The 16-year-old looked acutely psychotic — but her problem was something else entirely. (Robert Carter/For The Washington Post)

As the jet hurtled toward New York’s John F. Kennedy International Airport on New Year’s Day 2013, the clinical psychologist watched her 16-year-old daughter warily, praying there would be no recurrence of the girl’s inexplicable and bizarre behavior. The previous night, while walking down a street in Spain where the family had spent Christmas, the teenager suddenly began yelling that the traditional New Year’s Eve fireworks were actually bombs.

On the flight home, the girl seemed entirely normal. Her mother thought the high school junior might have had a panic attack, stressed by her upcoming college search and impending wisdom teeth extraction.

But the uneventful flight brought a short-lived relief. Five days later, the teenager was hospitalized for treatment of what appeared to be a severe psychotic break. And for the next six weeks, the news seemed to get worse as a more ominous diagnosis emerged — and with it the specter of death.

“Every day seemed like a horror story,” said Carmen, a psychoanalyst who practices in New York and whose last name, along with that of her daughter, Mia, is being withheld at her request to protect her professional privacy.

For Lara Marcuse, a neurologist at Mount Sinai Hospital in Manhattan who treated Mia during her hospitalization, those weeks were filled with tension and anxiety that deepened as she worried that the teenager might not survive her sudden illness.

“If she was my age,” said Marcuse, who is 44, “Mia would either be dead, in a coma or in a state psychiatric center.”

Instead Mia, now 18, has fully recovered. She recently had a part in her high school play, is anticipating graduation and looking forward to entering college in September.

‘Mom, help me!’

Mia’s outburst about bombs occurred as her family was wrapping up their annual visit with relatives in her mother’s native city of Barcelona. Mia began screaming “out of nowhere” and complained that her mouth was “distorted.”

“I didn’t understand what was going on,” Carmen recalled. Mia calmed down quickly and, with the help of a Valium, went to sleep. The next morning, as the pair awoke early to fly home to New York, Carmen quizzed her daughter. Mia said she felt fine and brushed off questions about what had been wrong.

Carmen remembers feeling glad that Mia had a previously scheduled appointment with the therapist who had been helping her cope with her father’s sudden death several years earlier. But the following day Carmen received a frantic call at her office from Mia, who yelled “Mom, help me!” but refused to elaborate. She raced home to discover that Mia was too afraid to leave their apartment and go to the therapist by herself, something she’d done dozens of times. They went together, and by the time they arrived Mia seemed herself, if somewhat anxious. The therapist agreed that Mia should be watched closely.

The next few days were punctuated by bizarre episodes. Mia called her uncle “Dad” and started speaking as though her mouth was full of cotton. She told her mother she was “dancing to the music of Shakira” when no music was playing. She complained that one of her arms was longer than the other, and she stood in an oddly hunched posture to compensate.

Mia said that she remembers feeling dizzy and as though she had “super hearing”: Sounds seemed excessively loud. “It was like I was looking at a kaleidoscope and everything was distorted,” she recalled. Colors looked so bright they were painful.

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Carmen, who was increasingly alarmed, was also puzzled. “These episodes didn’t make any sense. She would go in and out of them. It didn’t follow anything I’d ever seen.”

She did not ask Mia whether she had taken drugs. “My daughter is so strait-laced,” she said, “and I knew she hadn’t gone to a party.”

Four nights after they returned from Spain, Mia’s condition worsened dramatically, and she became increasingly agitated and unable to sleep. The next morning, after conferring with her therapist, her mother took her to Mount Sinai’s emergency room. “She was kind of slipping through my fingers,” Carmen recalled.

Mia was admitted to the psychiatry service and doctors began tests: A toxicology screen for drugs was negative, as were tests for pregnancy, Lyme disease and HIV. An MRI scan of her brain was clear, which ruled out a brain tumor. The diagnosis was atypical psychosis, and Mia was started on high doses of antipsychotic drugs.

“She certainly looked like someone having her first psychotic break,” said Marcuse, who is co-director of Mount Sinai’s epilepsy center. Two days after Mia was admitted, Marcuse was asked to look at her case by a colleague who had known Mia’s father.

As Marcuse flipped through her chart, one test caught her eye. Mia’s electroencephalogram, or EEG, a test that measures electrical signals in the brain, showed what Marcuse said was “a very subtle finding”: a slight slowing in the right frontal and temporal lobes of her brain.

“If she was 70, that would have meant nothing,” Marcuse said. “And if she had schizophrenia or a psychiatric illness, [the reading] should have been normal.”

Marcuse said a second, longer EEG was performed. It found a more pronounced right-sided slowing, which Marcuse called “very abnormal.”

