The cause is a mystery — and the symptoms can be distressing. Bell's palsy comes on without warning, bringing partial paralysis or weakness to a person's face.

The condition affects thousands of people in the U.S. every year. And now Tiffany Chen, the partner of Robert De Niro, is speaking out about her experience with Bell's palsy for the first time.

In an exclusive interview with "CBS Mornings" co-host Gayle King, Chen revealed her diagnosis, which began after the birth of the couple's daughter in early April. 

"When I got home, it was like everything was starting to just fall down on itself. Like, my face was melting on itself," Chen says in the interview. "Then a week after giving birth, that was when it all hit."

After calling her doctor, she tried to eat, but more difficulties set in.

"I went to put just a fork of food in my mouth, and everything came out. I couldn't eat," she said. "And then I was starting to slur. So I said, 'There's something really going on here.' ... I lost all facial function the minute I got into the hospital."

Here's what to know about the condition.

What is Bell's palsy?

The National Institutes of Health defines Bell's palsy as "a neurological disorder that causes paralysis or weakness on one side of the face." This happens when one of the nerves that controls facial muscles becomes injured or stops working properly. 

In some cases — including Chen's — it can affect both sides of the face. 

This led to some confusion in Chen's case when she was readmitted to the hospital.

"The one thing that threw everybody off was that it was perfectly symmetrical how everything had fallen and how everything just wasn't working," Chen recounted. "A couple times I heard the doctor say, 'Do you think it could be Bell's?' and they said, 'Nah, Bell's is only on one side, she's perfectly symmetrical.'"

Symptoms of Bell's palsy typically appear suddenly over a 48- to 72-hour period, and can range from mild to severe. Though symptoms generally start to improve after a few weeks, with or without treatment, the recovery time varies from person to person. 

"Usually, you will recover some or all facial function within a few weeks to six months. Sometimes the facial weakness may last longer or be permanent," the NIH says on its website. 

The condition affects about 40,000 people in the United States every year, the NIH estimates.

"It can affect anyone of any gender and age but seems to be highest in those in people 15 to 45 years old," the NIH states. It says risk factors for Bell's palsy include: 

• Pregnancy
• Preeclampsia (a high blood pressure condition during pregnancy)
• Obesity
• Hypertension
• Diabetes
• Upper respiratory ailments

Pregnant women appear to be at three times greater risk of Bell's palsy, especially in their third trimester or after giving birth, like in Chen's experience.

"It happens all the time. Patients, because they recover most of the time — not all the time — they're treated and they go on. But it's a devastating process until the recovery takes place, and not everyone recovers," Dr. Babak Azizzadeh, founder and director of the Facial Paralysis Institute, said on "CBS Mornings" Friday. 

Symptoms of Bell's palsy

According to the NIH, signs and symptom of Bell's palsy include: 

• Sudden weakness or paralysis on one side of your face
• A drooping eyebrow and mouth
• Drooling from one side of your mouth
• Difficulty closing an eyelid

The Mayo Clinic's list of symptoms also includes:

• Difficulty making facial expressions
• Pain around the jaw or in or behind your ear on the affected side
• Increased sensitivity to sound on the affected side
• Headache
• Loss of taste
• Changes in the amount of tears and saliva you produce

Before Chen noticed differences in her face, she says she felt a sharp pain behind her left ear after delivering her daughter. 

"They kind of just looked at me blank," she said of her doctors' reaction when she described that symptom. "They said it's probably because of the fluid build up after giving birth."

She also said her tongue started to feel strange. "It felt like a little tingly, just starting to get a little bit numb," she told King.

Since Bell's palsy symptoms can appear similar to other conditions that cause facial weakness, experts say it's important to seek medical attention immediately to rule out things like a brain tumor or stroke.

"The minute you see your face not moving correctly, you've got to go to the emergency room or urgent care," Azizzadeh advised. "Studies have shown very early treatment of (Bell's palsy) reduces long-term problems."

He adds many people also suffer in silence with the condition "because it's (viewed as) embarrassing," and it can affect mental health.

"People stop smiling and they get depressed because smiling really releases endorphins and makes us happy," he says. "And when you stop smiling, you go down and go into major psychosocial issues."

What causes Bell's palsy? 

Bell's palsy is the most common cause of facial paralysis, but its exact cause is unknown, according to the NIH.

"Viral and immune disorders are frequently suggested as a cause for this disorder," the National Organization for Rare Disorders says on its website. "There may also be an inherited tendency toward developing Bell's palsy."

Is there a treatment for Bell's palsy?

Most people who develop Bell's palsy recover over time without treatment. There are different treatment options for Bell's palsy, including medications to help recover facial nerve function and relieve pain.

Chen said she tried steroids, which are often prescribed for the condition, but has mostly relied on acupuncture, a treatment technique where fine needles are inserted into the skin.

"Acupuncture without electrical stimulation is fantastic," Azizzadeh says. "There aren't a lot of western studies looking at it, but it's been used for millennia."

Some patients receive physical therapy or facial massage, or may need eye protection if they are unable to close an eye. 

In rare cases, the NIH says, "cosmetic or reconstructive surgery may be needed to correct some damage such as an eyelid that will not fully close or a crooked smile." 

"Long term, if patients don't get recovery, we've pioneered surgeries and treatments with Botox and physiotherapy that can actually help people recover well," Azizzadeh adds.