While it's been held that elephants never forget, humans aren't so lucky. As we age, the neurons in our brains shrink, and the chemicals that transmit messages between these neurons are emitted in lesser quantities. As a result, everyone will likely experience the momentary panic of not knowing where the car keys are or realizing that an important appointment was missed. This is known as age-associated memory impairment.
When this cognitive decline begins, many may start to worry that they have Alzheimer's or some other dementia. But dementia is a significant mental decline that persists over time, and while memory loss is the most distinctive symptom, several other parts of the brain are affected as well. Dementia isn't diagnosed until memory loss is present in conjunction with one or more of these limited brain functions:
- Aphasia, or loss of the ability to produce or understand language
- Apraxia, or the inability to make certain movements despite a healthy body
- Agnosia, or problems recognizing familiar persons and objects, even though the senses are functioning
- Executive dysfunction, or is an inability to plan, organize or reason
These symptoms were often thought to be the effects of mental illness and carried a shameful stigma. The effects can be off-putting and uncomfortable for those around the person exhibiting them. But these symptoms are caused by a variety of disorders plaguing the brain. The most common disorder that leads to dementia is Alzheimer's disease, which accounts for approximately 50 to 70 percent of all dementia diagnoses [source: Agronin].
As with other dementias, Alzheimer's disease causes the brain to degenerate as nerve cells are destroyed and the connections between them reduced. It strikes first in the hippocampus, where recent memories are stored, and progresses through the brain until it affects judgment, speech patterns and eventually, all forms of memory. Alzheimer's is distinguished by the presence of neurofibrillary tangles and amyloid plaques in the brain, but doctors aren't sure if these abnormalities are the cause or a byproduct of the disease. Doctors also aren't completely sure what causes Alzheimer's, though genetics likely plays a role, particularly abnormalities in a protein called apolipoprotein E. After receiving a diagnosis of Alzheimer's disease, the patient could continue to live for up to 10 years, though 3 to 5 years is more common [source: Beers]. You can read more about Alzheimer's in How Alzheimer's Disease Works.
About 30 percent of patients with Alzheimer's disease also have vascular dementia. Vascular dementia is the second most common cause of dementia, accounting for slightly more than 20 percent of dementia cases [source: Judd]. Vascular dementia is usually the result of damage done to the brain by a stroke, which occurs when a blood clot or a hemorrhage cuts off the brain's blood supply. Sometimes just one stroke is enough to cause damage, which is known as single-infarct dementia. More commonly, however, each small stroke isn't damaging, but the cumulative effect destroys so much brain tissue that memory, language and other cognitive functions are affected. This is known as multi-infarct dementia.
Not every stroke results in dementia, but when one does, the symptoms of dementia appear soon afterwards. Patients with vascular dementia may improve a bit between strokes, but worsen suddenly if another one occurs. Vascular dementia is similar to Alzheimer's in that memory loss and the ability to complete basic tasks are compromised, but factors such as judgment and personality aren't as affected. Vascular dementia also includes a few non-cognitive symptoms, including loss of bladder or bowel control and weakness or paralysis in the face or on one side of the body.
Unfortunately, we're not done with the causes of dementia yet, so turn the page to find out what other disorders can plague the brain.