OCD and ADHD Dual Diagnosis Misdiagnosis and the Cognitive ‘Cost’ of Obsessions

by Amitai Abramovitch, PhD and Andrew Mittelman

Dr. Amitai Abramovitch is a neuropsychologist and a Research Fellow at the OCD and Related Disorders Program at Massachusetts General Hospital and the Department of Psychiatry at Harvard Medical School. Dr. Abramovitch can be reached at aabramovitch@partners.org.

Andrew Mittelman is a Research Coordinator at the OCD and Related Disorders Program at Massachusetts General Hospital. He can be reached at amittelman@partners.org.

Both obsessive compulsive disorder (OCD), and attention-deficit hyperactivity disorder (ADHD), are considered fairly common and serious neuropsychiatric disorders. To the untrained eye, some of the symptoms associated with attention and concentration can appear remarkably similar, especially in children and adolescents. However, ADHD and OCD are notably different in terms of brain activity and their clinical presentation. ADHD is considered be an externalizing disorder, meaning it affects how people outwardly relate to their environment. Individuals with ADHD may exhibit inattention, lack of impulse control, and risky behaviors. OCD on the other hand. is characterized as an internalizing disorder, meaning individuals with OCD respond to anxiety producing environments by turning inward. Individuals with OCD exhibit frequent obsessive and/or compulsive thoughts and behaviors. In addition, generally speaking, people with OCD tend to demonstrate a more inhibited temperament and tend to avoid risky or potentially harmful situations. Furthermore, individuals diagnosed with OCD are overly concerned with the consequences of their actions and tend to not act impulsively. Not surprisingly, people with OCD exhibit unusually low rates of novelty seeking behavior and cigarette smoking.

Considerable evidence has suggested that ADHD and OCD are characterized by abnormal brain activity in the same neural circuit. Specifically, both conditions exhibit opposite patterns of brain activity in the frontostriatal system1, the segment of the brain responsible for higher order, motor, cognitive, and behavioral functions. However the similarities between OCD and ADHD are limited to only which part of the brain is affected; patients with OCD exhibit significantly increased activity (hypermetabolism) in the frontostriatal circuits, meaning this part of the brain is overactive in people with OCD, while patients with ADHD exhibit decreased activity (hypometabolism), meaning this part of the brain is less active in people with ADHD.

While the disorders are associated with very different patterns of brain activity, the resulting cognitive effects are actually similar, especially in executive functions2 such as response inhibition, planning, task switching, working memory, and decision making. Sufferers of both OCD and ADHD have consistently and significantly underperformed in tests of executive functions.

Some research has suggested that OCD and Obsessive Compulsive Spectrum Disorders fall upon a compulsive-impulsive continuum. In other words there exists a gradient of disorders ranging from behavioral impulsivity to compulsivity. OCD appears to lie at one end of this spectrum, while ADHD exists at the other. This is surprising considering that over 35 studies have reported that an average of 21% of children and 8.5% of adults with OCD actually have ADHD as well.

This begs the question, can one person be both impulsive and careful — be both a risk taker and avoid risks — and exhibit opposite patterns of brain activity at the same time? As a secondary question, if this indeed is possible, how can we account for the significant decrease in reported comorbidity rates in adulthood? Is it the case that two thirds of the children diagnosed with both disorders become cured from one of the conditions? These two questions were at the focus of our research into the association between ADHD and OCD.

In order to answer the first question, we examined our hypothesis that different mechanisms in OCD and ADHD may result in similar cognitive impairments, in other words, though the disorders are associated with very different patterns of brain activity, they may result in the same effects on a person’s cognitive functioning. This hypothesis is in line with other research suggesting that very different disorders are characterized by impairments in executive functions, although they may differ in patterns of brain activity and clinical picture. For example, despite very different symptoms, post-traumatic stress disorder, major depressive disorder, panic disorder, schizophrenia, and bipolar disorder are all characterized by impairments in executive functions and abnormal patterns of brain activity. In addition, across conditions, trait and state anxiety has been associated with cognitive impairments. Thus, we have proposed an “Executive Overload model of OCD.”

The Executive Overload model suggests that sufferers of OCD experience an “overflow” of obsessive thoughts. This overflow (which was found to correlate with increased frontostriatal brain activity), results in an overload upon the executive system, which is reflected in executive impairment, resulting in changes to a person’s behaviors and abilities. In general, anxiety has been known to put strain on the executive system, and we argue that obsessions may be similar to anxiety in regards to their associated cognitive ‘cost.’ Specifically, individuals with OCD are demonstrating deficits that we believe are actually caused by the symptoms themselves.

A good analogy for the Executive Overload model of OCD would be the RAM memory on a personal computer. The more software programs that a computer has operating in the background, the less processing power is available to support complex computations (think of Microsoft Word crashing because you have too many other programs open). In OCD, a person may perform a certain task while at the same time experiencing a surge of intrusive thoughts, such as, “am I doing this right?” or, “did I make a mistake?” etc. Thus, the more obsessive, intrusive thoughts that an individual experiences in a given moment, the fewer resources would be available for other tasks (such as listening to a teacher in class, or concentrating during a business a meeting), specially complex ones. In other words, cognitive impairments in OCD are largely state-dependent; thus, our model predicts that treating and reducing OCD symptoms ought to be accompanied by an improvement of executive functioning.

