The Comorbidity Changes With Age

What the research found

A 2023 study in the Journal of Clinical Medicine looked at 472 children and adolescents with high-functioning autism spectrum disorder. All were aged between three and eighteen. All were seen at a single tertiary children’s hospital in Rome. “High-functioning” here means their measured intelligence sat in the broadly average range; children with an IQ below 70 were not included. The average nonverbal IQ in the group was 90.6.

The headline figure is straightforward. Of the 472 children, 153 also carried a second psychiatric diagnosis. That is 32.2 per cent, or close to one child in three. A psychiatric comorbidity, in plain terms, is a separate diagnosable condition sitting alongside the autism itself. It is not a symptom of autism and not a milder or more severe form of it. It is a distinct thing that happens to co-occur.

The most common of these second conditions was attention-deficit/hyperactivity disorder. Among the children who had any comorbidity, 58.2 per cent had ADHD. Next came anxiety and obsessive-compulsive disorders, together accounting for 21.6 per cent. The remainder were much rarer: mood disorders and oppositional defiant disorder at 5.2 per cent each, tic disorders at 4.6 per cent. So two conditions, ADHD and anxiety, do most of the work.

The more interesting result is what happened when the researchers split the children by age. The pattern was not flat. Among preschoolers aged three to five, ADHD was the dominant second diagnosis, present in 20.4 per cent, while anxiety and OCD were almost absent at 0.4 per cent. Among school-age children aged six to eleven, ADHD remained common at 21 per cent, but anxiety had climbed to 10.6 per cent. Among adolescents aged twelve to eighteen, the picture had reversed: anxiety and OCD reached 21.8 per cent, while ADHD had fallen to 7.3 per cent.

Read together, these numbers describe a shift. In the youngest children, the accompanying difficulty tends to be attention and activity. By adolescence, it tends to be anxiety. The authors are careful about what this can and cannot mean. Their study is cross-sectional, meaning it photographs different children at different ages at one moment in time. It does not follow the same child across the years. So it cannot prove that an individual child’s ADHD gives way to anxiety as they grow. It shows that older and younger children in this clinic looked different, which is a weaker claim than a claim about change over a lifetime.

What it means for you and your child

The first thing to hold onto is that a second diagnosis is common but not the rule. Two-thirds of the children here had autism and nothing else on the psychiatric side. If your child has been given one diagnosis, that does not mean others are waiting in the wings. It means roughly a one-in-three chance, in a group like this one, of a further condition being present. That is worth knowing, not worth dreading.

The second thing is that the type of difficulty you watch for may reasonably change as your child gets older. In the early years, the co-occurring problem that showed up most was ADHD: trouble with attention, with sitting still, with regulating activity. In the teenage years, it was anxiety. This does not mean every young child will develop anxiety later, and it does not mean an anxious teenager was necessarily inattentive as a toddler. It is a pattern across the group, offered as a prompt to stay alert rather than a forecast for any one child.

For anxiety in particular, the practical value is early recognition. Anxiety in an autistic adolescent can be easy to miss or to misread as ordinary autistic reticence. If the number rising to more than one in five by the teenage years reflects something real, then the years leading into adolescence are a sensible time to pay attention to worry, avoidance, rituals and low mood, and to raise them with a clinician rather than waiting.

It is also worth being clear about the limits of this single study, because they bear directly on how much weight to give it. The children came from one hospital in one country, which means the exact percentages may not transfer cleanly to other settings. Children with intellectual disability were deliberately excluded, so the findings speak only to autistic children in the average IQ range. Sleep problems were not counted at all. And because the design is a snapshot, the age pattern is suggestive rather than proven. None of this makes the study weak. It makes it one solid data point, not a final word.

What you can take from it is modest and useful. Comorbidity is common enough to keep in mind and uncommon enough not to assume. The kind of difficulty that accompanies autism may look different at five than at fifteen. And the most actionable move is not to predict, but to keep the conversation with your child’s clinicians open as they grow, so that a second condition, if it appears, is named early and supported well.