Jamie-Lynn Sigler detailed her 10-year-old son's "nightmare" ICU visit that led to an autoimmune disorder diagnosis.

According to the TV star, her son has been diagnosed with acute disseminated encephalomyelitis (ADEM).

"4 weeks ago, what seemed like a normal virus for our son, turned into a nightmare," Sigler shared Tuesday on Instagram. "Beau has what we believe to be, ADEM. To say this has been hard, is an understatement, and I’ve never felt more broken…"

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ADEM is a very rare, neurological immune response that sometimes kids can get after contracting a simple virus. "They compared it to getting struck by lightning," Sigler explained in an episode of "MeSsy with Christina Applegate & Jamie-Lynn Sigler," emphasizing how rare the condition is.

‘SOPRANOS’ STAR JAMIE-LYNN SIGLER REVEALS HOW MS PREVENTS HER FROM GIVING HER KIDS ‘ALL THEY WANT’

Her 10-year-old son had not been feeling well in the days leading up to the diagnosis. At one point, Beau had a 105-degree fever, and Sigler's husband rushed him to the hospital as he screamed his head hurt. Sigler's son went to the ER three separate times over the next days, only to be sent home after tests were run.

After landing an appointment with an infectious disease doctor, Sigler recalled taking her son to the ER a fourth time.

"On the drive [to the infectious disease doctor] he was screaming in pain, in a way that no mother should ever see her kid scream," she recalled, deciding in that moment to head to the ER instead.

The doctors told her to take him to the infectious disease appointment first. As soon as Sigler and Beau entered the doctor's office, they were accompanied back to the ER. The 10-year-old was then admitted into the ICU.

During his time in the ICU, Beau lost his ability to speak and, at one point, his ability to move his legs.

"It was and has been like the darkest, hardest, most f---ed up thing I've ever been through," she explained. "Every time we'd get like a little bit of good news, you'd get slapped in the face with something else."

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Beau is still in the hospital and will likely remain there for another month, according to the "Sopranos" star.

While Beau is not in the ICU anymore, Sigler admitted he's still struggling with psychosis.

"Who is my kid? Is he ever coming back? What is our life going to be? I don't know," she explained.

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Sigler thanked everyone for the support in her Instagram post, writing, "The doctors nurses, and therpaists [sic] have been INCREDIBLE here, and we have the highest of hopes for our son that he makes a full recovery, but we also know that whatever life turns into, we have a f--king army around us, and we will be ok."

"Thank you to everyone , you know who you are. I will never be able to repay you for the way you have held us and supported us. And to my husband Cutter, your patience and strength during this has been unparalleled and to our other son Jack, while you are only 6, the way you just "got it" and turned into a care taker for your brother with each hospital visit reminds me of the beauty in this world."