Mechanisms of Post-Traumatic Stress Disorder and Chronic Pain Comorbidity
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Introduction: There is a known comorbidity between chronic pain and post-traumatic stress syndrome (PTSD), with the two disorders not only sharing common symptoms and affected brain areas but also seem to affect one another. This review aims to summarize the current available literature regarding the mechanisms behind this high rate of comorbidity between chronic pain and PTSD. Methods: A systematic search was conducted in three databases (Pubmed, Scopus, and Embase). Inclusion criteria required the papers to contribute primary information about the mechanisms involved between chronic pain and PTSD. Results: Results could be separated in two main categories, psychological symptoms affecting the comorbidity and neurological studies on the brain areas altered and involved in the comorbidity. Pain catastrophizing, avoidance, hyperarousal, dissociation, anxiety sensitivity symptoms were the main psychological mechanisms through which PTSD symptoms affected pain outcomes. Worse pain outcomes again lead to worse PTSD outcomes and creating a negative spiral in which both disorders mutually maintain and reinforce each other. In addition to this mutual maintenance of chronic pain and PTSD, co-occurrence of other disorders such as depression and insomnia seem to result in worse outcomes for both chronic pain and PTSD. Altered activation patterns were found mostly in areas involved in both pain and stress. Altered activity and connectivity was found in areas in the cingulate cortex, insular cortex, frontal cortex, amygdala, and hippocampus. The alterations in activity and connectivity were mostly related to changes in pain modulation and pain experience in patients with comorbid chronic pain and showing PTSD symptoms. Discussion: Function of various brain areas related to pain and stress show altered activity and connectivity in patient with post-traumatic stress symptoms (PTSS) and chronic pain. The altered brain function is connected to the altered behavioral and modulatory behaviors observed in these individuals. Symptoms of PTSD seem to increase both PTSS severity and chronic pain severity, while chronic pain symptoms seem to increase symptoms severity of PTSS. This creates a vicious cycle of mutual maintenance and reinforcement between the two disorders.