Rachel Schneider was initially told she had a panic disorder. Years later, a different explanation for her disturbing condition emerged. (Josh Schneider)

Rachel Schneider was 13 years old when it happened the first time. She and a few other children had been singing prayers at her Hebrew school when suddenly Schneider felt that something inside her head had gone wrong and that she wasn’t there. Her father brought the terrified girl to an emergency room, where doctors dismissed the episode as a panic attack. They said that Schneider was fine.

It happened again while Schneider was on vacation in Quebec City with her parents and sister. Inside a souvenir shop, she was startled by a man who came up behind her holding a small marionette and speaking rapidly in French. Schneider screamed and ran out of the store. Her mother tried to comfort her, but Schneider could not be consoled.

“Suddenly it was like the entire room was pouring into my brain, and the lights, and the sound,” Schneider said recently. Speaking of that first incident, she said she felt as if a chasm had opened up inside her head, and she was seeing but not seeing, and hearing but not hearing.

“It changed things,” Schneider said. “It was the biggest one of them I’d ever had.”

Soon Schneider was diagnosed with panic disorder. For the rest of her adolescence, she avoided going out at night or being in large crowds at concerts or parades, afraid that her symptoms — feeling numb and being sensitive to loud sounds and sensitive to light — would produce another attack. Once, she had an attack in an airport after a long flight. Jet-lagged and hearing her name blared over a loudspeaker, Schneider thought that she was going crazy — but it had just been her father, who was having her paged to make sure that she had made it safely.

At 27, Schneider, a copywriter who lives in New York, finally sought more answers.

“I was seeing a psychiatrist, and she wouldn’t listen to me when I kept saying, ‘I have sensory stuff; there’s something sensory going on,’ ” Schneider said. “She would say, ‘No, you have panic disorder.’ . . . So I found somebody new.” Her new psychologist sent Schneider to an occupational therapist, who confirmed the psychologist’s suspicions: Schneider had sensory processing disorder, or SPD.

For people with this disorder, the brain has trouble receiving and responding to sight, touch, sound, smell, taste, the vestibular sense (balance and gravity), the proprioception sense (body orientation and position in space) and interoception (sensations felt within the body). Defined by occupational therapist and developmental psychologist A. Jean Ayres in 1972 as “sensory integration dysfunction” — the disorder would be renamed in 2004 by Lucy J. Miller, one of Ayres’s former research assistants — Ayres coined the term to describe a disorder of children who seemed affected by poor sensory processing, resulting in hyperactivity, behavior problems, speech and language delays, muscle tone and coordination problems, and learning difficulties.

The American Psychiatric Association considers SPD a sub-symptom or co-disorder with autism spectrum disorder, labeling it as oversensitivity in DSM-V, the most recent Diagnostic and Statistical Manual of Mental Disorders.

Schneider had always been an excellent student, but she had suffered from poor coordination and from anxiety about going places in the dark. She believed that her symptoms had worsened with the onset of puberty, resulting in the misdiagnosis of panic disorder.

As an adult, Schneider, now 33, has had trouble sleeping and has needed deep touch pressure, a form of tactile sensory input provided by firm holding, stroking or squeezing, to calm down. Many people with SPD have similar symptoms: sensitivity to noise, sensitivity to the fit and texture of clothing, sensitivity to touch and to smells. Poor balance and low muscle tone are other effects, along with problems coordinating some physical activities. Some adults with SPD are unable to have sex or drive a car.

According to Teresa ­May-Benson, executive director of the Spiral Foundation, an SPD advocacy organization, the first challenge is identifying a person’s affected sensory systems. “Is it a movement, or vestibular, problem? Is it a tactile problem? Is it poor proprioception, muscle information? Is the person oversensitive, having what we would call a modulation problem?”

For Schneider, occupational therapy made an immediate difference, starting with the 17-pound weighted blanket that allows her to sleep more comfortably.

“That was one of the first tools I got,” Schneider said, “because everybody [said], ‘You’re not sleeping, because your body is thrashing. It doesn’t know where it is in space.’ ” Other habits she picked up include using only natural light in the apartment she shares with her husband and using a deep-pressure brush to help stimulate her sense of touch.

For her, an episode is much less overwhelming now than when she was 13.

But the episodes of dissociation can still frighten her. Sitting on a couch at her home, she teared up slightly, noting, “You can tell I’m getting emotional talking about it. I’m emotional because they’re that scary still.”

What new techniques reveal

Since Schneider was diagnosed, researchers have been able to get a better picture of the brains of people diagnosed with SPD, using more-advanced technology.

In 2013 and 2016, a University of California at San Francisco research team led by Elysa Marco and Pratik Mukherjee published two studies on children with SPD.

The first showed that the white-matter microstructure in the brains of boys with SPD was different from that of those in a control group, particularly in the areas linked to sensory processing. The second study repeated these findings on a larger group of both girls and boys.

“I think our work really strongly shows that these differences are brain-based,” Marco said.

To Schneider, this research was validating. “It was a ‘eureka’ moment for me,” she said. “I knew it existed, I was talking about it for a couple years before then, but suddenly for the first time I had proof. I had proof that I’m different, and here it is, here’s why I’m different.”

Schneider, who has an active social life and no obsessive behaviors, does not believe she has autism spectrum disorder. She and many SPD researchers believe that her condition — which Miller estimates may affect 1 in 20 people — should be recognized as an independent diagnosis. Much of the research has been focused on proving that SPD can occur independently, and separate scientific work groups have shown that between 37 and 80 percent of children diagnosed with SPD have no accompanying disorders.

Asked for comment on this issue, Darrel Regier, vice chair of the APA task force that produced the DSM-V, said, “I think there is certainly a possibility” that SPD would be approved as a separate disorder.

“If there’s a longer period of consideration,” he said, “if there’s more extensive evidence of people who really do have a separate condition, and they [can be proven] to not have autism spectrum disorder or some other neurological condition, I think it could certainly be approved.”

Schneider hopes that approval comes soon.

“Over- or underresponsivity to information, yes,” she said, but “I don’t think [autism spectrum disorder] captures my experience right, at all.” In 2015 she published a book about SPD, “Making Sense: A Guide to Sensory Issues.” She says she knows there are more adults like her who struggle with sensory processing and want to know what causes it.

“I have faith that I can do pretty much anything if I can only figure it out,” Schneider said, “because, if it really matters to me, even if I have to do it differently from most people, I will still find a way to do it.”