Abstract
Amnesia is the total or partial loss or impairment of memory as a result of disease, injury or severe emotional trauma. The individual suffering from amnesia is unable to recall events that happened in the preceding few minutes, months or years.
Amnesia is of two types:
- Anterograde amnesia
- Retrograde amnesia
In aterograde amnesia, the person is unable to recall recent events such as something that happened a few hours ago. In retrograde amnesia, the person cannot recall remote memories.
Amnesia has several causes such as depression, poor concentration, age and physical illness with the Korsakoff’s syndrome being one of the illnesses that can cause amnesia.
Amnesia in Korsakoff’s syndrome
The Korsakoff’s syndrome is caused by a lack of vitamin B1 which is also referred to as thiamine. This vitamin is involved in a number of reactions which provide energy to the neurons in the brain. Lack of thiamine results in lack of these reactions and consequently lack of the end products produced by the reactions. Unavailability of this end products leads to sub-normal functioning of the body leading to symptoms that are characteristic of the syndrome. Vitamin B1 deficiency is common in people who are alcoholic, who undergo dialysis, pregnant women with hyperemesis gravidarum and patients who are on a long term vitamin-free diet after surgery.
This syndrome can occur together with wernicke’s syndrome which is characterized by ataxia, confusion and paralysis of the eye muscles that are responsible for eye movement. When these two occur together they are referred to as wernicke’s – korsakoff’s syndrome.
This condition is characterized by an impaired temporal order recall, anterograde amnesia that is the inability to recall relevant events, confabulation where the patient makes up stories to fill in the missing gaps created by the memory loss. The patient is usually unaware of any memory defect and this is one of the hallmarks of the disease. The mamilliary bodies and the thalamus in the diencephalons are the parts responsible for the amnesia. To be precise, the amnesia is caused by atrophy of the nuclei in the midline of the thalamus.
The diagnosis of anterograde amnesia in Korsakoff’s syndrome is done by examining the mental state of the individual so as to recognize confabulation and assess the ability to absorb new information.
This syndrome is treated by the intra-venous administration of thiamine. Improvement takes months of treatment and even then, the person does not make a full recovery as the memory loss is irreversible.
Unlike the anterograde amnesia that is seen in the Korsakoff’s syndrome, the amnesia associated with bilateral medial-temporal lobectomy is retrograde. This means that the patient who has undergone a bilateral media-temporal lobectomy is unable to recall events that happened before the onset of amnesia. Retrograde amnesia following this lobectomy shows that structures and pathways in the anterior temporal lobes play a very important role in memory for past events.
There has been unresolved issues as to what extent and manner retrograde amnesia affects such facts (semantic) memory and autobiographical (episodic) memory. Available information, however, shows that retrograde amnesia can vary in its severity.
Research findings indicate that damage to the hippocampal formation is responsible for retrograde amnesia and that additional temporal cortical damage produces more extensive and severe retrograde amnesia.
References
John PJ, Pinel. (2008) Biopsychology -7th Edition. Boston, Mass. : Pearson/Allyn and Bacon.
Kirwan CB, Bagley PJ, Galvan VV, Squire LR. From the cover: Detailed recollection of remote autobiographical memory after damage to the medial temporal lobe. PNAS, 2008; 105(7): 2676-2680.