Christina Applegate has said she is 'so sorry' that her daughter has been diagnosed with Postural Orthostatic Tachycardia Syndrome, a neurological condition.
PoTS is a condition caused by a problem in the body's nervous system, leading to an abnormal increase in heart rate upon standing, or even sitting up.
It causes a number of symptoms, including dizziness, palpitations and fatigue.
For some people, symptoms can be mild, but for others it can severely affect their quality of life.
Speaking on her famous mom's podcast, MeSsy, Sadie Grace LeNoble, 13, opened up about the condition.
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"I have something called POTS," she said. "I have no clue what it actually is but it's something to do with the autonomic nervous system and it affects my heart. When I stand up, I get really, really dizzy and my legs get really weak, and I feel like I'm going to pass out."
At its worst, Sadie can faint and even become unconscious.
"That's only on really bad days when it's hot out," she continued.
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"Last year, in sixth grade, I would go to the nurse multiple times a day for it because I always felt like I was going to pass out.
"In class, if I were to stand up then I would be like, 'I have to go to the nurse. I can't do this.' Or I'll be in PE, and I'll be like, 'I have to go to the nurse'."
Sadie explained that her school initially thought she was exaggerating to get out of doing PE, adding: "They were like, 'You're doing this to get out of class. It's probably just anxiety. Go back to class.' They wouldn't do anything for it.
"Them not doing anything about it definitely hurt me physically and emotionally.
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"Because I was just like, 'This is rude, and I feel sick and you're telling me to go to PE and run laps around the football field. I can't do that'."
Dead To Me star Christina explained she feels 'so bad' that she hadn't realized her daughter was suffering so much.
"I just didn't see it at home, babe. At home you were fine. But it's kind of like us," she said. "We get out in the world, and the stresses and the anxiety of the world bring upon our symptoms much worse than they would be if we were in the safety and the coolness of our own homes.
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"And I'm so sorry Sadie Grace."
Christina herself suffers with neurological condition multiple sclerosis (MS), which affects the brain and spinal cord. It can cause a range of symptoms, including issues with vision, balance, sensation and movement.
Sadie explained that having PoTS has helped her understand her mom's condition much better.
"I feel like if I didn't have this thing it would be a lot harder to understand what my mom's going through," she said.
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"Like, when my mom's like, 'Oh, I'm kind of in pain right now. Oh, I'm having tremors.' If I didn't have this, I probably would be like, ''don't really care. I don't know what you're talking about'."
Topics: Celebrity, Health, Christina Applegate
Lucy Devine