Fans of the legendary music group U2 were shocked and saddened to learn of the rare diagnosis of the band's drummer, Larry Mullen Jr.

According to Fox News, the drummer recently revealed that due to that rare diagnosis, he often experiences trouble playing music. 

The condition, classified as rare, is known as dyscalculia, a learning disorder. It's a sub-version of the more commonly known learning disability, dyslexia.

The condition, Mullen said, "pains" him when he experiences it while playing music, which is made much more difficult by the rare condition.

What did he say?

In a recent interview with "Times Radio," Mullen opened up about his condition and described how it has made his music career that much more challenging.

"I’ve always known that there’s something not particularly right with the way that I deal with numbers," he explained. "I’m numerically challenged. And I realized recently that I have dyscalculia, which is a sub-version of dyslexia. So I can’t count [and] I can’t add."

He admitted that counting bars during a performance is not unlike climbing Mt. Everest, as far as challenging circumstances go.

"When people watch me play sometimes, they say, 'you look pained.' I am pained because I’m trying to count the bars. I had to find ways of doing this — and counting bars is like climbing Everest," Mullen said.

Fox News noted:

Dyscalculia, according to the Cleveland Clinic, is "a learning disorder that affects a person’s ability to understand number-based information and math. People who have dyscalculia struggle with numbers and math because their brains don’t process math-related concepts like the brains of people without this disorder."

Symptoms of the condition usually crop up in childhood, but many don't realize what they have until much later in life.

Helping the cause

Mullen is an advocate for those suffering from dyslexia and gave the interview as he promoted his new documentary, "Left Behind."

Fox News noted:

According to the film's website, it tells "the inspiring story of a group of determined moms who band together to create the first public dyslexic school in the largest school district in America, New York City."

Mullen reportedly wrote and produced the music for the documentary.

As he spoke about the film, he revealed that his son also suffers from dyslexia.