No one except Gregory Kapothanasis knows exactly what upset him today. On this hot day in July, he went to his day program for adults with developmental disabilities, as he has done without incident five days a week for the past four years. But then things unraveled. According to the program’s report, he grabbed a staff member’s arm hard enough to bruise it. Then, on the bus during the daily outing, he started screaming and hitting his seat. Now, several hours later, he is finally home, but there is a stranger in his living room. Bouncing from one couch to another, clutching a faded beige blanket stolen from his aunt’s dog, Kapothanasis still seems out of sorts.

His mother, Irene — who has cared for him, with the help of home aides, for all of his 24 years — is playing over the day’s events, trying to figure out what triggered him. His outburst is disturbingly reminiscent of a difficult period that peaked six years ago but is uncharacteristic of the young man today. Kapothanasis loves interacting with other people, going to the beach and dining at DiMillo’s, a floating restaurant in a decommissioned car ferry in Portland, Maine.

Kapothanasis has autism and speaks only a few words: He can’t explain what happened this morning. Did he have constipation and discomfort, as his doctor suggested? Did he get bored of the day’s program, causing him to act out? Had something occurred on the bus previously that made him fear that part of his day? All his mother can do is wonder — and try to make his evening better.

Kapothanasis places several pieces of gum in his mouth, something his mother says soothes him. Then he curls up for a nap and dozes off. The time it takes him to calm down is short compared with the protracted meltdowns that routinely overtook him during his teen years. He became aggressive starting in puberty: He would bite and hit himself, or hit and grab other students, teachers and bus drivers. His mother had to supervise his interactions with his siblings, even though, as triplets, they had always been close. In December 2011, when he was 18 years old and over 6 feet tall, his behaviors had escalated so much that his school enrolled him at the inpatient clinic at Spring Harbor Hospital in Maine.

“Look, we all have scars from Gregory, we’ve all felt his wrath. And it came to the point where he had to be institutionalized,” his mother says. He spent five long weeks at the hospital. “It was probably one of the darkest moments in our lives,” she says.

When Kapothanasis arrived at the hospital, he didn’t seem sad, cried only on occasion and didn’t respond to things not visible to others — signs he did not have depression or psychosis. But he startled easily, paced and rocked in place, and sweated heavily. “The phrase ‘cat on a hot tin roof’ was pretty descriptive,” recalls Matthew Siegel, director of the developmental disorders program at the hospital.

After several weeks of close observation, Siegel and his colleagues managed to piece together Kapothanasis’ behaviors into a clear diagnosis, based on criteria outlined in the “Diagnostic and Statistical Manual of Mental Disorders.” In addition to his autism, “he was kind of screaming anxiety — if you’re looking for it,” Siegel says. Until then, it seems no one had been. When Kapothanasis entered the clinic, he was on his third antipsychotic medication. Some are approved to treat aggression as a feature of autism, but none treat anxiety. “It’s fair to say that he was not being treated for anxiety,” Siegel says.

There are many reasons it took nearly six years for Kapothanasis to get the help he needed. Doctors may have assumed that his aggression and tendency to hurt himself were part of his autism, Siegel says. Traits that characterize autism — including social deficits, stereotyped movements and restricted interests — can mask or mimic symptoms of anxiety. During a visit to an outpatient clinic, for example, Siegel points out a nonverbal young woman with autism who repeatedly traces a pattern in the air with her hands. At first glance, her gestures resemble ‘stimming,’ the repetitive behaviors often seen in autism. But she does it at specific times, Siegel says, suggesting a ritual related to obsessive-compulsive disorder — a form of anxiety.

Compounding the problem, many people on the spectrum, like Kapothanasis, cannot tell their caregivers or doctors what they are feeling or thinking. Those who can may still struggle to identify and understand their own emotions — a phenomenon called alexithymia — or to articulate them to others. Because of these factors, the clinical questionnaires designed to ferret out anxiety traits in neurotypical individuals are woefully inadequate for many people with autism. The tests may also miss children with autism, who can have unusual phobias, such as a fear of striped couches or exposed pipes.

“People on the spectrum have really unique, distinct ways of perceiving the world, and also have distinct experiences, which is why we’ll see classic things like social phobia and generalized anxiety, but also maybe these more distinct, more autism-related manifestations,” says psychologist Connor Kerns, assistant research professor at the A.J. Drexel Autism Institute in Philadelphia. Kerns and others are working on new ways to measure both ordinary and unusual forms of anxiety in people with autism. This work could help clinicians better detect the anxiety that hides behind autism, reveal the underlying mechanisms and lead to better treatment.