Alzheimer's: Causes and Risk Factors
Learn about the causes of Alzheimer's and how the disease affects the brain.
The brain is a complex system much a like a computer. There's information coming in, information being processed and turned into memories, and information going out. All of this is done by means of hundreds of billions of nerve cells, each capable of branching out and connecting with hundreds of thousands of other nerve cells. But, unlike wiring in a computer, nerve cells don't touch each other. Instead, dozens of chemical messengers communicate across tiny gaps called synapses. In Alzheimer's, something goes wrong with this system, although researchers have yet to identify exactly what the malfunction is.
How does Alzheimer's destroy the brain?
In a brain affected by Alzheimer's, there are abnormal bundles of twisted fibers in the nerve cells - called tangles, and abnormal protein deposits surrounding the nerve cells - called plaques. The combination of tangles and plaques seems to interfere with brain function, especially memory. Although many older people develop some plaques and tangles, people with Alzheimer's have them to a much greater extent.
Searching for causes
- Abnormal proteins. Amyloid is a protein normally produced in the body. But, in people with Alzheimer's, there's an abnormal buildup of this protein. The buildup forms plaques around the nerve cells interfering with their function. This alone may cause Alzheimer's-related brain tissue damage. Or, there may be a defect in the protein, or an abnormal response to the protein, either of which may trigger an inflammatory response leading to damaged brain cells. It's possible a combination of these and other factors are at play in the development of Alzheimer's.
- Faulty genes. A genetic defect may be responsible for the development of an unusual form of protein. Forty-percent of people with Alzheimer's have a protein called ApoE4. But, not everyone with the gene for ApoE4 develops Alzheimer's and not everyone with Alzheimer's has the gene, so it can't be the only cause. Other genes have also been implicated. For example, research has found that people with Down Syndrome develop tangles and plaques in their brains similar to those found in brains with Alzheimer's.
- Defective chemical messengers. One nerve cell communicates with another by releasing chemicals called neurotransmitters. People with Alzheimer's have a lower than normal level of a neurotransmitter called acetylcholine. Other neurotransmitters have also been shown to be lower in some people with Alzheimer's, but none to the same degree.
- Abnormal glucose metabolism. Nerve cells depend on glucose, a sugar molecule, for energy. When the metabolism of glucose is disturbed, the cells may not be able to manufacture neurotransmitters such as acetylcholine, or they may react abnormally to such chemical messengers. Eventually they die. Researchers are trying to find out whether the abnormal glucose metabolism seen in people with Alzheimer's is a cause or effect of the disease.
- Calcium overload. Nerve cells need calcium to transmit signals. But, too much calcium can kill a cell; so calcium levels are normally regulated by various mechanisms in the body. A breakdown in any one of the mechanisms could cause the degeneration of nerve cells seen in Alzheimer's.
- Environmental toxins and viruses. Environmental factors that have been studied include aluminum, zinc, food-borne poisons and viruses.
Risk factors for Alzheimer's are divided into ones that are well established and others that are possible, but still being studied. None of the risk factors predict development of the disease. They simply suggest an increased risk.
- Increasing age. Simply put, the older you are, the greater the risk.
- Family history and genetics. Early onset Alzheimer's is thought to be caused by a defective gene passed down through the family. There may also be a genetic link between Down syndrome and Alzheimer's. The increased presence of the unusual protein called ApoE4 gene can be detected by laboratory tests.
- Environmental factors. Environmental factors probably influence any genetic predisposition. Various studies have linked Alzheimer's to specific environmental factors, such as zinc and food borne poisons.
- Head injury. The Alzheimer's Association says that there appears to be a strong link between serious head injury and future risk of Alzheimer's. However, that link may only apply to people who already have a genetic susceptibility to Alzheimer's disease.
- Vascular disease. Heart disease, stroke and high blood pressure all damage blood vessels that carry oxygen and nutrients to the brain and may contribute to the development of Alzheimer's. More and more studies are finding associations between cardiovascular diseases and Alzheimer's.
- Gender and education. Female gender and low education level may play a role in the likelihood of developing Alzheimer's.
Not one cause but many?
In the end, Alzheimer's may turn out to be a medical whodunit with many culprits all interacting and influencing each other, but not one of them causing the disease on its own. Whatever the cause or combination of causes, researchers have a way to go before solving the mystery.