Cerebral metabolism in major depression and obsessive-compulsive disorder occurring separately and concurrently

Background: The frequent comorbidity of major depressive disorder (MDD) and obsessive-compulsive disorder (OCD) suggests a fundamental relationship be tween them. We sought to determine whether MDD and OCD have unique cerebral metabolic patterns that remain the same when they coexist as when the), oc cur independently. Methods: [F-18]-fluorodeoxyglucose positron emission tomography (PET) brain scans were obtained on 27 subjects with OCD alone, 27 with MDD alone, 17 w ith concurrent OCD+MDD, and 17 normal control subjects, all in the untreate d state. Regional cerebral glucose metabolism was compared between groups. Results: Left hippocampal metabolism was significantly lower in subjects wi th MDD alone and in subjects with concurrent OCD+MDD than in control subjec ts or subjects with OCD alone. Hippocampal metabolism was negatively correl ated with depression severity across all subjects. Thalamic metabolism was significantly elevated in OCD alone and in MDD alone. Subjects with concurr ent OCD+MDD had significantly lower metabolism in thalamus, caudate, and hi ppocampus than subjects with OCD alone. Conclusions: Left hippocampal dysfunction was associated with major depress ive episodes, regardless of primary diagnosis. Other cerebral metabolic abn ormalities found in OCD and MDD occurring separately were not seen when the disorders coexisted. Depressive episodes occurring in OCD patients may be mediated by different basal ganglia-thalamic abnormalities than in primary MDD patients. (C) 2001 Society of Biological Psychiatry.