By Will Dunham

WASHINGTON (Reuters) - For elderly people with elevated blood pressure, further spikes in blood pressure levels can affect their ability to think clearly, U.S. researchers said on Monday.

The findings offer another reason for people with high blood pressure, also known as hypertension, to get the condition under control -- for the sake of their cognitive functioning as well as many other health reasons, they said.

Researchers led by North Carolina State University psychology professor Jason Allaire tracked 36 people, average age 73, in the Detroit area, having each of them take blood pressure reading and take a series of cognitive tests twice daily for 60 days. The tests assessed things like thinking abilities, pattern recognition and problem solving.

For people whose systolic blood pressure was typically 130 or above, their cognitive scores suffered on days when they had blood pressure spikes, the researchers said.

Systolic blood pressure, the top number in blood pressure readings, refers to the pressure when the heart beats while pumping blood. People with systolic blood pressure of 140 or higher are considered to have high blood pressure, and from 120 to 139 are considered to be "pre-hypertensive," or on the threshold of high blood pressure.

People with healthy blood pressure levels did not see their cognitive functioning drop in the twice-daily tests when their blood pressure rose on occasion, the researchers found.

The researchers noted that previous studies had shown that simply having high blood pressure was related to worse cognitive performance in the elderly, but had not evaluated the impact of blood pressure spikes.

"If you have blood pressure that wildly fluctuates and you have high blood pressure, you might be in double trouble for poorer cognitive functioning," Allaire, whose findings appear in the Journal of Gerontology: Psychological Sciences, said in a telephone interview.

Several studies have found a link between high blood pressure and dementia, which is marked by a loss of memory and other cognitive abilities, including the ability to speak, identify objects or think abstractly. A study published in July found that treating high blood pressure in the very elderly may help reduce their risk of developing dementia.

The researchers did not ask the study participants if they were taking blood pressure medication such as beta blockers.

People with high blood pressure have elevated risks of serious health problems such as heart attack and stroke. Blood pressure tends to increase with age.

Stress can cause a person's blood pressure to rise, and also can interfere with one's ability to think clearly. Allaire said it is unclear whether the cognitive functioning declines that the study detected was due to the physiological changes caused by the blood pressure spikes or to stress that may have brought caused the blood pressure to rise.

"The question is: is it the increased blood pressure or is it the stress? And the answer is: it could be both," he added.

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