Brain morphology predicts long-term placebo response in multiple sclerosis patients
Although significant insights have been gained into the neural mechanisms of acute placebo responses, less is known about the mechanisms of longer-term placebo responses, such as those seen in clinical trials, or the interactions between these mechanisms and brain disease. We examined neuropathological and morphological brain correlates of placebo responses in a randomized clinical trial of a controversial endovascular treatment ("liberation therapy") for multiple sclerosis. Patients were randomized to receive either balloon or sham extracranial venoplasty and followed for 48 weeks. The trial did not support therapeutic efficacy of venoplasty, but a subset of both venoplasty- and sham-treated patients reported an improvement in health-related quality of life that peaked at 12 weeks following treatment, suggesting a placebo response. Placebo responders had higher lesion activity than placebo non-responders. Although placebo responders did not differ from non-responders in terms of total normalized brain volume, regional grey or white matter volume or cortical thickness, graph theoretical analysis of cortical thickness covariance showed that placebo non-responders had a more homogenous cortical thickness topology with a more small-world-like architecture. In placebo non-responders, lesion load inversely predicted cortical thickness in primary somatosensory and motor areas, association areas, precuneus and insula, primarily in the right hemisphere. In placebo responders, lesion load was unrelated to cortical thickness. The neuropathological process in MS may result in a cortical configuration that is less suited to functional integration and less capable of generating a sustained placebo response.