Processing of memories involves the following:
Registration (taking in new information)
Encoding (forming associations, time stamps, and other processes necessary for retrieval)
Deficits in any of these steps can cause amnesia. Amnesia, by definition, results from impairment of memory functions Memory Loss Memory loss is a common complaint in the primary care setting. It is particularly common among older people but also may be reported by younger people. Sometimes family members rather than the... read more , not impairment of other functions (eg, attention, motivation, reasoning, language), which may cause similar symptoms.
Amnesia can be classified as follows:
Retrograde: Amnesia for events before the causative event
Anterograde: Inability to store new memories after the causative event
Sense-specific: Amnesia for events processed by one sense—eg, visual memory
Amnesia may be
Transient (as occurs after brain trauma)
Fixed (as occurs after a serious event such as encephalitis Encephalitis Encephalitis is inflammation of the parenchyma of the brain, resulting from direct viral invasion or occurring as a postinfectious immunologic complication caused by a hypersensitivity reaction... read more , global ischemia, or cardiac arrest)
Progressive (as occurs with degenerative dementias, such as Alzheimer disease Alzheimer Disease Alzheimer disease causes progressive cognitive deterioration and is characterized by beta-amyloid deposits and neurofibrillary tangles in the cerebral cortex and subcortical gray matter. Diagnosis... read more )
Memory deficits more commonly involve facts (declarative memory) and, less commonly, skills (procedural memory).
Etiology of Amnesias
Amnesia can result from diffuse cerebral impairment, bilateral lesions, or multifocal injuries that impair memory-storage areas in the cerebral hemispheres.
Predominant pathways for declarative memory are located along the medial parahippocampal region and hippocampus as well as in the inferomedial temporal lobes, orbital surface of the frontal lobes (basal forebrain), and diencephalon (which contains the thalamus and hypothalamus). Of these structures, the following are critical:
Nuclei of the basal forebrain
Mediodorsal thalamic nuclei
The amygdaloid nucleus contributes emotional amplifications to memory. The thalamic intralaminar nuclei and brain stem reticular formation stimulate the imprinting of memories. Bilateral damage to the mediodorsal nuclei of the thalamus severely impairs recent memory and the ability to form new memories.
Amnesia may be caused by
Thiamin deficiency (by causing Wernicke encephalopathy Wernicke Encephalopathy Wernicke encephalopathy is characterized by acute onset of confusion, nystagmus, partial ophthalmoplegia, and ataxia due to thiamin deficiency. Diagnosis is primarily clinical. The disorder... read more or Korsakoff psychosis Korsakoff Psychosis Korsakoff psychosis is a late complication of persistent Wernicke encephalopathy and results in memory deficits, confusion, and behavioral changes. Korsakoff psychosis occurs in 80% of untreated... read more ) in patients with chronic alcohol abuse or severe undernutrition
Global brain anoxia or ischemia
Embolic occlusion at the top of the basilar artery, causing ischemia in the anterior medial temporal lobes
Degenerative dementias Dementia Dementia is chronic, global, usually irreversible deterioration of cognition. Diagnosis is clinical; laboratory and imaging tests are usually used to identify treatable causes. Treatment is... read more such as Alzheimer disease Alzheimer Disease Alzheimer disease causes progressive cognitive deterioration and is characterized by beta-amyloid deposits and neurofibrillary tangles in the cerebral cortex and subcortical gray matter. Diagnosis... read more
Various drug intoxications (eg, chronic solvent sniffing, amphotericin B or lithium toxicity)
Psychologic trauma or stress
Wernicke-Korsakoff syndrome is a form of amnesia that combines Wernicke encephalopathy Wernicke Encephalopathy Wernicke encephalopathy is characterized by acute onset of confusion, nystagmus, partial ophthalmoplegia, and ataxia due to thiamin deficiency. Diagnosis is primarily clinical. The disorder... read more and Korsakoff psychosis Korsakoff Psychosis Korsakoff psychosis is a late complication of persistent Wernicke encephalopathy and results in memory deficits, confusion, and behavioral changes. Korsakoff psychosis occurs in 80% of untreated... read more .
Posttraumatic amnesias for the periods immediately before and after concussion or moderate or severe head trauma seem to result from medial temporal lobe injury. Moderate or severe trauma may affect larger areas of memory storage and recall, as can many diffuse cerebral disorders that cause dementia.
Psychologic disturbances of memory (as occurs in dissociative amnesia Dissociative Amnesia Dissociative amnesia is a type of dissociative disorder that involves inability to recall important personal information that would not typically be lost with ordinary forgetting. It is usually... read more ) result from extreme psychologic trauma or stress.
Age-associated memory impairment (benign senescent forgetfulness) refers to the memory loss that occurs with normal aging. People with benign senescent forgetfulness gradually develop noticeable problems with memory, often first for names, then for events, and occasionally for spatial relationships. Benign senescent forgetfulness has no proven relationship to dementia, although some similarities are hard to overlook.
Amnestic mild cognitive impairment (amnestic MCI) may be present in people who have a subjective memory problem, who do worse on objective memory tests, but who otherwise have intact cognition and daily function. People with amnestic MCI are more likely to develop Alzheimer disease than age-matched people without memory problems.
Diagnosis of Amnesias
Bedside neurologic testing
Simple bedside tests (eg, 3-item recall, location of objects previously hidden in the room) and formal neuropsychologic tests Diagnosis (eg, word list learning tests such as the California Verbal Learning Test and the Buschke Selective Reminding Test) can help identify verbal memory loss. Assessment of nonverbal memory is more difficult but may include recall of visual designs or a series of tones.
Clinical findings usually suggest causes and any necessary tests.
Treatment of Amnesias
Treatment directed at the cause
Any underlying disorder or psychologic cause of amnesia must be treated. However, some patients with acute amnesia improve spontaneously. Certain disorders that cause amnesia (eg, Alzheimer disease, Korsakoff psychosis, herpes encephalitis) can be treated; however, treatment of the underlying disorder may or may not lessen the amnesia.
Cholinergic drugs (eg, donepezil) may improve memory slightly and temporarily in patients with Alzheimer disease; these drugs are often also tried when another dementia is the cause. Otherwise, no specific measures can hasten recovery or improve the outcome.
Amnesias have various causes, including traumatic brain injury, degenerative dementias, metabolic disorders, seizures, and psychologic trauma or stress.
Diagnose amnesia clinically using bedside tests (eg, 3-item recall) or formal tests (eg, word list learning tests).
Treat the cause of amnesia.