1. Experiencing general chronic pain or somatic chronic pain are associated with increased risk of later suicidal behaviour at 10 years follow-up.
2. For general chronic pain, this association remained significant within dizygotic twin pairs, but non-significant within monozygotic twin pairs.
Evidence Rating Level: 2 (Good)
While experiencing chronic pain is a known risk factor for suicide, there are numerous chronic pain conditions that often co-exist together, such as irritable bowel syndrome, fibromyalgia, and chronic fatigue syndrome. It is unclear whether increased suicide risk is associated with specific pain conditions or chronic pain more generally. As well, both chronic pain and suicidality could have genetic and familial risk factors that confound the association between the two. Therefore, this population-based twin cohort study based in Sweden aimed to identify the association between general versus specific chronic pain factors, and suicidal behaviour, while controlling for confounding in twin pairs. The study population was drawn from a 2006 study of twins born in Sweden between 1958 and 1985. This included self-reporting symptoms of chronic pain on 9 pain scales, such as joint pain, lower back pain, and chronic widespread pain. National health records were used to identify suicidal behaviour, defined as intentional self-harm, injury of undetermined intent, and death by suicide, at 10 years follow-up. In total, there were 17,148 participants, including 7126 monozygotic twins and 10,022 dizygotic twins. The prevalence of suicidal behaviour was 1.6%. The results showed that participants scoring 1 standard deviation above the mean on the pain scales were at a higher risk of suicidal behaviour at follow-up, with ORs ranging from 1.13 to 1.47 depending on the scale. After combining all pain conditions into a multiple regression model however, only chronic fatigue syndrome (OR 1.26, 95% CI 1.14-1.40) and bladder pain (OR 1.19, 95% CI 1.08-1.31), were significantly associated with suicidal behaviour. This suggests that the experience of chronic pain more broadly is a major contributor to this association with suicidality. Furthermore, those scoring 1 SD above the mean on the general factor of pain and the specific factor of somatic symptoms were at higher risk of suicidal behaviour (OR 1.51, 95% CI 1.34-1.72 for general factor, OR 1.80, 95% CI 1.45-2.22 for somatic symptoms factor), while the specific factor of neck-shoulder pain had no significant association. However, with regards to the general pain factor, the association with suicidal behaviour remained significant within dizygotic twins (OR 1.46, 95% CI 1.11-1.93) but not within monozygotic twins (OR 0.89, 95% CI 0.59-1.33), suggesting that monozygotic twins were at approximately equivalent risk of later suicide regardless of pain factor score. A similar pattern was observed for the somatic symptoms factor (OR 1.46, 95% CI 0.89-2.39 for dizygotic; OR 1.02, 95% CI 0.49-2.11 for monozygotic). Overall, this study demonstrated that general and somatic chronic pain are associated with suicidal behaviour, but that this association is not significant within monozygotic twin pairs.
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