While their affective response does not differ from healthy patients, people with Alzheimer’s disease (AD) are less sensitive at detecting thermal pain, according to investigators at Vanderbilt University School of Nursing, in Nashville, Tenn.
Publishing in BMC Medicine 2016 May 10, the investigators wrote that the findings suggest people with AD may experience more pain and sustain more damage to tissue and organs before identifying and reporting injuries.
The researchers conducted a cross-sectional study, in which 40 adults with AD and 40 adults without the disease were compared on their psychophysical responses to various levels of thermal stimuli. The two groups were similar in median age (AD, 75 years; control, 70 years) and equally matched in terms of sex, with 20 women and 20 men in each group. The participants were selected from a sample of 97 people who were completing a larger study of mechanisms underlying AD-related alterations in pain responsiveness.
The investigators hypothesized that compared with age- and sex-matched controls with normal cognitive function, people with AD would be less sensitive to thermal pain and find it to be less unpleasant. They also posited that greater AD severity would be associated with higher thresholds for perceiving thermal stimuli and lower levels of unpleasantness, in a linear fashion.
Each patient had a thermode (heating element) placed on the thenar eminence of their right hand, and their responses to heat exposure were assessed using the Medoc Q-Sense or Medoc functional MRI-compatible ATS-Contact Heat-Evoked Potential Stimulator Model Pain and Sensory Evaluation System, both manufactured by Medoc Ltd. The systems were programmed to deliver heat according to a modified protocol successfully used in a previous study of mechanical pressure pain in patients with AD (Brain 2006;129:2957-2965), beginning with a baseline temperature of 30° C, with an upward ramp rate of 1° C per second.
The participants were asked to report pain intensity (“how strong the pain feels”) and the pain’s unpleasantness (“how unpleasant or disturbing the pain is for you”). To rate the intensity, in separate trials, the patients were instructed to press a button to stop the heat stimulus when they perceived the sensations of “warmth,” “mild pain” and “moderate pain.” An unpleasantness scale of 0 to 20 (0=neutral, 20=very intolerable), which the investigators noted has been successfully used in research in older adults with AD, was used to measure the pain’s unpleasantness.
The patients with AD were observed to require significantly higher temperatures to evoke responses to the three intensity descriptors (Wald chi-square [df=1], 7.71; P=0.005). This was true for all three thresholds—“warmth” (Cohen’s d, 0.64; P=0.002), “mild pain” (Cohen’s d, 0.51; P=0.016) and “moderate pain” (Cohen’s d, 0.45; P=0.043). The two groups did not differ in reports of unpleasantness, the investigators found.
Todd Monroe, PhD, assistant professor of nursing and medicine at Vanderbilt University and first author of the study, said he and his colleagues were not surprised to find that people with AD are less sensitive to thermal pain, but did not expect to find that both groups reported similar feelings of unpleasantness in response to thermal pain. “Clinically, we interpret this to mean that while it may take longer for people with AD to feel or perceive pain, that pain is equally distressful relative to cognitively intact controls,” he said.
“Clinicians should be aware that, compared with cognitively healthy people, people with AD experience the same distress from pain but it may take greater intensity for people with AD to realize and report pain,” Dr. Monroe noted. “This means that clinicians should consider any slight pain or discomfort in someone with AD as a potentially more serious problem.”
Chad Boomershine, MD, director of Boomershine Wellness Centers, in Brentwood, Tenn., and assistant clinical professor of medicine at Vanderbilt University, who was not involved with the study, said he was “not at all surprised” by the study’s findings.
“AD patients often cannot tell us that they are in pain, much less where the pain is. Consequently, we always have to suspect pain in AD patients, especially if they show agitation or behavioral change,” Dr. Boomershine said.
Dr. Boomershine added that the findings “will reinforce the notion that we need to suspect pain in this population and use scheduled analgesics,” instead of waiting for AD patients to ask for pain medication.