Major depressive disorder (MDD) has consistently been regarded as the most frequently diagnosed comorbid disorder in patients with obsessive-compulsive disorder (OCD). More than half of OCD patients have the lifetime comorbidity of MDD, which is more likely to develop after the onset of OCD. MDD may occur in response to the chronic distress and functional impairments associated with OCD, resulting in either a negative impact on the quality of life of these patients or poor responses to treatments and unfavorable prognoses. However, obsessions, particularly aggressive obsessions, and excessive anxiety have been identified as contributing factors to the occurrence of comorbid MDD, which may support the possible role of an altered relationship between the orbitofrontal cortex and amygdala/parahippocampal region in the development of lifetime MDD in OCD patients. Thus, based on the heterogeneity of OCD, the cognitive (typical) type of OCD, which is characterized by the presence of obsessions or cognitive processes resulting in provoked anxiety and compulsions, is more likely to develop comorbid MDD than the motoric type of OCD that accompanies compulsions to alleviate tension related to sensory phenomena such as feelings of incompleteness and urges to reach a specific sensation of feeling "just right".
 Even though comorbid MDD does not markedly impact on the phenomenological or psychopathological features of OCD, bidirectional interactions between these disorders need to be considered in order to establish adequate treatment strategies for such OCD patients. Preceding pharmacotherapies such as SSRI are indispensable in these treatments because of the possible refractoriness associated with such a concurrently depressive condition to CBT. Furthermore, SSRI augmentations with antidepressants that enhance noradrenergic function may sometimes be effective in the treatment of OCD with comorbid MDD. The influences of environmental factors and/or personality pathology need to be evaluated in order to assess the addition of further treatment options such as environmental manipulations, family-focused interventions, cognitive therapies, or interpersonal psychotherapy, especially for OCD patients with treatment-refractory MDD.
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