Primary OCD, as defined by the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV),² consists of recurrent urges to perform an action (compulsions) or recurrent intrusive thoughts (obsessions), or both. Furthermore, for the symptoms to qualify as true OCD, the compulsions or obsessions must cause significant dysfunction, be recognized by the sufferer as coming from his or her mind (not externally planted), and be egodystonic (i.e., unpleasant and not pleasurable).

An important clinical point is that the obsessions in OCD must be recognized by the individual as senseless, at least at some point in the disorder. A patient with OCD realizes that his or her compulsions do not result from mind control or from some other form of thought insertion, such as might be seen in schizophrenia. Because the obsessions are unpleasant and often of a violent or sexual nature, OCD sufferers attempt to ignore, control, or suppress the obsessions, often with a compulsion. For example, a husband with an obsessional worry about harming his new bride might be forced or compelled to repeatedly check on her welfare to assure himself that he has not harmed her. To summarize, obsessions are recurrent, persistent, resisted by the individual, and unpleasant. Often, obsessions center on certain themes such as contamination, aggressive thoughts concerning harming others or oneself, the need for symmetry or exactness, excessive somatic worries such as about AIDS or terminal cancer, and sexual or religious worries. A common feature of OCD is pathologic doubt and the person’s inability to convince himself or herself that he or she has made a correct decision or that the environment is safe.

In contrast, compulsions are repetitive, purposeful, intentional actions sometimes performed in response to an obsession. It is important that the person recognizes or has recognized the senselessness of the actions. Performing the behavior often reduces anxiety. If an OCD sufferer resists performing a compulsion, invariably inner tension mounts until the compulsion is yielded to and the tension disappears. Common compulsions include checking, cleaning, ordering, counting, repeating, and hoarding. The DSM-IV has placed qualifying criteria on compulsions, which must be distressing to the individual, time consuming (>1 hour/d), and cause impairment in function. The phenomenology of obsessions and compulsions varies, depending on whether a person has pure OCD, OCD accompanied by motor tics, or OCD with motor and vocal tics [a disorder known as Gilles de la Tourette syndrome (GTS)].^15,21 For example, OCD/GTS subjects have increased touching compulsions and only rarely have washing compulsions—the most common compulsion in pure OCD.

Most cases of OCD begin in middle to late adolescence. The course can vary from chronic and unremitting to a more episodic illness featuring episodes of remission and relapse. Obsessive-compulsive behavior was, until the last few decades, thought to be rare; however, the National Institute of Mental Health (NIMH) Epidemiologic Catchment Area Survey revealed a U.S. prevalence of 2% to 3%.^26,46 Obsessive-compulsive behavior is also often accompanied by other psychiatric disorders. For example, one study found that 30% of OCD patients suffered from a major depressive episode, 27% had simple phobias, 14% had panic disorder, 9% had agoraphobia, and 5% had GTS. The exact gene or genes for OCD and its pattern of transmission are unknown. However, Pauls et al.^41,42,43 established that OCD is linked to GTS and chronic motor tics. OCD in GTS seems to respond in a similar fashion as primary OCD to drug treatment with SSRIs and behavioral therapy.^16,48

In addition to pure OCD as defined earlier, there are many OCD-related disorders that some have labeled an OCD spectrum. This spectrum includes a collection of disorders or behaviors that resemble OCD in some way and often respond to treatment with antiobsessional agents. Dysmorphophobia is the fixed idea that a part of one’s anatomy is disfigured or wrongly proportioned.²⁰ Trichotillomania is the compulsive pulling of one’s hair, often seen in young to middle-aged women.^1,53 There is interesting research into the relationships between the eating disorders, particularly anorexia nervosa, and OCD. In addition, some obsessions involve sexual themes, and some forms of fetishism respond to treatment with serotonin-reuptake inhibitors.⁵⁶ Similarly, some obsessions involve violent, aggressive themes, and new research has shown that periodic impulse dyscontrol disorders respond to treatment (or, paradoxically, can be made worse by) serotonin-reuptake inhibitors.^8,35,37

Structural studies of OCD subjects have yielded inconsistent but intriguing results. Luxenberg et al. in 1988,³³ using computed tomography (CT) scans, found decreased volume of the caudate heads in OCD subjects compared with controls. Follow-up studies by Garber et al.¹² and Kellner et al.,²⁷ however, using more sophisticated magnetic resonance imaging (MRI), failed to confirm these initial findings.

With regard to functional neuroimaging studies [positron emission tomography (PET), single photon emission computed tomography (SPECT)], a fairly consistent picture emerges. Numerous studies conducted in different centers with both PET^{3,4,5,6,34,39,50,54} and SPECT¹⁹ have consistently found abnormalities in the orbitofrontal white matter and basal ganglia that change with pharmacologic or behavioral treatment. This is one of the more consistent and remarkable findings in the recent history of biologic psychiatry and serves as the foundation for the ongoing revolution in understanding OCD.

It is important to realize that the differences noted in OCD are always found only on comparing group means. Unfortunately, we lack the ability to diagnose OCD on an individual basis using PET or SPECT. In addition, for almost all patients studied, as their OCD symptoms improve with either pharmacologic or behavioral treatment, the brain metabolism also changes to a more “normal” pattern.¹⁹ The changes in brain metabolism thus appear to mirror the clinical improvement.