Migraine and functional neurological disorder (FND)-a review of comorbidity and potential overlap.

Migraine and functional neurological disorder (FND) are two of the most common conditions in neurological practice. It is assumed that the two conditions have distinct underlying mechanisms. However, it can be clinically challenging to disentangle their relative contributions to a patient's symptoms. In addition, apart from the relationship between persistent postural perceptual dizziness (PPPD) and migraine, the frequency of co-occurrence has not been characterized in detail. Contemporary conceptualizations of FND have driven a re-evaluation of its relationship to other neurological disorders, including migraine. We carried out a narrative review of the literature examining the co-occurrence of migraine and FND. We also explored their comorbidities, aetiological risk factors and mechanisms, focusing especially on areas of potential overlap. Our review suggests increased frequency of migraine in people with functional seizures compared to epilepsy, but data from people with functional motor symptoms is mixed. Robust epidemiological studies evaluating the frequency of FND in migraine are lacking. Similar to other neurological disorders, migraine is an established trigger of FND. Female gender, adverse childhood experiences and comorbid psychiatric and functional disorders, such as irritable bowel syndrome and fibromyalgia, are more common in both conditions than in controls, but perhaps more so in FND. Mechanistic research in both conditions highlights converging frameworks of dysregulated allostatic/stress responses in the context of predictive processing models of the brain. This has implications for pharmaceutical and rehabilitation treatments. The relationship between migraine and FND is poorly studied. An overview of their overlap offers a model of non-dualistic thinking within a clinical neuroscience framework for future studies.