For older people with mild cognitive impairment or dementia, low blood pressure might be linked to faster mental decline, according to a new study.

There is not much data on blood pressure in people with cognitive impairment, lead author Dr. Enrico Mossello of the University of Florence in Italy told Reuters Health by email.

This new study, he added, is the first to suggest that cognitive declines might happen faster in older people on blood pressure medicine whose systolic pressure - the top number - is low.

Between 2009 and 2012, Mossello and his coauthors analyzed 172 older people. Most had dementia; about a third had only mild cognitive impairment. Almost 70 percent were taking medication for high blood pressure.

The researchers recorded participants’ blood pressure and their performance on a mental test. They repeated all the measurements six to 18 months later - by which time mental function had declined for the whole group, on average, and disability had increased.

The researchers divided participants into three groups based on daytime readings of systolic blood pressure, which is the “120” of a healthy “120 over 80 millimeters of Mercury” blood pressure reading.

People in the lowest third of systolic blood pressure scores (below 128 mm Hg) had bigger decreases on their mental performance tests than those in the middle and high blood pressure groups, according to results in JAMA Internal Medicine.

When the researchers took blood pressure medications into account, only those on the medications who also had lower blood pressure experienced more cognitive decline.

Naturally low blood pressure may not be harmful, but these results suggest that excessive lowering of blood pressure with antihypertensive drugs seems to affect cognition negatively, Mossello said.

“The idea has crystallized that all high blood pressure is bad,” said Dr. Rudolf Westendorp of the faculty of health and medical sciences at the University of Copenhagen in Denmark.

“The dogma is that blood pressure should always be below 140/85, but that is simply not true,” Westendorp, who coauthored an editorial accompanying the new research, told Reuters Health by email.

Frail older people experience more dizziness on standing because blood pressure drops below a minimum that keeps the brain oxygenated, he said.

“Many have tripped or collapsed with often fatal consequences,” he said. “That's why doctors should taper blood pressure lowering medication when older patients develop this type of symptoms and prevent these unwanted side effects.”

This does not mean that high blood pressure is better, Mossello stressed. He said patients should never stop their blood pressure medication until a doctor orders it.

“There are many dementia patients with high blood pressure, hopefully treated, who will experience blood pressure decrease in the course of the disease and need attention to have their therapy adjusted and avoid overtreatment,” Mossello said.

He emphasized that daytime blood pressure readings were more predictive than office readings. People may get nervous at doctors' offices, which falsely increases their blood pressure readings.

“Probably systolic blood pressure values between 130 and 145 are fine for most older patients with dementia,” he added.