A persistent headache turns out to be something ominous

"It was on the right side of my head," recalled Klein, now 33, "and it just kind of lingered." Klein tried ignoring it, then took over-the-counter pain relievers, which didn't help. When he noticed that his eyes seemed light-sensitive, Klein assumed he knew what was wrong.

"This must be a migraine," he remembers thinking.

But over the next two dizzying weeks, Klein would learn that his headache wasn't a migraine, but the harbinger of a condition that would irrevocably reorder his life.

"It was really hard to let it sink in and then to tell people that I love," he said of the diagnosis, which was made with unusual speed. "One of the hardest things is that there's so much ambiguity" about the outcome.

By the time Klein and his girlfriend, Kelly O'Connell, returned to their apartment in Washington, Klein's headache had lasted a week. He wasn't concerned, but O'Connell was adamant that he see a doctor. A week-long headache seemed excessive to her.

Because he had been in excellent health, Klein said, he did not have a primary-care doctor. He headed for a walk-in clinic in nearby Montgomery County, figuring he'd get a prescription, which would quickly resolve the problem.

"I figured that what I needed was a triptan," said Klein, a salesman for a major pharmaceutical company, referring to a class of drugs commonly used to treat migraines. When he met the doctor assigned to his case, Klein said, he requested a triptan prescription.

But as the doctor conducted a basic neurological exam and asked Klein to track his fingers with his eyes, Klein reported that the eye movement "hurt - a lot." The doctor stopped the exam. He told Klein that he wouldn't be giving him a prescription but would be sending him to an ophthalmologist for further evaluation.

"He said, 'I don't think it's a migraine,' " Klein recalls. " 'I think your optic nerve is inflamed.' " Located at the back of the eye, the optic nerve transmits images from the retina to the brain's visual centers.

Determining whether there was a problem with Klein's optic nerve would require dilating his eyes. Klein said he felt relieved that the ophthalmologist to whom he was referred was able to see him the same day.

The ophthalmologist quickly determined that Klein's optic nerve was severely swollen, a condition called optic neuritis. But the underlying cause, he told Klein, would require testing more specialized than he could provide.

Klein was referred to neuro-ophthalmologist David M. Katz, who practices in North Bethesda, Md. Neuro-ophthalmologists - the United States has about 500 of them, compared with more than 19,000 ophthalmologists - are trained in both neurology and ophthalmology; they specialize in unusual visual problems that originate in the nervous system, not the eyes.

Klein, who had grown increasingly concerned about both the absence of an explanation and his worsening vision, obtained an appointment with Katz early the following week.

"Over the weekend, the vision in my right eye fell off a cliff," he said. Colors looked bleached, as though he had looked at the sun too long.

While examining Klein, Katz was struck by the severity of the inflammation. Optic neuritis has multiple causes, including Lyme disease, multiple sclerosis and syphilis. Sometimes no underlying reason can be found. To narrow the possibilities, Katz ordered an MRI scan and a battery of blood tests.

Klein remembers Katz telling him that he might have multiple sclerosis. Klein was familiar with the disease; his grandmother and a first cousin had the progressive autoimmune disorder, which occurs when the myelin sheath that encloses nerve fibers in the central nervous system is damaged, impairing the transmission of signals to and from the brain, the spinal cord and the other parts of the body. There are several forms of MS, which can disrupt movement, vision, cognition and ultimately the ability to speak.

Other autoimmune problems, Klein added, are "sprinkled throughout my family."

Klein said that Katz also told him he was ordering tests for another illness, which the doctor did not initially name.

Katz recalls that while he considered MS a possibility in Klein's case, the severity of his optic neuritis, worse than that seen in MS patients, pointed to another diagnosis: NMO, or neuromyelitis optica. The rare autoimmune disease of the central nervous system, also known as Devic's disease, is much less common. There are an estimated 4,000 cases of NMO in the United States compared with about 400,000 MS cases.

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NMO occurs when the immune system mistakenly attacks healthy cells in the spinal cord and eyes, causing eye pain and vision loss as well as weakness, numbness and in some cases paralysis of the arms and legs. Loss of bladder and bowel control can also occur.

Until 2000, when a blood test to detect NMO was invented by scientists at the Mayo Clinic, there was no way to distinguish MS from NMO; the latter was considered a severe variant of the former.

Doctors now believe that they are separate diseases and are treated differently. MS is treated with MS-specific medications that slow the progression of the disease but do not cure it. No drug has been specifically approved to treat NMO; patients are often treated with immunosuppressive drugs that can prevent future attacks but are not curative. These include Rituxan, a chemotherapy drug that is also used to treat leukemia.

NMO attacks tend to be more severe than those caused by MS and can occur in clusters, sometimes separated by years. Minorities, especially African Americans, are at greater risk. Women outnumber men roughly 6 to 1, and the disease strikes both children and adults, with two peaks, in adolescence and around age 40.

Testing confirmed Katz's suspicion. A blood test revealed an antibody to NMO-IgG, the diagnostic hallmark of the disease and a spinal tap provided further confirmation.

On March 15, less than a week after his initial appointment, Klein and O'Connell met with Katz, who broke the news.

"We said, 'What the hell is [NMO]?' " Klein recalled. "We were kind of expecting MS." Klein said he asked Katz if the prognosis for NMO was better. The doctor, he said, "sidestepped the question."

But a quick internet search alarmed the couple. O'Connell, he said, was "absolutely devastated" by the diagnosis, and Klein said he struggled to come to terms with it.

Still reeling from the unexpected news - three weeks earlier, Klein had been perfectly healthy - he began the emotionally wrenching task of informing relatives and close friends.

"The first person I called was my father," he said. "I said, 'Dad, I'm sick,' " Klein recalled. The rest of the conversation remains something of a blur.

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In hopes of restoring Klein's vision in his right eye, which was rapidly spiraling toward blindness, Katz prescribed a course of high-dose intravenous corticosteroids, which eased the swelling and restored his sight. Within a month of his diagnosis, he saw an NMO specialist at Johns Hopkins. Klein began receiving infusions of Rituxan, which so far has prevented another attack. Sometimes, people with NMO have only one attack, but recurrence is the rule.

Despite Klein's family history of MS and other autoimmune diseases, doctors do not believe that NMO is inherited.

Last year, he and O'Connell moved to Colorado to be close to her sister and to accelerate their plans for a family. O'Connell is pregnant; their baby is due in January.

Klein continues to work in pharmaceutical sales. He periodically sees an NMO specialist at the University of Colorado in Denver and has become involved with the Guthy Jackson Charitable Foundation, an NMO advocacy and support group based in California. That affiliation has connected him with other patients.

"After a year and a half, I feel very confident about the course of my treatment," Klein said. The passage of "time - and letting go of what I don't have control over" - coupled with the belief that "I'm doing as much as I can" has enabled him to come to terms with his illness.

"I feel very lucky at the speed with which I got the diagnosis," Klein added. He suffered only a 5 percent vision loss and hopes to be among the fortunate NMO patients who has a single episode.

Speed matters, said Katz, who has treated several NMO patients during his 20-year career. One of them, who had been misdiagnosed for a decade with MS before she saw Katz, was blind in one eye and began to rapidly lose vision in the other. Testing revealed she had NMO. Proper drug treatment helped restore the sight in her failing eye and also improved her mobility.

The ability of doctors to recognize the disorder - or to send them to a specialist who might - is crucial, Katz noted, but the disease remains unrecognized or misdiagnosed.