A randomized, placebo-controlled, double-blinded mechanistic clinical trial using endotoxin to evaluate the relationship between insomnia, inflammation, and affective disturbance on pain in older adults: A protocol for the sleep and Healthy Aging Research for pain (SHARE-P) study

Chronic pain is prevalent in older adults. Treatment, especially with opioids, is often ineffective and poses considerable negative consequences in this population. To improve treatment, it is important to understand why older adults are at a heightened risk for developing chronic pain. Insomnia is a major modifiable risk factor for chronic pain that is ubiquitous among older adults. Insomnia can also lead to heightened systemic inflammation and affective disturbance, both of which may further exacerbate pain conditions in older adults. Endotoxin exposure can be used as an experimental model of systemic inflammation and affective disturbance. The current study aims to understand how insomnia status and endotoxin-induced changes in inflammation and affect (increased negative affect and decreased positive affect) may interact to impact pain facilitatory and inhibitory processes in older adults. Longitudinal data will also assess how pain processing, affective, and inflammatory responses to endotoxin may predict the development of pain and/or depressive symptoms. The current study is a randomized, double-blinded, placebo-controlled, mechanistic clinical trial in men and women, with and without insomnia, aged 50 years and older. Participants were randomized to either 0.8ng/kg endotoxin injection or saline placebo injection. Daily diaries were used to collect variables related to sleep, mood, and pain at two-week intervals during baseline and 3-, 6-, 9-, and 12-months post-injection. Primary outcomes during the experimental phase include conditioned pain modulation, temporal summation, and affective pain modulation ∼5.5 hours after injection. Primary outcomes for longitudinal assessments are self-reported pain intensity and depressive symptoms. The current study uses endotoxin as an experimental model for pain. In doing so, it aims to extend the current literature by: (1) including older adults, (2) investigating insomnia as a potential risk factor for chronic pain, (3) evaluating the role of endotoxin-induced affective disturbances on pain sensitivity, and (4) assessing sex differences in endotoxin-induced hyperalgesia.