Christina Applegate is "so sorry" that her 13-year-old daughter, Sadie Grace LeNoble, has been diagnosed with the neurological condition Postural Orthostatic Tachycardia Syndrome

Sadie also has Misophonia, which causes decreased tolerance to specific sounds.

Speaking on the 'MeSsy' podcast with her parent, who has Multiple Sclerosis herself, Sadie said: "I have something called POTS.

"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."

On severe days, she can faint and even become unconscious.

She said: "That's only on really bad days when it's hot out."

On how it impacts her ability to do physical activities, she said: "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.'"

The school thought she was making up her symptoms just to get out of PE, because they didn't know what she was suffering from.

Sadie said: "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."

She would do her sports classes and suffer big time for it, which also impacted her mental health.

She continued: "Them not doing anything about it definitely hurt me physically and emotionally.

"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.'"

Her mom said: "She wears layers of clothes on 90-degree days and she hates PE - sorry school, not a big fan of PE or physical things.

"I was like, 'Oh, I kind of felt that way too.' I feel so horrible that we didn't pay attention to it."

Apologising to her daughter, Christina said: "I just didn't see it at home, babe. At home you were fine. But it's kind of like us."

Christina 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."

She said: "And I'm so sorry Sadie Grace."

Her daughter replied: "It's OK."

Sadie said her neurological issues make her have more empathy and understanding of her mom's battle with autoimmune disease MS.

She added: "I feel like if I didn't have this thing it would be a lot harder to understand what my mom's going through.

"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, 'I don't really care. I don't know what you're talking about.'"