Eric Dane is detailing some of the symptoms he’s experienced since being diagnosed with amyotrophic lateral sclerosis (ALS).

On Monday, June 16, the 52-year-old actor spoke to Diane Sawyer about his health for a segment on Good Morning America, reflecting on the first moments he noticed changes in his body.

“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 recalled.

“But a few weeks later I noticed that it had gotten a little worse,” he said. “So 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.’”

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After nine months, Dane finally received his crushing ALS diagnosis.

“I will never forget those three letters. It’s on me the second I wake up,” he said. “It’s not a dream.”

Today — a year and a half since his symptoms began — the Euphoria star said he’s lost control of his right arm and he’s concerned about losing mobility elsewhere.

“I have one functioning arm. My dominant side. My left side is functioning, my right side has completely stopped working,” he shared, noting that function on his left side is slowly deteriorating. “It’s going. I feel like maybe a couple, few more months, and I won’t have my left hand either. It’s sobering.”

He added that although he’s able to walk right now, “I’m worried about my legs.”

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Dane first revealed his ALS diagnosis exclusively with PEOPLE in April.

“I have been diagnosed with ALS,” he shared. “I am grateful to have my loving family by my side as we navigate this next chapter.”

The actor is married to Rebecca Gayheart and the couple share two children, Billie Beatrice, 15, and Georgia Geraldine, 13.

“I kindly ask that you give my family and I privacy during this time,” he told PEOPLE.

ALS, also known as Lou Gehrig’s disease, is a rare degenerative disease that causes progressive paralysis of the muscles. Patients first experience twitching or weakness in a limb, often followed by slurred speech. According to the Mayo Clinic, because the disease affects the nerve cells in the brain and spine that control muscle movement, patients slowly lose their ability to speak, eat, walk, and breathe independently.

There’s no cure for ALS, and people usually live three to five years after diagnosis, according to the Muscular Dystrophy Association. However, some patients can live decades.