2.4 AMNESTIC DISORDERS
Victor et al. (1971) defines the amnestic syndrome as
an abnormal mental state in which memory and learning are affected out of
all proportion to other cognitive functions in an otherwise alert and
responsive patient. Amnestic disorders have been broadly classified into
transient amnesias and persistent memory disorders. Transient amnesias
include transient global amnesia, transient epileptic amnesia,
posttraumatic amnesia and alcoholic blackouts, while persistent memory
disorders include Korsakoff syndrome and amnestic syndromes that follow
herpes encephalitis, severe hypoxia, vascular disorders or head injury (Kopelman,
2000).
Transient global amnesia (TGA)
TGA is a syndrome of
severe, forgetful confusion characterized by memory loss and total
disorientation except for self-identity, followed by gradual resolution
within 24 hours. The memory loss includes a profound impairment of
long-term episodic memory, while working memory is spared. TGA presumably
results from transient dysfunction of the temporal lobes or diencephalons.
Commonly, the presumed cause is transient ischemia (Plum, 1992). TGA may
arise during coughing, exercise, sexual intercourse, driving, medical
procedures or after severe emotional stress; and other known precipitants
include seizures, migraine, and medications like sildenafil (Ullrich and
Urion, 2003; Gandolfo et al., 2003).
Posttraumatic amnesia
TBI is the common cause of amnestic
syndromes seen in clinical practice (Cummings and Mega, 2003). Duration of
the coma after TBI is the best predictor of the severity of posttraumatic
memory and cognitive deficits, and duration of post-traumatic amnesia is
correlated with posttraumatic intellectual disorders (Levin, 1989).
Alcoholic blackouts
Alcoholic blackouts are among
the most frequently reported symptoms in the progression of alcoholism,
and are defined as the temporary, complete inability to form long-term
memory as a result of a high blood alcohol level. Goodwin et al. (1969)
described en bloc and fragmentary types of blackouts. The former are
instances of full and permanent memory loss for intoxicated events,
whereas the latter are episodes for which retrieval of experiences is
facilitated by provision of cues. The risk of blackouts is associated with
increased alcohol consumption, age of drinking onset, the number of
alcoholic relatives, and the individual's capacity to control drinking
behavior (Jennison and Johnson, 1994). Nelson et al. (2004) found evidence
of a substantial genetic contribution to liability for alcohol-induced
blackouts. Hypoglycemia may be another contributory factor to blackouts (Kopelman,
2000).
Korsakoff syndrome
Many cases of the Korsakoff
syndrome are diagnosed following an acute Wernicke encephalopathy, but the
disorder can also have an insidious onset and such cases are more likely
to come to the attention of the psychiatrists (Kopelman, 2000). Patients
with the syndrome have difficulty learning new information and usually
have a retrograde amnesia that extends 3-20 years prior to the onset of
the amnesia. Typically, patients remain amnestic for 1-3 months after
onset and then begin to recover over a 1- to 10- month period. A
personality change, usually emotional indifference or apathy, frequently
accompanies the amnesia (Cummings and mega, 2003).
Management of amnesia
It has been observed that the
natural history of many of the amnestic disorders, regardless of the
cause, entails a gradual lessening of the severity of the retrograde and
anterograde amnesia with time (Burke, 2001). Memory impairment in the
Korsakoff syndrome has shown mild response to treatment with clonidine,
methysergide, and methylphenidate (McEntee and Crook, 1990). Physostigmine
has been found useful in cases with amnestic disorder due to herpes
simplex encephalitis (Catsman-Berrevoets et al., 1986, Peters and Levin,
1977). Psychosocial approaches used in the management of amnesia are
categorized as restorative therapies which attempt to bring back normal
function by repeated training, and compensatory approaches which attempt
to help the person overcome the deficit by the use of other intact
cognitive abilities (Burke, 2001). |
