Last blog, we talked about head trauma as a risk factor for Alzheimer's disease, stimulated by recent press regarding potentially hundreds of NFL players suffering dementia. But to put the NFL issue in perspective, there are many other modifiable and preventable risk factors for dementia with far more importance in terms of impact, factors that affect perhaps millions of people.

Consider major surgery. Older people often come into the doctor, perhaps brought in by a spouse, with the history that they were "fine" until they had a major surgery (perhaps for a hip fracture, or cardiac bypass surgery), and after the surgery, they came out of it demented. It was as if they had gotten Alzheimer's disease from the surgery.

How could this happen? Fact is, it doesn't.

People don't get Alzheimer's disease from a single major surgery. Most likely, they had it before the surgery, but it was mild, or subclinical, or not diagnosed. The Alzheimer's process starts in the brain perhaps 20 years before enough damage has been done to cause dementia.

But, prospective research shows that major surgery does cause cognitive decline. In some studies, up to 50% of older people suffered cognitive impairment in the first week after major surgery, and about 10-30% of these individuals still have cognitive decline 6-18 months after the surgery, suggesting it is permanent.

While there is some suggestion that cardiac bypass surgery is worse than other types of surgery, most evidence indicates that it doesn't matter what type of surgery is done, as long as its "major" surgery (not minor surgery).

Could the culprit be the anesthesia? Researchers investigating conclude that no one particular type of anesthesia is worse than others.

Work in animal models suggests that the culprit is the hypothermia induced by most anesthetics. In animal models, anesthetics not only cause hypothermia, they also cause the "neurofibrillary tangles," a sign of neuronal damage and death that is an essential feature of Alzheimer's disease. A correlation has been shown between the degree of hypothermia and the number of neurofibrillary tangles. The chemistry has even been worked out a bit. Certain key enzymes are more sensitive to colder temperatures than others, and these are the ones that promote the tangles.

There seems to be a bit of a paradox however. Hypothermia has been reported to be useful in preventing organ damage in some cardiac surgeries. But we don't know the long term effects on the brain quite yet.

So what can be done?

One approach may be new drugs that can prevent the inhibition of the key enzymes, or other drugs that can protect the neurons from the damaging effects of anesthetic induced hypothermia.

Preventing cognitive decline after surgery is important. There are more than 7 million surgeries in older people in the United States every year. If just 5% of these people suffer permanent cognitive decline as a result, that is 350,000 people each year!

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