At that point, Marcuse said, she was virtually certain she knew what was wrong with Mia — and it wasn’t a psychiatric disorder, although it looked like one. Mia was transferred to neurology and became Marcuse’s patient.

Carmen remembers feeling elated by this turn of events. “You think, ‘Well, that’s it, they know what it is and they’re going to cure her and she’s going to be out of here in three days,” she said.

But Marcuse knew that nothing could be further from the truth. Mia’s illness is “like walking someone through Hades,” the neurologist said. “And most people get worse before they get better.”

Like a rock rolling downhill

Marcuse believed that Mia had limbic encephalitis, an inflammation of the brain; the disease can be triggered by an infection or an autoimmune reaction in which the body attacks itself. Sometimes there is an underlying cancer, or a benign ovarian tumor called a teratoma; in other cases the trigger is unknown but tests reveal the presence of certain antibodies that attack the brain. (The disorder is also called anti-NMDA receptor encephalitis or antibody-mediated encephalitis.)

Treatment depends on the underlying cause: If cancer or a tumor are present, they must be removed. If there is no tumor but antibodies are attacking the brain for an unknown reason, drugs to neutralize the antibodies are essential.

Early symptoms of the disease include impaired memory, confusion, personality changes and hallucinations that can resemble schizophrenia or another psychotic illness. Patients then progress to seizures, which can be prolonged and life-threatening. Without treatment, the patient can lapse into a coma, suffer permanent neurological damage or die.

“It’s like trying to stop a rock from rolling downhill,” Marcuse said of the course of the illness. She has treated about six cases in her career — three in the last year alone — and said that the illness continues to be mistaken for schizophrenia, in part because doctors have not heard of it. A 2012 editorial in the British Journal of Psychiatry echoed that warning.

Time is critical to prognosis: The sooner treatment is started, the less likely it is that damage will occur. An international multi-center study published last year found that treatment started within four weeks of symptoms was a predictor of a good outcome.

A spinal tap and other tests confirmed Mia’s encephalitis diagnosis, but scans did not find cancer or an ovarian tumor. Mia was immediately started on high-dose steroids and other medications, augmented by a cocktail of psychiatric drugs to diminish the hallucinations and other symptoms. But instead of getting better, Mia continued to deteriorate.

She remained extremely agitated and unpredictable. She kicked Marcuse in the stomach and screamed, “You are going to hell, I hate you!” She wandered around the ward announcing she was Santa Claus or that she was pregnant. One night she talked nonstop. Soon she could no longer tie her shoes or draw a clock face. For a while she became virtually catatonic, unable to talk or to feed herself. Part of her face was paralyzed.

Mia remembers little, other than her inexplicable fear of Marcuse.

After a week, it was clear that first-line treatment was failing. “I was very concerned for her,” said Marcuse, who found the facial paralysis alarming because it typically occurs when the illness is more advanced and harder to treat. “It was a very draining time.”

There was little choice except to try the other remaining treatment: infusions of a potent anti-cancer medicine called rituxan.

“It’s a risky medication,” Marcuse said. After extensive consultations with Carmen, Mia received the first of four weekly doses on Jan. 24.

A striking turnaround

A few days after the first dose, Mia became less agitated, and her speech was less odd. After the second dose she suddenly turned to her mother and announced, “Mom, I’m lucid!” She was able to tie her shoes and her cognitive abilities began to return. She was discharged from Mount Sinai on Feb. 8, after her third infusion. On April 1 she went back to school.

Mia will be closely monitored for several years, Marcuse said, because the disease sometimes recurs. Luckily, she shows no signs of residual cognitive or motor impairment, as Marcuse had initially feared. The cause of her encephalitis remains unidentified, because doctors have not found a tumor or other trigger.

Both mother and daughter credit the neurologist for what Carmen calls Mia’s “amazing recovery.” But her daughter’s ordeal has led Carmen to think back to the patients she saw on psychiatric back wards earlier in her career. She wonders if any of them were actually suffering from encephalitis, not schizophrenia. “I thought, ‘Oh my God, did we ever miss one?’ ”

Marcuse has had similar thoughts and says that had Mia’s illness occurred a few decades ago, she almost certainly would have been misdiagnosed as having schizophrenia. “This is not a new disease, and people have been dying of this for a long time,” she said. “And the fact of the matter is that it’s still getting missed.”

For Mia, who used her ordeal as the subject of her college essay, the past year has been life-altering. “I realized all the love that my family had for me, which was a nice lesson — but I wish I could have learned it a different way.”

Submit your solved medical mystery to sandra.boodman@washpost.com. Read previous mysteries here.