For @goddessofwincest, who knows why.
Cas diagnoses Sam with Postural Orthostatic Tachycardia Syndrome. (AO3)
It’s been going on for weeks before Sam can’t hide it anymore. Truth is, Dean’s already noticed: the shortness of breath, the color suddenly draining from Sam’s face when he stands, the clutching at his chest when he thinks Dean’s not looking. Dean’s not said anything either, never pushing more than to just check Sam’s okay, and usually being met with gruff dismissal and assurances his brother’s fine.
It’s only when Sam almost passes out from nothing more than standing that they can’t ignore it anymore.
“Sammy,” Dean says, darting to his brother’s side from where he’s just tried to rise from a chair in the bunker’s library. “Hey, I got you.” He helps Sam ease himself back down, dizzy and blinking to clear his head. “What’s wrong?”
“Nothing, Dean. I’m fine.”
“No, you’re not. You almost passed out, man, and don’t lie to me. This isn’t the first time.”