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Thursday, October 27, 2011

Transient Global Amnesia

serendip

Transient Global Amnesia


Miranda White

A little while ago, my father and grandfather were driving in our car together. All of a sudden, my grandfather said that he was feeling dizzy and thought the beginnings of a migraine were coming on. My grandfather is extremely healthy and has an amazing memory, so my father was shocked when not long after, when grandfather asked where Ruthy, his recently deceased wife, was. When my father reminded him that she had died of cancer last year, my grandfather broke into tears, as if he was being told for the first time. In addition, he couldn't even remember what he had just eaten for dinner or any other events of the day. My father drove him straight to the emergency room, worried that he had perhaps just suffered a minor stoke. By the time that he got to the hospital, he was already beginning to regain some of the memories that had been lost. The doctors reassured him that it was not a stroke, but rather a memory disorder called transient global amnesia.

Transient global amnesia (TGA) is a type of amnesia involving the sudden, temporary disturbance in an otherwise healthy person's memory. The other main kinds of amnesia are called anterograde and retrograde amnesia. Anterograde amnesia is a type of memory loss associated with a trauma, disease, or emotional events. It is characterized by the inability to remember new information. (1) Retrograde amnesia is associated with the loss of distant memories usually preceding a given trauma. (2) In transient global amnesia, generally both distant memories and immediate recall are retained, as are language function, attention, visual-spatial and social skills. However, during the period of amnesia, people suffering from the disorder cannot remember recent occurrences nor can they retain any new visual or verbal information for more than a couple minutes. (3) Though patients generally remember their own identities, they are often very confused by their surroundings and the people around them. They continuously ask questions about events that are transpiring, for example where they are, who is with them, what is happening. However, once they are told, they immediate forget the answer, and repeat the question again. (4)

The period of amnesia can last anywhere from one to twenty-four hours. Some people suffer from a headache, dizziness, and nausea while others have only memory loss. TGA generally affects fifty to eighty-year-old men, about 3.4 to 5.2 people per 100,000 per year. (5) People afflicted with transient global amnesia always recover and can remember the memories that were lost during the episode. (6) Once they regain their memory, some people, such as my grandfather, can recall both the episode and the feeling of not being able to remember. However, others never recover the memories of the attack nor the events immediately before.
The cause of TGA remains in dispute. There is convincing evidence that external emotional stresses, such as sexual intercourse, immersion in cold water, or strenuous physical exertion, can trigger the associated loss of memory. (7) For example, my grandfather suffered from TGA directly after taking his sister to the hospital. TGA may be the result of a transient ischemic attack, a "mini-stroke." Transient ischemic attacks are caused by a temporary interruption of the blood flow to the brain. (8) Another possible cause of transient global amnesia is a basilar artery migraine, a type of migraine caused by the abnormal constriction and dilatation of vessel walls. (9)

Patients suffering from transient global amnesia have undergone medical imaging techniques, for example magnetic resonance imaging (MRI) and positron emission topography (PT), in order to find out what biological changes cause a temporary lapse in memory. The symptoms of transient global amnesia seem to be the result of dysfunction in such regions of the brain as the diencephalon and medial temporal lobes. (4) The diencephalon is composed of the thalamus, epithalamus, subthalamus, and hypothalamus. The thalamus is associated with memory, and changes in its structure have been proven to result in amnesia. (10) Some MRIs have shown evidence of changes in the medial temporal lobes, indicating that patients had suffered from a transient ischemic attack. Nonetheless, many people that have undergone such tests have not shown any changes in the functioning of their brains. (4)

These findings are in line with our neurobiological understanding of memory. Under normal functioning, there are three kinds of memory: working memory, declarative memory, and procedural memory. Working memory allows for short-term recollection, for example, it is responsible for your being able to remember the gist of the sentence you just read. It is associated with the temporary storage of verbal and visual information. The verbal working memory is localized to the frontal regions of the left hemisphere, while spatial working memory involves mainly the right hemispheres. Procedural memory is responsible for cognitive and motor skills, all learned, habitual actions, for example, my ability to type this paper without looking at the keyboard or my ability to ride a bicycle. (12) The anatomical basis for procedural memory appears to be the basil ganglia, thalamus, and the frontal lobes. Declarative memory, associated with the hippocampus, is all experiences and conscious memory, including people, events, objects, facts, figures, and names. The region of the brain termed the medial frontal lobe is particularly responsible for declarative memory function.

There is much evidence proving that damage to the medial frontal lobe, severely affects a person's ability to recall and form long-term memories. The most well-known clinical example involves a patient called H.M. H.M. was afflicted with epilepsy. Surgeons removed both of his medial temporal lobes in an attempt to cure him from his disease. However, in so doing, they profoundly damaged his memory. He could no longer form new memories, though all his memories from before the surgery were retained - in other words he had anterograde amnesia. (11) Therefore, it appears that the lack of functioning and blood supply of the medial temporal lobe produces the symptoms of transient global amnesia, and results in the inability to make and recall autobiographical memories.

Transient global amnesia fortunately has a very positive prognosis - its effects are never permanent and the episodes last for a relatively short period of time. However, the inability to remember can be extraordinarily frightening. It is a natural experiment because it shows fairly clearly that certain parts of the brain are involved with certain kinds of memory. We often see ourselves as unitary beings, but in fact we are made up of many different processes that make up who we are. Although much of the neurobiology associated with memory remains quite mysterious, transient global amnesia helps highlight the particular machinery of our personal narratives.

References

1)Anterograde amnesia

2)HealthyMe Amnesia

3)E Medicine, Transient Global Amnesia

4)Transient Global Amnesia Case Studies

5)Neuroland, TGA

6)Transient Global Amnesia

7)HealingWell, What Happened to Afterglow

8)Transient Ischemic Attack

9)Basilar Artery Migraine Page

10)The Diencephalon

11)Medial Temporal Lobe

12)The Cognitive and Habit Subsystems , A great image of the anatomy of the brain.

1 comment:

  1. People, who suffer from various memory disorders, as well as those who want to prevent the disorders, want to know how to enhance memory.

    Memory Disorder Clinic

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