Calling her multiple sclerosis diagnosis “overwhelming,” actress Selma Blair says she’s relieved to at least connect her symptoms to a disease.

“I have MS and I am ok,” Blair, 46, wrote on Instagram Saturday as she went public with her story for the first time.

“I am disabled. I fall sometimes. I drop things. My memory is foggy. And my left side is asking for directions from a broken gps. But we are doing it. And I laugh and I don’t know exactly what I will do precisely but I will do my best.”

Like many MS patients, it was a long journey to get answers: Blair received the diagnosis in August, but she believes she’s probably had multiple sclerosis for at least 15 years, she wrote. That’s not a surprising scenario, said Bruce Bebo, executive vice president of research at the National Multiple Sclerosis Society.

“It’s pretty common for people, when they get diagnosed, to reflect back on symptoms that they had during their lifetime and now see them for what they were,” Bebo told NBC News.

“They may have been ignored, they were perhaps minor… You live your life, move forward and you don’t think much about it.”

What is MS?

Multiple sclerosis is a disease of the central nervous system that affects a person’s brain and spinal cord. The cause is a mystery, but the condition damages the material that surrounds nerve cells, slowing down or blocking communication between the brain and the body.

Almost 1 million people live with MS in the U.S., the National Multiple Sclerosis Society estimated. Most patients are diagnosed between the ages of 20 and 50, and the disease is two to three times more common in women than in men.

"MS is a more common disease than a lot of people think. It affects your neighbors, it affects your friends," Bebo noted.

What are the symptoms?

The signs depend on where in the body the disease strikes, Bebo said. No two people have the same symptoms, and each person’s experience can change over time, the National Multiple Sclerosis Society noted.

When the inflammation that causes MS attacks the optic nerves, common symptoms can include vision problems, such as blurry vision, double vision or even temporary blindness, Bebo said. Ozzy Osbourne’s son Jack became about 90 percent blind in one eye before his MS diagnosis.

If the attack affects parts of the brain that control sensation, patients may experience numbness or tingling in their hands or feet.

If certain pathways in spinal cord are attacked, it can affect people’s ability to move and walk. Their feet will drop and they may easily trip, Bebo said.

“Silent symptoms,” those not readily apparent to an observer, can include cognitive changes like Blair’s “foggy” memory, fatigue, mood swings and depression.

Less common signs include speech problems, trouble swallowing, tremors and seizures.

For many people, the symptoms will come and go, making the disease frustratingly unpredictable.

“It’s one of the real cruel parts about MS,” Bebo noted. “So you don’t know: One day, you’re going to feel great and the next day, you’re going to have an attack… and maybe won’t able to perform your job that day or take care of your children.”

How is MS diagnosed?

There isn’t one definitive test that can determine whether a person has MS or not, Bebo said. Rather, it’s a series of tests and a process of elimination.

Blair said she finally received her diagnosis after getting an MRI. Indeed, areas where nerve cells have been damaged by MS can show up on the scan. Other diagnostic tools include blood tests, a spinal tap to test a person’s cerebrospinal fluid, and optical coherence tomography — a scan of the retina.

What is the treatment?

There’s no cure for the disease.

People with relapsing-remitting MS, where symptoms are followed by periods of recovery, have more than a dozen medication options.

“Altogether, the experience of someone who is living now with relapsing MS is much improved and the prognosis for their future is much better than it was 20 years ago,” Bebo said.

The options are more limited for patients with progressive MS, where the symptoms steadily worsen, but there is hope. Last year, a drug called Ocrevus became the first to be approved specifically for primary progressive MS, a very aggressive form of the disease.

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