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The Relationship Between Autism Spectrum Disorder and ADHD

Here's how to understand the diagnosis and treatment options.

In recent years, there has been a move to diagnose some kids with both Autism Spectrum Disorder (ASD) and Attention Deficit Hyperactivity Disorder (ADHD) at the same time.

In the United States, these disorders are defined by a psychiatric guide to diagnoses called the Diagnostic Statisticians’ Manual (DSM), which is currently in its fifth iteration (DSM-5). Is double diagnosis an accurate approach? Does it reflect biology in these kids? Let’s look further into how this situation came about.

To make medical diagnoses, physicians traditionally rely on three things: the history given to them by patients, an office exam, and medical tests. Examples of medical tests doctors use to help make diagnoses include X-Rays, MRIs or CT scans (to look at anatomy), EKGs or EEGs (to look at overall electrical activity patterns in electrochemically active organs like the heart or brain), and bloodwork and tissue biopsies (to look at metabolic and cellular information). Increasingly, doctors do DNA tests that contribute genetic information to the diagnostic process.

Tests are particularly useful for making a diagnosis when the underlying biology of a medical condition is well understood. When doctors are successful at reaching a diagnosis, they generally call the condition a “disease.” But this word doesn’t apply very well to the vast majority of neuropsychiatric conditions — including both ASD and ADHD. That is why these conditions are called “disorders” (or even “spectrum disorders”) and not “diseases.”

Two for the price of one!
Source: VitaLinka/Shutterstock

Behavioral disorders — ASD, ADHD, and all other psychiatric disorders — are generally not caused by a single biological abnormality or genetic mutation. Instead, each of these disorders can be caused by literally hundreds of genetic or biological variants, most often in combination, and they have other contributors too. Behavioral disorders may be caused by prenatal infection or other prenatal trauma, postnatal infection or other medical conditions, malnutrition, physical and emotional trauma, stress or other environmental issues both in the past and present — and of course, interactions between all these factors put together.

Given that medical tests aren’t very useful when it comes to diagnosing behavioral disorders, what do doctors have to go on? They have to rely on the history offered to them by patients and the office exam. To make neuropsychiatric diagnoses, doctors ask about behavioral symptoms and observe patients in the office firsthand, and then they categorize: If a patient has behavioral symptom A, behavioral symptom B, and behavioral symptom C, then (assuming the condition is severe enough) they qualify for the diagnosis of Behavioral Disorder X.

These behavioral diagnostic categories are useful. For one thing, they have proved to be very handy for guiding appropriate clinical treatment, including both medication and non-medication treatments. They are also used by researchers, insurance companies, and school administrators — but they have limitations, one of which is that human behavior can never really be compartmentalized so neatly. For example, although there are several types of anxiety disorders listed in the DSM, the general condition of “anxiety” is found across many other behavioral disorders. So although ASD is not considered an “anxiety disorder” people with ASD are often anxious. So are people with ADHD, as are people with migraines, asthma, cardiac arrhythmia, etc.

To put it another way, behavioral issues that patients face are not so easily cataloged as medical books (including the DSM) might tempt a person to believe. The DSM is just a tool designed to categorize human behavior in a clinically useful way — but it is inherently artificial, and must be taken with a grain of salt (and preferably used by a well-trained clinician with plenty of practical experience and good judgment).

For years, the DSM stated that the diagnoses of ASD and ADHD were mutually exclusive. In the latest version of DSM, however, co-diagnosis is allowed. As it turns out, on the basis of simple symptom categorization, there is a fair amount of overlap between these two behavioral conditions. Here are some statistics: There is a 5-10% incidence of ADHD overall in children, and a 2-5% incidence of ASD. Based on DSM-5 criteria, around 50% of children diagnosable with ASD could also be diagnosed with ADHD, and around 15% of children diagnosable with ADHD could be also be diagnosed with ASD.

Why do these two disorders have so much overlap? For one thing, there is overlap in their behavioral signs and symptomatology. Both disorders make kids appear less engaged. Inattentive behavior in ADHD may cause affected children to miss social cues, for example, which can resemble mild ASD. Kids with ADHD have a higher rate of rejection by their peers, which can lead to them being more withdrawn, which may also resemble ASD. Children with both disorders may have, or develop, problems with empathy, as well as facial recognition.

Children with ASD and ADHD both have high rates of depression and anxiety. Impulsive behavior and deficits in planning (executive functioning) are common to both disorders. Both disorders include problems with pragmatic language, as well as with cognitive flexibility. Both disorders are more often found in the presence of intellectual disability or more specific learning disabilities such as in language, reading or motor skills. There may, in fact, be some underlying genetic or other biological factors common to both disorders: In many families where there is a child with ASD, there is a higher incidence of other family members who have ADHD (and vice versa).

Although we already mentioned that medical tests including brain MRIs are usually not that helpful in making psychiatric diagnosis, at a research level some interesting differences and similarities have been found in brain scans of people with these disorders. Note that these differences are not generally discernable on a single scan and so are not very useful when examining an individual patient at this time — this is research data averaged over hundreds or thousands of test subjects.

That said, there is a larger total brain and white matter volume in a subset of patients with ASD, but this is not typically seen in ADHD, whereas in both disorders a smaller corpus callosum (a connection between the right and left hemispheres of the brain) has been found in some patients. In functional neuroimaging, where different parts of the brain “light up” depending on the rate of metabolism while a person performs a wide range of tasks, the most consistent finding in both disorders is reduced activation of front and side brain regions (the frontal and parietal lobes).

Effective behavioral treatments for the two disorders also have some similarities and some differences: In ASD the major goal is usually to promote social skills development, whereas in ADHD the major goal is usually on managing executive tasks; but controlling impulsive behavior is often a therapeutic focus in both disorders. Many of the therapeutic techniques used in each disorder are similar. For example, the positive reinforcement of desired behaviors, achieving consistent quality sleep, engaging in regular exercise, and assembling appropriate educational support are important strategies for treating both disorders.

There is also overlap in the medications used to treat these two disorders. As with behavioral treatment, medication management focuses on targeting specific symptom improvement. So whether your child has ASD or ADHD, if the goal is to improve attention, a medication that has been FDA-approved for attention (ADHD) may be helpful. Conversely, there are some medications that have been FDA-approved for impulsive behaviors such as aggression in ASD. Although less common, these medications may sometimes be useful for impulse control in severe ADHD.

In summary, if you or your child has been given a diagnosis of ADHD or ASD or any combination thereof, focus on treating actual behavioral challenges more than a diagnosis listed in a book or on a website. If your child has specific challenges with hyperactivity/impulsivity, then look for behavioral strategies or medications to address that particular problem. If social skills training is needed, focus on that.

Always remember that just because something has worked for some other person ostensibly with the same diagnosis, that treatment may or may not be appropriate or what works best for you or your child. Treatment must always be based on what’s in the best interest of the patient — who is an individual with an individualized behavioral condition, not a categorical diagnosis in a book.