loss of memory for all events during a discrete, specific period of time. Called also localized amnesia
continuous amnesia loss of memory for all events after a certain time, continuing up to and including the present.
the most common of the dissociative disorders
; it is usually a response to some stress, such as a threat of injury, an unacceptable impulse, or an intolerable situation. The patient suddenly cannot recall important personal information and may wander about without purpose and in a confused state.
Persons with a dissociative disorder may at times forget what they are doing or where they are; when they regain self-awareness, they cannot recall what has taken place. A less severe form than amnesia is sleepwalking. Dissociative disorders are very likely an attempt by the mind to shield itself from the anxiety caused by an unresolved conflict. The patient, upon encountering a situation that may be symbolic of this inner conflict, goes into a form of trance to avoid experiencing the conflict.
generalized amnesia loss of memory encompassing the individual's entire life.
lacunar amnesia partial loss of memory; amnesia for certain isolated experiences.
inability to recall events that occurred prior to the episode precipitating the disorder. Unlike anterograde amnesia
, it is the loss of memories of past events.
selective amnesia loss of memory for a group of related events but not for other events occurring during the same period of time.
transient global amnesia a temporary episode of short-term memory loss without other neurological impairment.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.