FDG-PET in Cotard syndrome before and after treatment: can functional brain imaging support a two-factor hypothesis of nihilistic delusions?

Cotard syndrome is a neuropsychiatric entity recognised by the presence of nihilistic delusions. Although different models have been proposed for the development of monothematic delusions, including Cotard syndrome, functional neuroanatomical models are lacking. A case report of a 19-year old male with autoimmune encephalitis and Cotard syndrome, in whom Positron Emission Tomography (F-FDG-PET) scans were performed before and after successful treatment with electroconvulsive therapy (ECT), is presented. Literature review on brain imaging is provided to discuss a functional neuroanatomical model of Cotard syndrome, in accordance with the two-factor theory of delusions. The patient's F-FDG-PET showed marked insular and prefrontal metabolic abnormalities. Except for insular hypometabolism, metabolic abnormalities improved after ECT. Previously reported structural neuroimaging studies in Cotard syndrome showed a predominance of right hemisphere lesions, in which frontal lobes were more frequently involved, followed by parietal and temporal lesions. Functional neuroimaging studies reported abnormalities in frontoparietal circuits as well as midline structures included in the "default mode network". Abnormalities in the functioning of the insular cortex and the prefrontal cortex could be related to the development of nihilistic delusions when a two-factor theory of delusions is considered.