Memory is the ability to retain and recall new information. Memory can be subdivided into short-term memory, which involves holding onto information for a minute or less, and long-term memory, which involves holding onto information for over a minute. Long-term memory can be further subdivided into recent memory, which involves new learning, and remote memory, which involves old information. In general, amnestic disorders more frequently involve deficits in new learning or recent memory.
There are a number of terms that are crucial to the understanding of amnestic disorders. In order to retain information, an individual must be able to pay close enough attention to the information that is presented; this is referred to as registration. The process whereby memories are established is referred to as encoding or storage. Retaining information in the long-term memory requires passage of time during which memory is consolidated. When an individual's memory is tested, retrieval is the process whereby the individual recalls the information from memory. Working memory is the ability to manipulate information from short-term memory in order to perform some function. Amnestic disorders may affect any or all of these necessary steps.
The time period affecting memory is also described. Anterograde amnesia is more common. Anterograde amnesia begins at a certain point in time and continues to interfere with the establishment of memory from that point forward in time. Retrograde amnesia refers to a loss of memory for information that was learned prior to the onset of amnesia. Retrograde amnesia often occurs in conjunction with head injury, and may result in erasure of memory of events or information from some time period (ranging from seconds to months) prior to the head injury. Over the course of recovery and rehabilitation from a head
About 7% of all individuals over the age of 65 have some form of dementia that involves some degree of amnesia, as do about 50% of all individuals over the age of 85.
Causes and symptoms
A number of brain disorders can result in amnestic disorders, including various types of dementia (such as Alzheimer's disease), traumatic brain injury (such as concussion), stroke, accidents that involve oxygen deprivation to the brain or interruption of blood flow to the brain (such as ruptured aneurysms), encephalitis, tumors in the thalamus and/or hypothalamus, Wernicke-Korsakoff syndrome (a sequelae of thiamine deficiency usually due to severe alcoholism), and seizures. Psychological disorders can also cause a type of amnesia called "psychogenic amnesia."
A curious condition called transient global amnesia causes delirium (a period of waxing and waning confusion and agitation), anterograde amnesia, and retrograde amnesia for events and information from the several hours prior to the onset of the attack. Transient global amnesia usually only lasts for several hours. Ultimately, the individual recovers completely, with no lasting memory
deficit. The cause of transient global amnesia is poorly understood; researchers are suspicious that it may be due to either seizure activity in the brain or a brief blockage in a brain blood vessel, which causes a brief stroke-like event that completely resolves without permanent sequelae (similar to a transient ischemic attack).
Symptoms of amnestic disorders may include difficulty recalling remote events or information, and/or difficulty learning and then recalling new information. In some cases, the patient is fully aware of the memory impairment, and frustrated by it; in other cases, the patient may seem completely oblivious to the memory impairment or may even attempt to fill in the deficit in memory with confabulation. Depending on the underlying condition responsible for the amnesia, a number of other symptoms may be present as well.
Diagnosis of amnestic disorders begins by establishing an individual's level of orientation to person, place, and time. Does he or she know who he or she is? Where he or she is? The day/date/time? An individual's ability to recall common current events (who is the president?) may reveal information about the memory deficit. A family member or close friend may be an invaluable part of the examination, in order to provide some background information on the onset and progression of the memory loss,
A variety of memory tests can be utilized to assess an individual's ability to attend to information, utilize short-term memory, and store and retrieve information from long-term memory. Both verbal and visual memory should be tested. Verbal memory can be tested by working with an individual to memorize word lists, then testing recall after a certain amount of time has elapsed. Similarly, visual memory can be tested by asking an individual to locate several objects that were hidden in a room in the individual's presence.
Depending on what types of conditions are being considered, other tests may include blood tests, neuroimaging (CT, MRI, or PET scans of the brain), cerebrospinal fluid testing, and EEG testing.
A neurologist and/or psychiatrist may be involved in diagnosing and treating amnestic disorders. Depending on the underlying condition responsible for the memory deficit, other specialists may be involved as well. Occupational and speech and language therapists may be involved in rehabilitation programs for individuals who have amnestic disorders as part of their clinical picture.
In some cases, treatment of the underlying disorder may help improve the accompanying amnesia. In mild cases of amnesia, rehabilitation may involve teaching memory techniques and encouraging the use of memory tools, such as association techniques, lists, notes, calendars, timers, etc. Memory exercises may be helpful. Recent treatments for Alzheimer's disease and other dementias have involved medications that interfere with the metabolism of the brain chemical (neurotransmitter) called acetylcholine, thus increasing the available quantity of acetylcholine. These drugs, such as donepezil and tacrine, seem to improve memory in patients with Alzheimer's disease. Research studies are attempting to explore whether these drugs may also help amnestic disorders that stem from other underlying conditions.
The prognosis is very dependent on the underlying condition that has caused the memory deficit, and on whether that condition has a tendency to progress or stabilize. Alzheimer's disease, for example, is relentlessly progressive, and therefore the memory deficits that accompany this condition can be expected to worsen considerably over time. Individuals who have memory deficits due to a brain tumor may have their symptoms improve after surgery to remove the tumor. Individuals with transient global amnesia can be expected to fully recover from their memory impairment within hours or days of its onset. In the case of some traumatic brain injuries, the amnesia may improve with time (as brain swelling decreases, for example), but there may always remain some degree of amnesia for the events just prior to the moment of the injury.
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Gabrieli, John D., et al. "Memory." In Textbook of Clinical Neurology, edited by Christopher G. Goetz. Philadelphia: W. B. Saunders Company, 2003.
Mesulam, M.-Marsel. "Aphasias and Other Focal Cerebral Disorders." In Harrison's Principles of Internal Medicine, edited by Eugene Braunwald, et al. New York: McGraw-Hill Professional, 2001.
Rosalyn Carson-Dewitt, MD