Obsessive-Compulsive and related disorders have in common persistent or recurrent uncontrollable ideas and/or behaviors that are dysfunctional. In most cases, there seem to be a neural substrate, particularly in the basal ganglia and the frontal lobe, as well as memetic (cultural/ideational) component. Memes are neural connections representing ideas, which undergo Darwinian selection in the brain (Leigh 2010, 2012).

An evolutionarily important event was the development of romantic love, which serves as an example of preoccupation, obsession, and compulsion, usually but not always within normal range (Leigh 2010). Helen Fisher studied romantic love extensively. She states, “The sex drive evolved to motivate individuals to seek a range of mating partners; attraction evolved to motivate individuals to prefer and pursue specific partners; and attachment evolved to motivate individuals to remain together long enough to complete species-specific parenting duties. These three behavioral repertoires appear to be based on brain systems that are largely distinct yet interrelated, and they interact in specific ways to orchestrate reproduction, using both hormones and monoamines. Romantic attraction in humans and its antecedent in other mammalian species play a primary role: this neural mechanism motivates individuals to focus their courtship energy on specific others, thereby conserving valuable time and metabolic energy, and facilitating mate choice” (Fisher et al. 2006).

The neural circuit underlying romantic love involves the right ventral tegmental area of the brain stem and right posterodorsal body of the caudate nucleus. The dopaminergic reward and motivation pathways contribute to aspects of romantic love.

The complex neurotransmitter network of the cortico-striatal-thalamo-cortical (CSTC) circuit involving dopamine, serotonin, glutamate, and gamma-amino butyric acid (GABA) may be dysfunctional in obsessive-compulsive syndrome (Harvey et al. 2001). The dysfunction in this loop may arise from an attenuation of feedback signal indicating reward which results in compulsive lever pressing in rats, which is further enhanced by lesions of the orbitofrontal cortex (Joel et al. 2005).

Compulsions may also develop from an operant conditioning paradigm as in compulsive gambling, which in turn may result in functional changes in the brain such as impairment of decision-making capacity (Fellows 2007; Hariri et al. 2006; Kalenscher et al. 2006).

Tourette’s syndrome is an example of brain dysfunction that involves dysregulation of both ideational and motoric function illuminating the role of the basal ganglia in both. In this syndrome, there are simple and complex motor tics, vocal tics, and frequently obsessive-compulsive symptoms. Its onset occurs before the age of 21 and the course is waxing and waning. Tourette’s syndrome occurs mainly in boys and is genetically transmitted with variable penetrance but it has also been associated with various infections and immunological conditions such as the PANDAS (pediatric autoimmune neuropsychiatric disorder associated with streptococcal infection). The neuropathology seems to involve a disturbance of the dopaminergic system in the basal ganglia.

Gene–environment interaction involving the serotonin transporter promoter gene (5-HTTLPR) has also been reported in OCD, i.e., those with the s/s allele and childhood trauma were more likely to develop OCD with dissociative experiences (Lochner et al. 2007). Clearly, same gene–environment interaction may lead to multiple vulnerabilities in the CNS, including OCD, dissociation, anxiety, and depression.

Normal thinking process may become abnormal if stuck in a loop, as in preoccupations, obsessions, and compulsions. Preoccupations, obsessions, and compulsions may occur unwantedly and unexpectedly, depending on the state of the brain and the memetic nature of the thought. In the obsessive-compulsive disorder (OCD), there may be a primary dysfunction of the cortico-striatal-thalamo-cortical network involving dopamine and serotonin discussed above. Symptoms of OCD may also occur when an idea becomes particularly reinforced in the brain because it is fed by energies from related preexisting ideas (memes) in the brain. For example, a suicide idea may be introduced by a film to a depressed brain that has many preexisting self-punishment and guilt ideas, thus the suicide idea may gain strength, proliferate, and recruit the motor neurons to carry out suicide. Other memes, like earworms which are unwanted melodies or sounds that keep on occurring in the mind, may replicate because of the strength of their vehicles (the rhythm, melody, form of presentation, color, smell, texture, etc.). Some such memes may represent “supernormal stimulus,” the kinds of stimuli that are evolutionarily determined to elicit strong preferences, or results of early imprinting (Burkhardt 2005).

The diagnosis of obsessive-compulsive and related disorders is made on the basis of the severity of the phenomena and the degree of distress. Earworms are nuisances but not necessarily distressing, ego-alien obsessions can be very distressing, and compulsions may be disabling.

In this chapter, we will discuss first OCD and the related disorders—hoarding disorder, trichotillomania, skin-picking disorder, substance/medication-induced OCD, and then body dysmorphic disorder separately as it is of particular importance in certain areas of consultation-liaison psychiatry.