Numb to the Pressure: Hypertension and Pain

Be thankful for your aches and pangs. People who hardly wince from injuries ranging from minor scrapes to major surgery could be candidates for high blood pressure.

By Angie Roberts

Sixty-one-year-old Thelma Brinkman has always had a high tolerance for pain. The most telling evidence of that came in her mid-50s, and again a few years later, when she was recovering from hysterectomy and hip replacement surgeries. In both cases, hospital caregivers hooked her up to a self-administering morphine pump. "They kept saying, 'You can use it, you can use it,'" Brinkman recalls. But the Avon, Indiana, grandmother didn't tap the pump nearly as often as the staff expected her to.

Brinkman has also been treated for high blood pressure since her early 20s. Is it a coincidence that she has both hypertension and a decreased sensitivity to pain? Probably not.

Various studies have demonstrated that people with high blood pressure have a higher tolerance for pain, for instance, after prostatectomy surgery or even during a silent heart attack. The studies suggest that even injuries as minor as a coffee burn or splinter could seem much less bothersome for hypertensive people than for their non-afflicted peers.

A decreased sensitivity to pain has also been noted in those likely to develop high blood pressure. And now, a new study looking for a stronger tie between the two conditions could help medical professionals better predict who may develop hypertension in the future.

The missing link

The American Heart Association says that 31.6 percent of the people who suffer high blood pressure don't even realize they carry the potentially deadly disease. The headaches, fatigue and anxiety symptomatic of hypertension are often ignored.

As with many health conditions, a combination of factors probably plays into hypertension's development, including environmental stressors, diet and genetics. People whose parents have high blood pressure are more likely to develop it than those whose parents don't. It's not a strong correlation, though. In Brinkman's family, both of her adult children have been diagnosed with hypertension. In many other families, one case of high blood pressure doesn't necessarily follow another. So it's difficult to know who's most at risk.

That's where the latest research of Ohio University health psychologist Christopher France, Ph.D., comes into play. He suspects the correlation between decreased pain sensitivity and hypertension is due to a faulty switch in the brain. That switch could be a key indicator of a person's susceptibility to hypertension.

When our bodies are injured, scientists believe that the brain dulls the pain by signaling the release of chemicals called opioids. Then, it turns off the opioid release.

This self-regulating system may not work for people who have hypertension or will likely develop it, says France. "They produce a significant opioid response to stress, but then they lack the normal ability to turn off the opioid release," he theorizes. "As a result, they end up having higher blood-opioid levels after stress."

And because the part of the brain that controls pain, the hypothalamus, also controls blood pressure, France believes the hypothalamus is the missing link between hypertension and pain sensitivity.

To test the theory, researchers are enrolling 144 people at each of three sites: Ohio University; the University of Minnesota; and the University of Birmingham in England. The Ohio and Minnesota sites will examine subjects with and without a family history of hypertension. The English site will study people with and without high blood pressure.

The first step is to determine if opioids are indeed causing decreased pain sensitivity. Researchers will measure participants' responses to electrical impulses both before and after receiving an opioid-blocking drug called naltrexone. If subjects feel little or no pain before receiving the drug, but feel pain when their opioid production is halted, that will explain how the pain is being blocked.

Predicting the future

The next step takes place over a three-year follow-up period. Researchers will continue monitoring the American subjects, looking for a correlation between changes in pain perception and changes in blood pressure. "Because most people who have a family history of hypertension do not go on to develop high blood pressure themselves, we hope that decreased pain perception may be a marker for those most likely to show increases in blood pressure," France explains.

The whole theory behind the study sounds counterintuitive to medical school training, says Wayne Ellis, M.D., an Indianapolis internist and pediatrician. "In general, I always thought the more opioids you're producing, the lower your blood pressure would be," remarks the physician, whose practice includes many hypertensive patients.

France agrees that it is counterintuitive. But, he explains, "Although higher opioids may lead to lower blood pressure, one may develop physiological tolerance to higher opioid levels over time. Thus, the expected lower blood pressure levels may not occur."

If France and his collaborators are right, their research may lead to early interventions, including lifestyle changes and cardiac monitoring, for those at greater risk. It could even lead to a new hypertension drug. Based on new evidence, researchers could theoretically develop a medication preventing opioid dysregulation, thus preventing hypertension.

It's "a rather speculative notion at this point," says France. And not the most important aspect of the research for someone like Brinkman, who worries that her predecessors may develop high blood pressure. Proving a link between hypertension and pain may not directly help her. But, she says, "It would help my grandchildren."

Angie Roberts is an Indiana-based writer specializing in health, environmental and family issues.

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