Neurological Disorders - Causes Of Memory Impairment: Improving Memory Understanding Age Related Memory Loss
Content provided by the Faculty of the Harvard Medical School
Neurological disorders
When neurons are damaged or cannot function effectively, memory impairment and cognitive decline can occur. Following are some examples of conditions that prevent neurons from functioning normally, causing a variety of mental impairments, including memory loss.
Alzheimer's disease
Alzheimer's disease is far and away the leading cause of dementia, accounting for 50%–70% of all dementia cases. Although it is extremely rare before age 60, Alzheimer's, like dementia in general, becomes much more common with age (see Figure 5). With Alzheimer's disease, there is a substantial loss of neurons along with the appearance of abnormal structures in the brain called "plaques" and "tangles," particularly in regions of the brain associated with memory. Eventually, these abnormalities and the loss of neurons extend into other areas of the cortex.
Figure 5: Risk of Alzheimer's disease with age
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It's important to note that the vast majority of people who develop Alzheimer's disease have no family history or known genetic predisposition. But in a small number of cases, the disease is known to run in families and show a clear hereditary component. In particular, inheriting the ApoE4 gene increases your chances of getting the disorder when you are older. A rare form of early-onset Alzheimer's, which typically appears in the 40s or 50s, is strongly determined by genes and has been traced to abnormalities involving three genes other than ApoE4.
Cerebrovascular disease and stroke
The second leading cause of dementia (after Alzheimer's disease) is cerebrovascular disease. This term refers to injury to the brain caused by hypertension, reduced blood flow, and damage to the system of blood vessels that serves the brain.
A stroke occurs when the blood supply to part of the brain is temporarily interrupted. Brain cells, like tissues elsewhere in the body, need a constant supply of oxygen to live; they get this oxygen from the blood. When blood flow is interrupted during a stroke, some brain cells may die. Even "silent" strokes, or transient ischemic attacks — those that cause few or no obvious persisting symptoms — can lead to dementia. A large study published in TheNew England Journal of Medicine in 2003 found that people who had suffered silent strokes were more than twice as likely to develop dementia within an average of three and a half years as people without strokes. People with silent strokes also exhibited a sharper decline in memory test performance as well as in overall intellectual functioning.
High blood pressure and diabetes increase your risk for stroke. On the other hand, certain lifestyle factors can reduce your risk for all of these illnesses. These include avoiding smoking, maintaining a normal weight, controlling blood pressure, and exercising regularly.
Head trauma
A blow to the head that is strong enough to cause a concussion — a brief alteration of consciousness — can also impair memory. The blow can damage brain cells, or it can stretch or tear the axons, the fine filament "tails" of the neurons that compose the white matter of the brain and spinal cord. Most people who suffer mild concussions recover their memories and other mental functions completely within a few hours or days. More severe head trauma, such as an injury sustained in a high-speed collision, frequently destroys brain tissue and injures nerve fibers throughout the brain. This type of damage is permanent. People who sustain multiple minor injuries from concussions — for example, prizefighters — are prone to later development of dementia and other brain disorders. There is also a growing awareness of reduced cognitive function caused by other sports activities that involve impact to the head.
Other neurological disorders
Among the many illnesses that interfere with brain activities — including memory — are Parkinson's disease, Huntington's disease, multiple sclerosis, epilepsy, CNS Lyme disease, and brain tumors.
Amnesia: Memory loss caused by injury or traumaAmnesia is the inability to form new memories or, in some cases, to remember existing ones. Amnesia occurs when key structures of the brain — such as the hippocampus, which is essential for consolidating memories — don't function properly. The malfunction can be caused by some types of stroke, concussive injury, chronic alcoholism, disruption of oxygen supply, or certain kinds of infections such as viral encephalitis. Amnesia is also a common side effect of electroconvulsive therapy used to treat major depression, although the effect is typically temporary. People with amnesia don't forget everything, and they retain their general level of intelligence. They have a normal attention span and can form short-term memories lasting perhaps a few minutes. Their procedural memory — which covers well-established skills such as driving a car or brushing teeth — remains intact, because retention of these skills doesn't depend on the hippocampus. The breakdown occurs with acquiring new long-term declarative memories, which often depend on the hippocampus. People with anterograde amnesia are unable to form new long-term memories after the moment of an accident or the onset of illness. People with retrograde amnesia have difficulty retrieving previously learned information, memories that had been acquired prior to the onset of the condition that caused the amnesia. The duration of amnesia depends on the cause. If the disruption of brain function is temporary (as in a blow to the head that causes a minor concussion), most of the lost memory will be restored, although memories formed just before and soon after the injury occurred are usually lost forever. There is also a rare condition called transient global amnesia (TGA). TGA refers to a brief period of time (usually hours) during which a person is able to engage in normal behavior but after which she or he does not remember the events that occurred during that period. The cause of TGA is unknown, although damage in the limbic system has been found in some individuals. TGA is more frequent in people who have migraine disorders. TGA does not appear to be related to later development of a more serious memory disorder such as Alzheimer's disease. Certain drugs and medications can also produce TGA-like episodes. |
| Last updated: | January 23, 2007 |
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Medical content reviewed by the Faculty of the Harvard Medical School. Harvard Health Publications, Copyright © 2007 by President and Fellows of Harvard College. All rights reserved. Used with permission of StayWell.
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