Amnesia classification
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]; Associate Editor(s)-in-Chief: Aditya Govindavarjhulla, M.B.B.S. [3]
Overview
Amnesia can classified based upon various neuropsychopathologies. It is important to consider the etiological, organic, progressive and clinical criteria in amnesia classification. Amnesia can be classified based on the temporal nature of the amnesia, which is anterograde if the person forgets what occurs after the inciting event, or retrograde if the person forgets everything that occured before the inciting event. Amnesia can also be classified based upon the nature, type, and pathophysiology of the amnestic episode.
Classification
Temporal classification of amnesia
Anterograde Amnesia
Anterograde amnesia is a form of amnesia, or memory loss, in which new events are not transferred from short-term memory to long-term memory. This may be a permanent deficit, or it may be temporary, such as is sometimes seen for a period of hours or days after head trauma or for a period of intoxication with an amnestic drug. The deficit makes its sufferers unable to recall an event which occurred only moments earlier when their attention has shifted to something else. Those who have theoretically pure anterograde amnesia are still able to access memories formed before its onset, but they exist in a transient world where anything beyond their immediate attention span disappears from their consciousness permanently. However, theoretically pure anterograde amnesia rarely surfaces: in reality, long-term cases nearly always occur with some degree of retrograde amnesia.
Anterograde amnesia is often informally, but incorrectly, called "short-term memory loss", conjuring up the idea, as in the movie Memento, that it is a problem of short-term memory. For this reason, formal (correct technical or scientific) usage demands the term anterograde amnesia, since the deficit is not in short-term memory, but rather in encoding into more permanent, long-term memory.
"Traveler's amnesia" is a temporary form of anterograde amnesia in which victims may, for instance, realise they have changed planes during a memory gap or discover that they rented a car. This condition is caused by some medications, notably imidazopyridines and benzodiazepines, especially when they are used as sleep aids. Although medical researchers characterize this side effect as "less common",[1], the benzodiazepine triazolam (Halcion) apparently has the greatest chance of inducing traveler's amnesia, whether taken exactly as directed, varying the dosage (say, when coming off the drug too quickly), drinking alcohol, or not getting enough sleep.[1] However, benzodiazepines alprazolam (Xanax) and nitrazepam (Mogadon) are also more likely to be at fault, the former on its own and the latter when the victim is sleep-deprived or in some way changing the dose.[1]
Criminals may use medications with anterograde amnesic effects for date rape. Unbeknownst to the victim, the perpetrator uses drugs such as flunitrazepam, temazepam, and other common substances, usually in a drink, to cause disorientation; incapacitation; unconsciousness; distortions in vision, time, sense, and identity; and an uninhibited state, the hallmark of which is anterograde amnesia. [2]
Amnesia automatism is usually induced by prescription drugs, frequently but not necessarily in association with moderate alcohol intake. Victims have memory gaps for a period shortly after taking the drug concerned, which causes embarrassment and fear for what might have happened. Disinhibited and uncharacteristic behaviour (sometimes together with carrying out quite complex tasks - e.g. cooking and serving a nice meal, but in the nude) is sometimes witnessed during such episodes, which adds further embarrassment and distress.
- Retrograde amnesia is the inability to recall some memory or memories of the past, beyond ordinary forgetfulness.
- These terms are used to categorize patterns of symptoms, rather than to indicate a particular cause or etiology. Both categories of amnesia can occur together in the same patient, and commonly result from drug effects or damage to brain regions most closely associated with episodic/declarative memory: the medial temporal lobes and the hippocampus.
- An example of mixed retrograde and anterograde amnesia may be a motorcyclist unable to recall driving his motorbike prior to his head injury (retrograde amnesia) and an inability to recall the hospital ward where he is told he had conversations with family over the past two days (anterograde amnesia).
Other types of amnesia
- Dissociative Amnesia results from a psychological cause as opposed to direct damage to the brain caused by head injury, physical trauma or disease. Dissociative Amnesia can include:
- Repressed memory refers to the inability to recall information, usually about stressful or traumatic events in a persons' life, such as a violent attack or rape. The memory is stored in the long term memory, but access to it is impaired because of psychological defense mechanisms. People retain the capacity to learn new information and later there may be some partial or complete recovery of memory. This contrasts with anterograde amnesia caused by amnestics such as benzodiazepines or alcohol, where an experience was prevented from being transferred from temporary to permanent memory storage because it was never stored in the first place.
- Dissociative Fugue (formerly Psychogenic Fugue) is also known as fugue state. It is caused by psychological trauma and is usually temporary, unresolved and therefore may return. The Merck Manual defines it as "one or more episodes of amnesia in which the inability to recall some or all of one's past and either the loss of one's identity or the formation of a new identity occur with sudden, unexpected, purposeful travel away from home" [4]. While popular in fiction, it is extremely rare.
- Posthypnotic amnesia is where events during hypnosis are forgotten, or where past memories are unable to be recalled.
- Lacunar amnesia is the loss of memory about one specific event.
- Childhood amnesia (also known as infantile amnesia) is the common inability to remember events from one's own childhood. Whilst Sigmund Freud attributed this to sexual repression, others have theorised that this may be due to language development or immature parts of the brain.
- Post-traumatic amnesia is generally due to a head injury (e.g. a fall, a knock on the head). Traumatic amnesia is often transient, but may cause permanent anterograde, retrograde, or mixed type memory loss. The extent of the period covered by the amnesia is related to the degree of injury and may give an indication of the prognosis for recovery of other functions. Mild trauma, such as a car accident that results in no more than mild whiplash, might cause the occupant of a car to have no memory of the moments just before the accident due to a brief interruption in the short to long-term memory transfer mechanism.
- Transient global amnesia is a well-described medical and clinical phenomenon. This form of amnesia is distinct in that abnormalities in the hippocampus can sometimes be visualized using a special form of magnetic resonance imaging of the brain known as diffusion-weighted imaging (DWI). Symptoms typically last for less than a day and there is often no clear precipitating factor nor any other neurological deficits. The cause of this syndrome is not clear, hypotheses include transient reduced blood flow, possible seizure or an atypical type of migraine. Patients are typically amnestic of events more than a few minutes in the past, though immediate recall is usually preserved.
- Source amnesia is a memory disorder in which someone can recall certain information, but they do not know where or how they obtained the information.
- Memory distrust syndrome is a term invented by the psychologist Gisli Gudjonsson to describe a situation where someone is unable to trust their own memory.
- Blackout phenomenon can be caused by excessive short-term alcohol consumption, resulting in anterograde amnesia.
- Korsakoff's syndrome can result from long-term alcoholism or malnutrition. It is caused by brain damage due to a Vitamin B1 deficiency and will be progressive if alcohol intake and nutrition pattern are not modified. Other neurological problems are likely to be present in combination with this type of Amnesia. Korsakoff's syndrome is also known to be connected with confabulation.