**2. Epidemiology**

The 12-month prevalence of OCD in the United States is 1.2%, with similar prevalence internationally (1.1–1.8%). Females are affected at a slightly higher rate than males in adulthood; although males are more commonly affected in childhood. The mean age at onset of OCD is 19.5 years and 25% of cases start by 14 years old. Onset after 35 years is unusual but does occur. Males have an earlier age of onset than females; nearly 25% of males have onset before the age of 10. The onset of symptoms is typically gradual; however, acute onset has also been reported [1].

If OCD is untreated, the course is usually chronic, often with waxing and waning symptoms. Some have an episodic course and a minority has a deteriorating course. Without treatment, remission rates in adults are low (i.e. 20%). Onset in childhood or adolescence can lead to a lifetime of OCD. However, 40% of individuals with childhood or adolescent onset of OCD may experience remission by early adulthood. The course of OCD is often complicated by the co-occurrence of other disorders. Compulsions are more easily diagnosed in children than obsessions, because compulsions are observable. However, most children have both [1].

#### **2.1. Prognostic factors**

Greater internalizing symptoms, higher negative emotionality, and behavioral inhibition in childhood are possible temperamental risk factors. Physical and sexual abuse in childhood and other stressful or traumatic events have been associated with an increased risk for developing OCD. Some children develop the sudden onset of OCD symptoms after streptococcal infection, and subsequently it is not distinguishable from OCD for the duration of their lives. In others, it has more motor symptoms and is amenable to antibiotic treatment if it is treated immediately.

## **2.2. Comorbidities**

Most OCD patients (76%) have a lifetime history of another anxiety disorder. Specifically, 63% have lifetime history of mood disorder, 41% have major depressive disorder, 23–32% has comorbid obsessive-compulsive personality disorder, 29% have lifetime history of tic disorder, and 12% have schizophrenia. Additional common diagnoses include bipolar, anorexia, bulimia and Tourette's [1].

## **2.3. Heritability**

OCD may be the most heritable psychiatric condition, with a monozygotic twin concordance rate of 0.52 and a dizygotic concordance rate of 0.21, with overall heritability for OCD estimated to be 48% [2]. The overall recurrence rate (another first degree family member getting OCD) is about 50%, which is higher with Tourette's and tics as well as childhood onset. It is lower with pure OCD of adult onset [3].
