Abstract
Obsessive–compulsive disorder (OCD) is an increasingly recognized disorder with a prevalence of 2–3% (Robins et al., 1984). Once thought to be psychodynamic in origin, OCD is now generally recognized as having a neurobiological cause. Although the exact pathophysiology of OCD in its pure form remains unknown, there are numerous reports of obsessive–compulsive symptoms arising in the setting of known neurological disease. In this paper, we review the reported cases of obsessive–compulsive symptoms associated with neurologic diseases and outline the known facts about the underlying neurobiology of OCD. Finally, we synthesize these findings into a proposed theory of the pathophysiology of OCD, in both its pure form and when it accompanies other neurological illness.