Eric Dane has lost the function of his right arm

He told Diane Sawyer on Good Morning America: "I have one functioning arm. ... My left side is functioning. My right side, [which is my dominant side], has completely stopped working.

"[My left arm] is going. I feel like maybe a couple, a few more months and I won't have my left. ... I'm worried about my legs."

The former Grey's Anatomy star initially attributed his first symptoms to "texting too much" on his phone.

He recalled: "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.

"A few weeks later, I noticed it'd gotten a little worse. 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.'"

After nine months, he received his diagnosis.

He said: "I will never forget those three letters, [ALS]. It's on me the second I wake up. It's not a dream."

Eric - who has daughters Billie, 15, and Georgia, 13, with wife Rebecca Gayheart - was once a competitive swimmer but he recently realised he is no longer "safe in the water" after one of his kids had to rescue him.

He said: "When I jumped in the ocean that day and realised I couldn't swim [or] generate enough power to get myself back to the boat, I thought, 'Oh, God.' "And then I realised in that moment I'm not safe in the water anymore."

Diane explained in a voiceover that Eric's daughter had to rescue him.

He said: "She dragged me back in the boat...

"I was like breaking down in tears, so I made sure she got back in the water with her friend and continued on with the snorkeling with the guide. But I was just heartbroken."