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The 9-year-old girl was strangely quiet throughout the meal. It was so uncharacteristic that in retrospect her father wasn’t sure why he didn’t notice. But he didn’t — until she started to scream. As tears ran down the girl’s face, she pointed to her foot, which was nestled in her lap.

The man scooped his daughter into his arms and hurried out to his car. The crying stopped by the time they reached the urgent-care center near their hotel in Miami Beach, but her foot was still too painful to walk on.

In the exam room, the father finally got a good look at his daughter’s foot. He was surprised: It looked completely normal. An E.R. doctor introduced herself and examined the limb. It looked OK to her too, and when the X-ray didn’t show anything, the doctor shrugged. If it keeps hurting, you should follow up with her doctor, she said.

The father was baffled. The girl had twisted her ankle a few weeks earlier, but she was swimming and playing in the hotel pool most of the afternoon — enjoying the sun in this midwinter break before heading back to their home in New York City. But that night of pain was only the beginning. His youngest was a tough kid, but after the trip to Florida she complained about her ankle and foot just about every day. She had to sit out most of basketball season. She said it hurt too much to run.

She had sprained that ankle several times over the past couple of years. Her doctors blamed weak ligaments and a sports-heavy schedule. They were an active family and recognized that injuries were just part of sports. But this was different. It seemed as if her ankle never got better after that last injury. Over the next few months, she saw a couple of orthopedists at the Hospital for Special Surgery. One ordered an M.R.I. of the ankle. That study was its own kind of nightmare. Holding her foot in the correct position was painful, the girl told them. And holding it still was practically impossible. Her foot had developed a tendency to roll to the side so that the arch and sole faced inward rather than down. And her toes were constantly in motion.

The M.R.I. was unremarkable. Physical therapy helped the stiffness but didn’t straighten out her foot. A cortisone shot did nothing. Same with an ankle brace. How could all this grow out of a sprain, the parents asked each other. A rheumatologist was certain that this was juvenile rheumatoid arthritis. The blood tests said otherwise. As spring arrived, the young girl started using crutches to get around. Any weight-bearing was excruciating.

Finally one evening, out of a sense of desperation, the father reached out to a high school friend, Joshua Hyman, who was now a pediatric orthopedic surgeon at NewYork-Presbyterian Morgan Stanley Children’s Hospital. He described his daughter’s pain and the strange stiffness that seemed to keep her foot on its side. There was a thoughtful silence on the phone, and then Hyman asked his friend if he was able to move his daughter’s foot — able to put it in a more normal position? He could, the father answered, but it felt as if the foot were fighting him. And as soon as the pressure was released, it would slowly turn back to its side. That’s how it felt best, his daughter told him.

There was another pause. “Has anyone mentioned something called dystonia to you?” Hyman asked.

A Wrong Message?

Dystonia is a movement disorder characterized by persistent or intermittent muscle contractions, which cause abnormal, often repetitive movements or positions, Hyman explained. It can affect any part of the body, and in some cases the entire body. It’s a problem with the brain and the nerves. They send the wrong message to the muscles. There are lots of possible causes, Hyman added: medications, infections, brain injuries, a handful of inherited diseases. Dystonia? the father asked. He wasn’t sure he’d ever heard the word. How did she get this? Why did she get this? Is there a treatment? Hyman was sympathetic but couldn’t answer his friend’s many questions. For that they needed a neurologist — probably one specializing in movement disorders.

It was late spring when the family got in to see Dr. Steven Frucht, director of the Fresco Institute for Parkinson’s and Movement Disorders at N.Y.U. Langone Health. As soon as he entered the exam room, Frucht noticed that the girl’s feet were asymmetric. She was sitting on a chair that was too tall for her feet to reach the floor, so they just hung down, at rest. The right foot was straight, as expected. The left pointed inward, toward the right, at a very unnatural angle. Frucht introduced himself and listened as the girl and her parents described her months of ankle pain.

He asked if she could put her feet flat on the floor. It was easy to do with the right foot, but the left seemed to curve inward so that only the outer edge of the foot rested on the ground. And the big toe on the left foot moved up and down constantly. The toes on the right were still. “Why are your toes moving?” Frucht asked. “Because it’s uncomfortable,” the girl replied.

Rapping Her Knuckles on the Table

Frucht asked if she had any problems with her hands. “Not really,” the girl answered. “Except I always get cramps when I write.” He gave her a pen and paper and asked her to write something for him. She had been writing for only a few seconds when she put down the pen and rapped her knuckles on the table. She then picked up the pen and started writing again. “Why did you do that?” Frucht asked. It feels better, she said.

Frucht shared with the family the diagnosis he’d been considering since he first saw the child. There is a form of dystonia that, while rare, is the most common form of inherited limb dystonia seen in people of Ashkenazi Jewish descent. It is called DYT-TOR1A — named for the gene that is abnormal in this disease.

“Whom could she have inherited it from?” the girl’s father asked. Neither parent nor anyone in either family had this problem. Frucht nodded; it is one of the mysteries of this disease. Only one parent has to have the gene to pass it on. Yet most people with this abnormal gene never develop the disease. No one understands why. Her father asked if it was her sports injuries that set her up for this. Probably not, the doctor replied, although there is still so much we don’t know. What we do know, he told them, is that this disease is often progressive. It had spread from her right hand to her left foot. And it may spread farther. Of course, they would need to do a test to see if she had this abnormality. Results would take about a week.

When the test came back positive, the doctor started the girl on a drug called Artane, which blocks some of the brain’s abnormal signals to the body. As the dose was increased, the girl and her parents could see the difference. Her walk wasn’t completely normal, but it was much better — so much better that she was able to go to summer camp.

But she had to keep increasing the dose of the medication. And by midfall she was starting to have side effects. Her performance in school, which had always been excellent, started to suffer. Frucht had told the family that there was another kind of treatment for this disorder, though he didn’t recommend it for younger children. It was something called deep brain stimulation. A small electrode could be implanted in the part of the brain where movement originates. Tiny electrical pulses would be generated at the site of the implant, and these pulses could block most of the abnormal signals that were driving the girl’s muscles to contract and jump. The parents were worried by the prospect of permanently implanting something in their daughter’s brain, but were even more worried by the progressive nature of this disease. The girl was excited by the prospect of not having to take a pill three times a day, but when one surgeon they saw said she would have to shave her head for the surgery, she immediately said no. She was quite proud of her long dark hair. Her parents kept looking. Finally they found a surgeon who had a lot of experience and would shave only a small patch of scalp.

It took awhile, but the surgery has been life-changing. Or perhaps, the family would say, life-normalizing. Because she’s back: She’s back on the basketball court, back to her good grades. She still has her head of beautiful dark hair; and even the tiny patch that was shaved for the surgery is back.

Lisa Sanders, M.D., is a contributing writer for the magazine. Her latest book is “Diagnosis: Solving the Most Baffling Medical Mysteries.” If you have a solved case to share, write her at Lisa.Sandersmdnyt@gmail.com.