GEOFF BENNETT: When actor Christina Applegate revealed earlier this week that she had been misdiagnosed with multiple sclerosis, it triggered not only an outpouring of empathy for her, but raised important questions about how often people are told they have the condition when they actually don't.

Our William Brangham has more.

WILLIAM BRANGHAM: Applegate was first given that M.S.

diagnosis five years ago after complaining of what became debilitating symptoms, including stabbing pains, fatigue, and difficulty walking.

On her podcast about living with what she thought was M.S.

-- the podcast is called "MeSsy" and it's co-hosted by actor Jamie-Lynn Sigler -- Applegate described the very moment she got this news from her doctor.

CHRISTINA APPLEGATE, Actress: She came into my room and she said: "Congratulations.

You don't have M.S.

Congratulations."

JAMIE-LYNN SIGLER, Actress: What does that moment feel like?

So loaded.

I cannot imagine.

CHRISTINA APPLEGATE: Well, I was like not happy at all.

JAMIE-LYNN SIGLER: Yes.

CHRISTINA APPLEGATE: You know, not -- everybody else was so happy.

JAMIE-LYNN SIGLER: I know.

When you told me, I feel like... JAMIE-LYNN SIGLER: My literal first words for you like, you can dance again, because that's just like where my head goes here.

CHRISTINA APPLEGATE: Yes.

Right.

JAMIE-LYNN SIGLER: You know what I mean?

Like... CHRISTINA APPLEGATE: Because you don't get misdiagnosed, and then all of a sudden I'm better.

WILLIAM BRANGHAM: And Applegate is not alone.

Statistics vary on the rate of misdiagnosis, but they range anywhere from 5 percent to one analysis estimating it's as high as 40 percent.

For more on all of this, we are joined now by Dr.

Michael Kornberg.

He's an associate professor of neurology at Johns Hopkins University.

Dr.

Kornberg, thank you for being here.

We should say at the outset, you are not Christina Applegate's doctor.

You have not reviewed her medical files.

But we have to assume she has plenty of resources, has access to excellent care.

How is it that this could have occurred?

How common is this?

DR.

MICHAEL KORNBERG, Johns Hopkins Medicine: So you mentioned the range of figures from various studies.

I would say that, in a specialty clinic where you are seeing neurologists who are experienced in treating people with multiple sclerosis, it's going to be on the lower end of that estimate.

But that still leaves somewhere to 5 to 10 percent of people who were given a diagnosis of M.S.

that later are told that that is an incorrect diagnosis.

The majority of cases, it is pretty clear, but obviously there's a sizable minority for whom that's not the case.

WILLIAM BRANGHAM: And why is that?

Why is M.S.

so hard to pin down?

DR.

MICHAEL KORNBERG: So there's a few issues.

The first is, it affects the brain and the spinal cord, which are not very accessible organs.

We're not doing biopsies for diagnosis.

There's no one single test that tells you have M.S.

There's not something in your blood or spinal fluid that everyone with M.S.

has and everyone without M.S.

doesn't have.

And so it is a matter of having typical neurologic symptoms and typical findings on MRI of the brain and spinal cord, sometimes supplemented with other tests.

But other conditions that cause neurologic symptoms that can cause some abnormalities on a brain MRI can be confused for M.S.

and lead to a misdiagnosis.

WILLIAM BRANGHAM: I know you're not a psychologist, but I imagine it must be just very shocking for a patient to spend many years thinking they have one thing, only to find out that that's not what has been ailing them.

DR.

MICHAEL KORNBERG: That is absolutely right.

So, the issue of misdiagnosis in M.S.

is something that has been studied.

It is a topic that we talk about in the M.S.

community.

And it is very distressing for people.

I think Christina Applegate articulated all of this and all of those common feelings very well.

So, when you are living with chronic disabling symptoms, it is not -- it is not reassuring to be told that you don't have M.S.

What you're being told is, the cause of your symptoms are not M.S., but you're still living with them.

It means it's something else.

But then also you're taking away the comfort that comes with just having a label for what's happening to you, what's going on in your body.

That is very comforting to people, even if they are still living with symptoms.

And as she mentioned in her podcast, she became very close to the M.S.

community.

This became kind of part of her identity, which often happens, and it can be very distressing when that is taken away from you.

WILLIAM BRANGHAM: Here's another tiny bit of her podcast.

Here's -- this is how she described where she finds herself today.

CHRISTINA APPLEGATE: And so I'm back at square one that I don't know what's going on with me.

And that's going to be an online investigation that my doctors are going to get into and just to find out, what has been happening since 2021?

Why did this happen?

I have every symptom of someone with M.S., every single one.

WILLIAM BRANGHAM: So, if someone is living with this suite of symptoms, or someone is living with a diagnosis of M.S.

and is hearing this news and might be uncertain about what they really have or whether they have been misdiagnosed, what would you counsel them?

DR.

MICHAEL KORNBERG: Yes, so, many things that we could say.

But, first, I do think it's important to say that, even though there is a minority, a sizable minority of people for whom this is an issue, that in the majority of cases it is a clear diagnosis.

And so I don't want everyone living with M.S.

thinking that, oh, my gosh, I probably was misdiagnosed.

Having said that, a few other things that we could say.

First, if you are having any question, obviously, it is never a bad idea to have a second opinion.

I think it is a good idea if you have M.S.

to be treated by a neurologist who has significant experience treating people with M.S.

Some other things that we can say is that there are some symptoms of M.S.

that are very typical and others that are you know what we call nonspecific and shared with other conditions.

And Christina Applegate mentioned a few.

Fatigue, very common in M.S., very common in other conditions.

Pain, very common in M.S., common in others.

And so if you're having some of these more nonspecific symptoms, if you get the sense that there is something unusual or atypical about your presentation, your case, then it doesn't hurt to get a second opinion.

It certainly doesn't hurt to just be open and honest about your concerns with your neurologist.

A good neurologist should be able to tell you whether you have a straightforward case of M.S.

or in instances where there's uncertainty.

WILLIAM BRANGHAM: Dr.

Michael Kornberg of Johns Hopkins, thank you so much for being here.

DR.

MICHAEL KORNBERG: Thank you very much for having me.