Obsessive-compulsive disorder (OCD) is a psychiatric disorder (more specifically, it is an anxiety disorder) most commonly characterized by a subject's obsessive, distressing, intrusive thoughts and related compulsions (tasks or "rituals") which attempt to neutralize the obsessions. Thus it is an anxiety disorder. It is listed by the World Health Organization as one of the top 10 most disabling illnesses in terms of lost income and diminished quality of life. OCD often causes feelings similar to those of depression.

Recurrent and persistent thoughts, impulses, or images that are experienced at some time during the disturbance, as intrusive and inappropriate and that cause marked anxiety or distress.
The thoughts, impulses, or images are not simply excessive worries about real-life problems.
The person attempts to ignore or suppress such thoughts, impulses, or images, or to neutralize them with some other thought or action.
The person recognizes that the obsessional thoughts, impulses, or images are a product of his or her own mind, and are not based in reality.
The tendency to haggle over small details that the viewer is unable to fix or change in any way. This begins a mental pre-occupation with that which is inevitable.
Repetitive behaviors or mental acts that the person feels driven to perform in response to an obsession, or according to rules that must be applied rigidly.
The behaviors or mental acts are aimed at preventing or reducing distress or preventing some dreaded event or situation; however, these behaviors or mental acts either are not connected in a realistic way with what they are designed to neutralize or prevent or are clearly excessive. Causes and related disorders

Psychological explanations
In the early 1910s, Sigmund Freud attributed obsessive-compulsive behavior to unconscious conflicts which manifested as symptoms.

Freud
There are many different theories about the cause of obsessive-compulsive disorder. Some research has discovered a type of size abnormality in different brain structures. The majority of researchers believe that there is some type of abnormality in the neurotransmitter serotonin, among other possible psychological or biological abnormalities; however, it is possible that this activity is the brain's response to OCD, and not its cause. Serotonin is thought to have a role in regulating anxiety, though it is also thought to be involved in such processes as sleep and memory function. This neurotransmitter travels from one nerve cell to the next via synapses. In order to send chemical messages, serotonin must bind to the receptor sites located on the neighboring nerve cell. It is hypothesized that OCD sufferers may have blocked or damaged receptor sites that prevent serotonin from functioning to its full potential. This suggestion is supported by the fact that many OCD patients benefit from the use of selective serotonin reuptake inhibitors (SSRIs) — a class of antidepressant medications that allow for more serotonin to be readily available to other nerve cells.

Biological explanations
OCD is manifested in a variety of forms.
Community studies have placed the prevalence between one and three percent, although the prevalence of clinically recognized OCD is much lower, suggesting that many individuals with the disorder are unaccounted for clinically.
A need for both sides of the body to feel even. A person with OCD might walk down a sidewalk and step on a crack with the ball of their left foot, then feel the need to step on another crack with the ball of their right foot. If one hand gets wet, the sufferer may feel very uncomfortable if the other is not. If the sufferer is walking and bumps into something, he/she may hit the object or person back to feel a sense of evenness. These symptoms are also experienced in a reversed manner. Some sufferers would rather things to be uneven, favoring the preferred side of the body.
An obsession with numbers (be it in math class, watching TV, or in the room). Some people are obsessed with even numbers while loathing odd numbers (they cause them a great deal of anxiety and often make the person uncomfortable or even angry) or vice versa.
Twisting the head on a toy around, then twisting it all the way back exactly in the opposite direction.(see even body section) Symptoms and prevalence
People with OCD may be diagnosed with other conditions, such as anorexia nervosa, social anxiety disorder, bulimia nervosa, Tourette syndrome, compulsive skin picking, body dysmorphic disorder, and trichotillomania. There is some research demonstrating a link between drug addiction and obsessive compulsive disorder as well. There is a higher risk of drug addiction among those with any anxiety disorder (possibly as a way of coping with the heightened levels of anxiety), but drug addiction among obsessive compulsive patients may serve as a type of compulsive behavior and not just as a coping mechanism. Depression is also extremely prevalent among sufferers of OCD. One explanation for the high depression rate among OCD populations was posited by Mineka, Watson, and Clark (1998), who explained that people with OCD (or any other anxiety disorder) may feel depressed because of an "out of control" type of feeling.

Related disorders
Obsessive-compulsive disorder tends to be slightly more common in females than in males. The lifetime prevalence of the disorder in women is 2.9 percent, versus 2.0 percent in men.
Violence is very rare among OCD sufferers, but the disorder is often debilitating to their quality of life. Also, the psychological self-awareness of the irrationality of the disorder can be painful. For people with severe OCD, it may take several hours a day to carry out the compulsive acts. To avoid perceived obsession triggers, they also often avoid certain situations or places altogether.
It has been alleged that sufferers are generally of above-average intelligence, as the very nature of the disorder necessitates complicated thinking patterns, but this has never been supported by clinical data.

Treatment
OCD primarily involves the brain regions of the striatum, the orbitofrontal cortex and the cingulate cortex. OCD involves several different receptors, mostly H2, M4, nk1, NMDA, and non-NMDA glutamate receptors. The receptors 5-HT1D, 5-HT2C, and the μ opioid receptor exert a secondary effect. The H2, M4, nk1, and non-NMDA glutamate receptors are active in the striatum, whereas the NMDA receptors are active in the cingulate cortex.
The activity of certain receptors is positively correlated to the severity of OCD, whereas the activity of certain other receptors is negatively correlated to the severity of OCD. Those correlations are as follows:
Activity positively correlated to severity:
Activity negatively correlated to severity:
The central dysfunction of OCD may involve the receptors nk1, non-NMDA glutamate receptors, and NMDA, whereas the other receptors could simply exert secondary modulatory effects.
Pharmaceuticals that act directly on those core mechanisms are aprepitant (nk1 antagonist), riluzole (glutamate release inhibitor), and tautomycin (NMDA receptor sensitizer). Also, the anti-Alzheimer's drug memantine is being studied by the OC Foundation in its efficacy in reducing OCD symptoms due to it being a NMDA antagonist. One case study published in The American Journal of Psychiatry suggests that "memantine may be an option for treatment-resistant OCD, but controlled studies are needed to substantiate this observation." The drugs that are popularly used to fight OCD lack full efficacy because they do not act upon what are believed to be the core mechanisms.

H2
M4
nk1
non-NMDA glutamate receptors
NMDA
μ opioid
5-HT1D
5-HT2C See also