Obsessive Compulsive Disorder (OCD)

Most people with OCD exhibit at least one comorbid disorder such as major depression, a phobia, social anxiety, autism spectrum disorder (ASD), attention-deficit hyperactivity disorder (ADHD), anorexia nervosa, Tourette’s syndrome, dementia, Parkinson’s disease, traumatic brain injury and Huntington disease. It has also been discovered that in abrupt early-onset OCD, an autoimmune reaction to Streptococcal infections in the basal ganglia may on some occasions be the cause. Initially termed,  ‘pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections (PANDAS), it is now known as pediatric autoimmune neuropsychiatric syndrome (PANS). The new definition of PANS is a broader view which includes neuropsychiatric reactions to non-streptococcal infections.

On a neurophysiological level, studies suggest a dysfunction in the cortico-striato-thalamo-cortical (CSTC) circuitry. The CTSC model postulates an imbalance in pathways connected to cortical brain regions. Hyperactivity in the Orbitofrontal Cortex (OFC) as a result of reduced inhibition in the thalamus has been linked to obsessive thoughts. The striatum supports the repetitive and ritualistic compulsions that follow. Neuroimaging studies further show abnormalities in these brain regions such as smaller hippocampus and larger palladium for adult OCD patients. In some studies, pediatric patients with OCD show larger thalamic volume and thinner inferior parietal cortex.

On the genetic level, studies suggest that there is a strong heritable component whereby OCD, like other conditions such as Tourettes syndrome, clusters in families. This is evident in first-degree relatives of people with OCD who are more likely to be at risk of OCD compared to relatives of controls. Several studies consistently obtain a risk percentage of 10% – 12 % for relatives of people with OCD and 1%-3% for relatives of controls. This finding is further supported by studies that find that the further away the degree of genetic relatedness is i.e., first-degree, second-degree and third-degree relatives, the less likely a relative of someone with OCD will also have OCD. Interestingly, in one study, compared to controls, non-biological relatives (spouses or partners of OCD probands with at least one child together) are 2.5 times more likely to have OCD. A proband is a person in the family who was first diagnosed with OCD. In familial OCD, hygiene and hoarding symptoms of OCD were more likely to be present. Familial risk of OCD were also found to be related to the onset of OCD in probands (the first person in the family to obtain a diagnosis of OCD) whereby the risk for relatives was greater in probands with early-onset OCD (≾19 year old) compared to late-onset OCD. In twin studies, twins who share 100% of their genes (monozygotic twins) are roughly twice as likely  as compared to twins who share 50% of their genes (dizygotic twins) to have OCD (87% vs 47% risk respectively). Overall, the heritability of OCD from twin studies is estimated at 48%. On a molecular and genetic level, Genome Wide Association Studies (GWAS) have identified several genes that have been implicated in the impairment of cortical-striatal function and possible functional glutamate-GABA imbalance in OCD.