AMNESIA
The term “amnesia” is used generally to describe impairment or loss of memory. It is often subclassified as being either retrograde or anterograde. With retro- grade amnesia, memories that had previously been stored are no longer available. With anterograde amnesia, information occurring in real time does not enter long-term storage. Memory is a complex process comprising three different functions: (1) registration of information, (2) storage by reinforcement, and (3) retrieval.
Registration of Information. If information is not registered initially, it
will not be remembered later. Failure to register is the explanation for
absentminded- ness, probably the most common abnormality of memory.
Storage by Reinforcement. Repetition of information to be remembered or
relating such information to other factors or events enhances later recall.
Retrieval. To recall the information, a person must search
the “memory bank,” where it has been stored. Inability to recall information on
request could result from a defect in any of the three aspects of memory
function.
 |
| Plate 2-21 |
The key anatomic regions for
registration and storage of memory traces are in an area often referred to as
the Papez circuit, in which the fornix connects the hippo- campus to the
mammillary bodies, which, in turn, are connected to the anterior nuclei of the
thalamus by the mammillothalamic tract. The anterior thalamic nuclei project to
the cingulate gyri, which then connect with the hippocampus, completing the
circuit. The memory system is primarily cholinergic. The left medial temporal
lobe is most concerned with verbal memory and the right temporal lobe with
visual recall.
The prototype of amnestic disorders
is Korsakoff syndrome, seen in chronic alcoholism and other states of
vitamin B deficiency. This syndrome affects the medial thalamus and mammillary
bodies and is characterized by an inability to record new memories and recall
events of the recent past. Some patients confabulate to fill in gaps in their
memory. Any bilateral destructive lesion of the thalami and medial temporal
lobes can cause a similar syndrome. Such lesions include gliomas that spread
bilaterally over the fornix and splenium of the corpus callosum; bilateral
posterior cerebral artery infarctions, often caused by embolism of the top of the
basilar artery; and herpes simplex encephalitis, a viral disease with
predilection for temporal lobe damage. Lesions within the Papez circuit affect
the “memory bank.” The patient is unable to recall items despite being given
cues or being asked to select the correct item to be recalled from a group of
alternatives. Uni- lateral lesions of the left medial temporal lobe and
thalamus can produce amnesia that may last up to 6 months.
Head trauma often disrupts functions
of memory. The severity of a head injury or concussion is often classified by the degree of retrograde
amnesia that results; the longer the period of retrograde amnesia, the
worse the injury. Head injury victims may also experience a period of
anterograde amnesia.
Transient Global Amnesia. Total global amnesia is a particularly common
memory disorder. In this benign syndrome, the patient seems bewildered and asks
repetitive questions about the environment and activities, and, despite
appropriate replies, asks the same questions moments
later. The patient cannot form new memories and is often unable to recall
events of the past days, months, and even years. Speech, reading, writing,
calculations, drawing, and copying are normal, as are the results of the rest
of the neurologic examination. Behavior and memory usually return to normal
within 24 hours, but the patient is never able to recall events during the
period of amnesia. Such attacks may recur, but the
cause of the syndrome remains obscure.