This progression has indeed been observed in patients undergoing OCD treatment where, in conjunction with clinical improvement, CBT resulted in decreased abnormal brain activity and improvement in cognitive symptoms. Our direct comparison of ADHD and OCD groups yielded an association between Obsessive Compulsive (OC) symptoms and executive function impairments only within the OCD group and not in the control or ADHD groups. We observed that deficient performance on tests of executive functions was correlated with the presence of OC symptoms, but only within the OCD group. In other words, for people with OCD, an increase in reported obsessive/compulsive thoughts and behaviors also meant a decrease in performance on executive function tests, such as ability to suppress responses.

However, within the ADHD group, more OC symptoms were actually correlated with better performance in tests of executive functions — one hypothesis has suggested that this may be because individuals with ADHD who also exhibit OC traits are better organized and attentive to details than individuals with ADHD who exhibit no OC symptoms.

In a second study, we examined the nature of ADHD symptoms throughout the lifespan. We noted that ADHD symptoms were correlated between childhood and adulthood in the ADHD and control groups, but not within the OCD group. This second study suggested that some attention problems in children and adolescents may actually stem from OCD symptoms, and are not ADHD related.

The second question regarding the co-occurrence between OCD and ADHD remains to be answered. Review of the literature suggests that two major findings are clearly observable. First, research reporting prevalence rates of ADHD-OCD co-occurrence exhibits significant inconsistency with reports ranging from 0% to 59% of individuals, with OCD diagnosed with concomitant ADHD. Whereas research suggests that one out of five children with OCD has co-occurring ADHD, only one out of every 12 adults with OCD has ADHD. So, what happens to half of the children with OCD who initially diagnosed with ADHD as well; does it disappear in adulthood? The answer appears to be both “yes” and “no.” It appears that preadolescent children with OCD go through a slower process of brain development in which their pattern of brain activity and associated symptoms may appear to fit the symptomatic description of ADHD. However, through adolescence this arrested development begins to abate as ADHD-like symptoms dissipate and brain activity changes to fit the adult patterns observed in adult OCD. Furthermore, we suspect that a full-blown dual diagnosis of ADHD and OCD in adults is in fact rather rare, and is usually associated with a mediating condition (notably chronic tic disorder, or Tourette Syndrome).

The ways that neuropsychological impairments manifest in a person’s behavior are universal. For example, a deficit in attention, regardless of the cause or condition, may cause an individual to appear as if she is not listening when spoken to directly (which is one of the DSM criteria for ADHD). In the light of deficits in attention and executive functions seen in both OCD and ADHD, it is easy to see how a clinician might potentially misdiagnose one condition as the other. In fact, chances of misdiagnosis may even be higher in children and young adolescents for whom diagnosis relies heavily on informants such as parents or teachers.

Consider the example of a child with OCD who sits in class obsessing over a stain on her sleeve. Frequently preoccupied by an overflow of obsessive-intrusive thoughts, this child cannot be attentive in class and would possibly receive increasingly lower grades. In turn, the teacher might perceive this student as inattentive and would report to the counselor and parents that the student may have ADHD. In an attempt to help the child focus more in class, a clinician may prescribe stimulant medication (such as Ritalin) after misdiagnosing the child with ADHD. Several studies suggest that stimulant therapy may exacerbate obsessive-compulsive thoughts and behaviors, or even induce them. Instead of improving, the misdiagnosed child would likely even deteriorate in condition. In fact, this may be intuitively explained; stimulant therapy increases frontostriatal brain activity, which is generally reduced in ADHD. In OCD, a disorder characterized by increased activity (which is correlated with symptom severity), stimulant medication will continue to activate an already hyperactive brain (specifically the frontostriatal system) potentially resulting in immediate exacerbation of symptoms. Another possible explanation, once suggested in the scientific literature, is that under the influence of stimulants individuals with OCD may experience improved attention toward obsessive thoughts, potentially resulting in an increase in obsessions, and an increase in compensatory compulsive rituals.

Implications for Practice

In light of the potential pitfalls of misdiagnosis, we recommend that clinicians examine two major diagnostic factors that may aid in establishing a more accurate diagnosis. First, clinicians ought to note the presence or absence of clinically significant levels of impulsivity and risk taking. Unlike those with ADHD from adolescence, people with OCD are very rarely impulsive and do not exhibit risk-taking behavior. This is especially true when OCD is the patient’s primary disorder. It is worth noting that 75% of all individuals diagnosed with ADHD are diagnosed with the impulsive/hyperactive (combined) type, associated with significant impulsive behavior, and ruling out the ‘pure’ inattentive type is more of a challenge. The second diagnostic marker is the ability to perform accurate and repetitive rituals governed by very specific and complex rules, something that people with ADHD will generally struggle with. In fact, attention to detail and the ability to strictly follow attention-demanding tasks are characteristic impairments of ADHD and are considered clinical diagnostic criteria.


Abramovitch A., Dar R., Mittelman A., Schweiger A., (2013). “Don’t judge a book by its cover: ADHD-like symptoms in obsessive compulsive disorder,” Journal of Obsessive Compulsive and Related Disorders, 2(1) 53–61.

Abramovitch A.,Dar R., Hermesh H., Schweiger A., (2012). “Comparative neuropsychology of adult obsessive-compulsive disorder and attention deficit/hyperactivity disorder implications for a novel executive overload model of OCD,” Journal of Neuropsychology, 6(2) 161–191.