He was Grey's Anatomy's McSteamy. Now Eric Dane is in the fight of his life.

Eric Dane captured our hearts as the ever-so-charming (and sometimes insufferable) plastic surgeon Mark Sloan on Grey's Anatomy.

For years, we watched McSteamy help save lives on the hit drama with that trademark sly smile. But in April, the actor opened up about being diagnosed with amyotrophic lateral sclerosis (ALS), a progressive neurodegenerative disease that affects nerve cells controlling voluntary muscle movement.

Now, he's spoken about it on television for the first time.

In a powerful interview with Diane Sawyer on Good Morning America, the actor shared the first moments he noticed something wasn't right.

"I started experiencing some weakness in my right hand, and I didn't really think anything of it at the time. I thought maybe I'd been texting too much and my hand was fatigued," he said.

"But the symptoms worsened.

"I went and saw a hand specialist who sent me to another hand specialist. I went and saw a neurologist, and the neurologist sent me to another neurologist and said, 'This is way above my pay grade.'"

Nine months after his first symptoms, he was diagnosed with ALS.

"I will never forget those three letters," Dane said. "It's on me the second I wake up… it's not a dream."

Eric Dane gave his first TV interview since his diagnosis. Image: YouTube/GoodMorningAmerica.

Today, about 18 months since his symptoms began, the Euphoria star said he's lost all use of his dominant arm and fears what's next.

"My left side is functioning, my right side has completely stopped working," he said.

"I feel like maybe a couple, few more months, and I won't have my left hand either. It's sobering."

While he can still walk, he worries that too won't last.

The diagnosis has upended his identity. Once a competitive swimmer and water polo player, Dane has come to the heartbreaking realisation that he can no longer trust himself in the water.

He recalled being on a boat with his 13-year-old daughter, Georgia, when he realised he didn't have the strength to swim back.

"I thought, 'Oh God' and I realised in that moment, I'm not safe in the water anymore," he told Sawyer. Georgia had to help him back to the boat.

"I was breaking down in tears… I was heartbroken."

Dane's best known for his role as Mark 'McSteamy' Sloan alongside Patrick Dempsey aka Derek 'McDreamy' Shepherd, left, on Grey's Anatomy. Image: ABC.

Through it all, it's his wife, Beverly Hills, 90210 actress Rebecca Gayheart, who's been by his side.

The pair announced their split in 2018 after nearly 15 years of marriage. But their divorce was never finalised.

At the time, they issued a joint statement: "After 14 years together, we have decided that ending our marriage is the best decision for our family. We will continue our friendship and work as a team to co-parent our two beautiful kids, as they are the most important thing in the world to us."

But a few months ago, they reconciled. In March, Gayheart requested to dismiss the divorce, seven years after first filing. Both she and Dane signed off on the paperwork, according to People.

The following month, Dane publicly revealed his ALS diagnosis.

Throughout his battle, he says Gayheart has been there for him every step of the way.

"I talk to her everyday," a tearful Dane told GMA.

"We have managed to become better friends and better parents. And she is probably my biggest champion, my most stalwart supporter, and I lean on her."

Dane with his daughters Georgia and Billie. Image: Getty.

Eric Dane and Rebecca Gayheart in 2017. Image: Getty.

The couple, who wed in a Las Vegas ceremony in 2004, share two daughters: Billie, 15, and Georgia, 13.

Dane admits the hardest part is grappling with the unfairness. His own father died by suicide when he was just seven. Now, he's facing the reality that he too may be taken from his daughters while they're still young.

But he's determined to keep fighting.

Dane has a new police drama, Countdown, on the way. He says he plans to keep working and to spend as much time as possible with the people he loves.

"I don't think this is the end of my story," he told GMA.

"I don't feel like in my heart this is the end of me."

Feature image: ABC/Getty.