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:

• Hippocampal gyri

• Hypothalamus

• 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 or Korsakoff psychosis) in patients with chronic alcohol abuse or severe undernutrition

• Traumatic brain injury

• Seizures

• Global brain anoxia or ischemia

• Encephalitis

• Embolic occlusion at the top of the basilar artery, causing ischemia in the anterior medial temporal lobes

• Degenerative dementias such as Alzheimer disease

• Hypothalamic tumors

• Psychologic trauma or stress

Wernicke-Korsakoff syndrome is a form of amnesia that combines Wernicke encephalopathy and Korsakoff psychosis.

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) 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. Objective cognitive tests are usually show normal or indicate minimal deficit. 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.

• Bedside neurologic testing

• Neuropsychologic testing

Simple bedside tests (eg, 3-item recall, location of objects previously hidden in the room) and formal neuropsychologic tests (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 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.