Amnesia

Amnesia (from Greek Ἀμνησία) is a condition in which one's memory is lost. The causes of amnesia have traditionally been divided into certain categories. Memory appears to be stored in several parts of the limbic system of the brain, and any condition that interferes with the function of this system can cause amnesia. Functional causes are psychological factors, such as mental disorder, post-traumatic stress or, in psychoanalytic terms, defence mechanisms. Amnesia may also appear as spontaneous episodes, in the case of transient global amnesia.^[1]

• Anterograde amnesia, is the loss of long-term memory, the loss or impairment of the ability to form new memories through memorization. People may find themselves constantly forgetting a piece of information, people or events after a few seconds or minutes, because the data does not transfer successfully from their conscious short-term memory into permanent long-term memory. Primarily in older men, transient global amnesia causes severe loss of memory for minutes or hours.
• Retrograde amnesia, the loss of pre-existing memories to conscious recollection, beyond an ordinary degree of forgetfulness. This type of amnesia first targets the patient's most recent memories. The amount of memories lost depends on the severity of the case. The person may be able to memorize new things that occur after the onset of amnesia (unlike in anterograde amnesia), but is unable to recall some or all of their life or identity prior to the onset. The effects of retrograde amnesia (RA) occurs on fact memory to a lower degree than it occurs on autobiographical memory, which can be affected over the whole lifespan of the patient by RA.^[2][3] There have also been some cases where retrograde amnesia is a result of hypoglycemia in insulin-dependent diabetic patients.^[4]

However, there are different types of memory, for example procedural memory (i.e. automated skills) and declarative memory (personal episodes or abstract facts), and often only one type is impaired. For example, a person may forget the details of personal identity, but still retain a learned skill such as the ability to play the piano.

In addition, the terms are used to categorize patterns of symptoms rather than to indicate a particular cause (etiology). Both categories of amnesia can occur together in the same patient, and commonly result from drug effects or damage to the brain regions most closely associated with episodic memory: the medial temporal lobes and especially 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), nor can he recall the hospital ward where he is told he had conversations with family over the next two days (anterograde amnesia).

The most influential case on anterograde amnesia was H.M.. This patient had to undergo bi-lateral removal of the hippocampus and amygdala in order to treat his severe epilepsy. In fact, H.M. was reported having up to 10 seizures per day. Although the epilepsy was fixed through surgery H.M. ended up with anterograde amnesia. ^[5]

The effects of amnesia can last a long time even after the condition has passed. Some sufferers claim that their amnesia changes from a neurological condition to also being a psychological condition, whereby they lose confidence and faith in their own memory and accounts of past events.

Another effect of some forms of amnesia may be impaired ability to imagine future events. A 2006 study showed that future experiences imagined by amnesiacs with bilaterally damaged hippocampus lacked spatial coherence, and the authors speculated that the hippocampus may bind different elements of experience together in the process of re-experiencing the past or imagining the future.^[6]

Discovery of amnesia

French psychologist named Theodule-Armand Ribot first observed that patients tend to lose recent memories because of retrograde amnesia. Because of this, medical experts started to call the gradients of memory loss as Ribot gradients. Aside from this particular type of amnesia, there is also anterograde amnesia, a medical condition where patients are unable to turn their immediate memory into long-term memory.^[7]

Types and causes of amnesia

• Post-traumatic amnesia is generally due to a head injury (example: a fall, a knock on the head). Traumatic amnesia is often transient, but may be permanent of either anterograde, retrograde, or mixed type. 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/long-term memory transfer mechanism. The sufferer may also lose knowledge of who people are. Having longer periods of amnesia or consciousness after an injury may be an indication that recovery from remaining concussion symptoms will take much longer. ^[8]
• Dissociative amnesia results from a psychological cause as opposed to direct damage to the brain caused by head injury, physical trauma or disease, which is known as organic amnesia. Dissociative amnesia can include:
  • Repressed memory refers to the inability to recall information, usually about stressful or traumatic events in persons' lives, such as a violent attack or disaster. The memory is stored in long term memory, but access to it is impaired because of psychological defense mechanisms. Persons retain the capacity to learn new information and there may be some later partial or complete recovery of memory. This contrasts with e.g. anterograde amnesia caused by amnestics such as benzodiazepines or alcohol, where an experience was prevented from being transferred from temporary to permanent memory storage: it will never be recovered, because it was never stored in the first place. Formerly known as "Psychogenic Amnesia".
  • 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." ^[9] While popular in fiction, it is extremely rare.
  • Post-hypnotic amnesia is where events during hypnosis are forgotten, or where past memories are unable to be recalled. The failure to remember those events are induced by suggestions made during the hypnosis. ^[10]
• 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. Sigmund Freud notoriously attributed this to sexual repression, while modern scientific approaches generally attribute it to aspects of brain development or developmental psychology, including language development
• 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. The hypotheses of the syndrome includes 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, with the amnesia being of the anterograde type. See Effects of alcohol on memory for a discussion of alcohol's disruption of memory formation.
• 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.It should be noted that the person's short term memory may appear to be normal, however the person may have a difficult time attempting to recall a past story, or with unrelated words, as well as complicated patterns

^[11]

• Drug-induced amnesia is intentionally caused by injection of an amnesiac drug to help a patient forget surgery or medical procedures, particularly those not performed under full anesthesia, or likely to be particularly traumatic. Such drugs are also referred to as "premedicants." Most commonly a 2'-halogenated benzodiazepine such as midazolam or flunitrazepam is the drug of choice, although other strongly amnestic drugs such as propofol or scopolamine may also be used for this application. Memories of the short time frame in which the procedure was performed are permanently lost or at least substantially reduced, but once the drug wears off, memory is no longer affected.
• Electroconvulsive therapy in which seizures are electrically induced in patients for therapeutic effect can have acute effects including both retrograde and anterograde amnesia.
• Prosopamnesia is the inability to recognize or remember faces, even in the presence of intact facial recognition capabilities. Both acquired and inborn cases have been documented.
• Situation-Specific amnesia can arise in a variety of circumstances (e.g., committing an offence, child sexual abuse) resulting in PTSD. It has been claimed that it involves a narrowing of consciousness with attention focused on central perceptual details and/or that the emotional or traumatic events are processed differently from ordinary memories.
• Anterograde amnesia refers to the inability to create new memories due to head trauma. That is not the only cause, it is also caused by Wernicke-Korsakoff Syndrome, cerebrovascular events, anoxia and other substances. The two brain regions related with this condition are medial temporal lobe and medial diencephalon. Anterograde amnesia can't be treated with pharmacological methods due to neuronal loss.^[12] However, treatment exists in educating patients to define their daily routines and after several steps they begin to benefit from their procedural memory. Likewise, social and emotional support is critical to improving quality of life for anterograde amnesia sufferers.^[13]
• Retrograde amnesia refers to inability to recall memories before onset of amnesia. One may be able to encode new memories after the incident. Retrograde is usually caused by head trauma or brain damage to parts of the brain besides the hippocampus. The hippocampus is responsible for encoding new memory. Episodic memory is more likely to be affected that semantic memory. The damage is usually caused by head trauma, cerebrovascular accident, stroke, tumor, hypoxia, encephalitis, or chronic alcoholism. People suffering from retrograde amnesia are more likely to remember general knowledge rather than specifics. Recent memories are less likely to be recovered but older memories will be easier to recall due to strengthening over time.^[14] Retrograde amnesia is usually temporary and can be treated by exposing them to memories from the loss. ^[15]

See also

References