Contact heat sensitivity and reports of unpleasantness in communicative people with mild to moderate cognitive impairment in Alzheimer’s disease: a cross-sectional study

Monroe et al. BMC Medicine (2016) 14:74 DOI 10.1186/s12916-016-0619-1 RESEARCH ARTICLE Open Access Contact heat sensitivity and reports of unpleasantness in communicative people with mild to moderate cognitive impairment in Alzheimer’s disease: a crosssectional study Todd B. Monroe1*, Stephen J. Gibson2, Stephen P. Bruehl3, John C. Gore4, Mary S. Dietrich5, Paul Newhouse6, Sebastian Atalla1 and Ronald L. Cowan7 Abstract Background: Compared to healthy controls, people with Alzheimer’s disease (AD) have been shown to receive less pain medication and report pain less frequently. It is unknown if these findings reflect less perceived pain in AD, an inability to recognize pain, or an inability to communicate pain. Methods: To further examine aspects of pain processing in AD, we conducted a cross-sectional study of sex-matched adults ≥65 years old with and without AD (AD: n = 40, female = 20, median age = 75; control: n = 40, female = 20, median age = 70) to compare the psychophysical response to contact-evoked perceptual heat thresholds of warmth, mild pain, and moderate pain, and self-reported unpleasantness for each percept. Results: When compared to controls, participants with AD required higher temperatures to report sensing 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). Conversely, there were no significant between-group differences in unpleasantness ratings (p > 0.05). Conclusions: The between-group findings demonstrate that when compared to controls, people with AD are less sensitive to the detection of thermal pain but do not differ in affective response to the unpleasant aspects of thermal pain. These findings suggest that people with AD may experience greater levels of pain and potentially greater levels of tissue or organ damage prior to identifying and reporting injury. This finding may help to explain the decreased frequency of pain reports and consequently a lower administration of analgesics in AD. Keywords: Pain, Dementia, Alzheimer’s disease, Unpleasantness, Psychophysics Background Poorly managed pain in people with Alzheimer’s disease (AD) is a significant public health concern. AD in general is a risk factor for the under-treatment of pain, due in part to a lack of understanding of the impact of AD on psychophysiological factors that influence the pain experience. In the presence of similar painful conditions, * Correspondence: 1 Vanderbilt University School of Nursing, Vanderbilt University Institute of Imaging Science, Vanderbilt Psychiatric Neuroimaging Program, Nashville, Tennessee, USA Full list of author information is available at the end of the article when compared to cognitively intact older adults, people with AD have been shown to receive less pain medication [1–3]. However, a recent large-scale study found that when compared to people without dementia, people with dementia reported pain less frequently and although they reportedly used acetaminophen more frequently, there were no significant differences in the use of opioids and NSAIDs [4]. Furthermore, in the presence of similar painful conditions, people with AD verbally report pain less frequently [5, 6] but exhibit similar pain-related behaviors when moved [6]. It is unknown if these findings reflect less © 2016 Monroe et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Monroe et al. BMC Medicine (2016) 14:74 perceived pain in AD or an inability to recognize pain or to communicate pain. Nociceptive studies of pain examine underlying neurophysiological processes that may lead to the perception of pain [7] whereas psychophysical studies examine selfreports of pain perception [8]. This paper will examine the latter. Pain is described as a multidimensional experience consisting of sensory, cognitive, and affective components [9]. If one or more of these components is altered in AD, the ability to detect and report pain may also be altered. Findings from acute experimental pain studies in AD are mixed. In response to contact heat stimuli [10, 11], experimental electrical shock [12, 13], mechanical pressure [14], CO2 laser [15], and ischemia [16], the self-reported pain threshold does not differ between healthy controls and people with AD. However, the magnitude of stimulus intensity required to reach tolerance (stimulus reported as “unbearable”) was significantly higher in AD, with pain tolerance increasing as AD severity worsened [16]. Conversely, for suprathreshold heat pain stimuli, people with AD reported more pain (increased sensitivity) when compared to controls [11]. Relative to controls, people with AD demonstrated a blunted autonomic response yet normal pain perception to a painful stimulus just above the pain threshold; however, when the pain stimulus was increased to twice the threshold, people with AD demonstrated a blunted perceptual response to pain (but no difference in autonomic response) [10]. These findings suggest that people with AD have a variable pain response that is threshold dependent [12]. Gibson and colleagues found that when compared to controls, individuals with AD demonstrated increased detection thresholds for just-noticeable sensation but the groups did not differ in their intensity ratings in response to fixed temperatures [15]. Cole and colleagues found that compared to healthy controls, people with AD displayed increased mechanical pressure pain thresholds (i.e., decreased sensory sensitivity) for justnoticeable pain and weak pain while also reporting just-noticeable pain as more unpleasant [17]. As an alternative to examining verbal reports of pain, some laboratories are examining facial affective responses to pain, demonstrating that facial coding can effectively be used to measure affective responses to evoked pain (presumably corresponding to pain unpleasantness) in people with AD [18]. Kunz and colleagues observed that, relative to controls, people with dementia demonstrated increased facial pain affect to both mechanical pressure and electrical shock [5, 14]. Similarly, Beach and colleagues recently found that in response to pressure algometry, people with AD demonstrated increased facial pain affect when compared to controls [19]. In summary, these experimental pain studies suggest that AD is variously associated with increased pain threshold Page 2 of 9 and tolerance with diminished sensitivity, no differences relative to he (...truncated)