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The aim of this study was to determine the basis of dissociative experiences in patients with fibromyalgia, testing a hypothesised link to ‘internal agency’ through interoception and autonomic control. Fibromyalgia is a complex polysymptomatic musculoskeletal disorder affecting 5% of the population. Common neuropsychiatric features include emotional fatigue, subjective cognitive dysfunction (e.g. ‘brain-fog’) and dissociation. The combination of brain-fog and dissociation is linked to higher levels of pain symptom intensity and decreased mental well-being. Patients describe such disturbances are as ‘nearly universal’ and important, yet the mechanisms underlying neuropsychiatric symptoms in fibromyalgia are poorly understood. Interestingly fibromyalgia is associated with dysautonomia, notably orthostatic intolerance. Moreover, fibromyalgia and dysautonomia (e.g. Postural Tachycardia Syndrome; PoTS) are both associated with connective tissue disorders, specifically joint hypermobility syndromes (Ehlers Danlos Syndrome hypermobile type; EDS-HT).Twenty-one patients with fibromyalgia (20 female; mean age 41.86 years) and twenty-two healthy controls (16 female; mean age 44.00 years) were recruited. Each participant completed the Dissociative Experiences Scale and the ASQoLS, which measures symptoms of autonomic dysfunction including orthostatic intolerance. All participants underwent assessment for joint hypermobility/EDS-HT using Brighton Criteria. Statistical comparison between groups was performed using independent samples t test. Mediation analyses were conducted according to the method of Baron and Kenny.Patients with fibromyalgia reported greater (mean, SEM) dissociative experiences (48.67, 9.99) than the control group (18.14, 2.72), (t40=2.95, p=0.005) and greater symptoms of orthostatic intolerance (41.90, 4.93) than controls (8.64, 1.02), (t41=6.611, p<0.001). The relationship between fibromyalgia (independent variable) and dissociation (dependent variable) remained significant after adjusting for the gender (r=0.391, p=0.011). This relationship was rendered non-significant when adjusted for symptoms of orthostatic intolerance (r=0.175, p=0.281), indicating full mediation. Interestingly all three variables (fibromyalgia, dissociative experiences and orthostatic intolerance) correlated significantly with the presence of joint hypermobility syndrome/EDS-HT.In a patient population vulnerable to neuropsychiatric symptoms, we found that dissociative experiences are fully mediated by symptoms of orthostatic intolerance. This is also the first study, to our knowledge, to explore dissociation and orthostatic intolerance in the context of fibromyalgia. Moreover, dissociation and brain-fog appear closely linked and are frequently reported by both patients with fibromyalgia and those with PoTS. Our observations are consistent with the hypothesised basis to dissociative symptoms in abnormalities in self-representation and internal agency linked to autonomic control and interoceptive prediction, revealed by subjective symptoms of orthostatic intolerance. This study suggests possibilities for recognition and treatment of neuropsychiatric symptoms.