Mikey opened up the door and weakly stepped out of the lab, he was leaning against the wall for support as he walked, Donnie got a hold of him just as Mikey passed out on his arms, everyone circled the two and Raph helped Donnie carry Michelangelo to the table to examine him the best possible, he wasn't completelly conscious but he was also far from awake, he couldn't move most of his body but he could move his eyes and fingers, he was so drained that he could barely hold his head up, his brothers had to practically support his neck like a new-born, Mikey was weezing, probably because his respiratory system had just been rearanged due to the mutation, it was a bit hard for him to breath.
Mikey's skin was still cooling down from all the burning, he had a high fever, nausea and a horrible ache on his heart and abdomen, he felt like his organs were just stirred like a nasty soup, his mind was somehow blank and clouded at the same time, he couldn't think about anything, he could only look around slowly while trying to figure out what was going on, he couldn't speak, he couldn't hear anything and his vision was blurry, his brain was in shutdown mode, all of his senses were turning off slowly as if he was falling asleep, Mikey felt the pains and aches slowly fading away, they felt so far away, he kept his eyes locked on a lamp nearby while his body felt more relaxed by the seconds.
Donnie and the others noticed Mikey blacking out and tried everything to keep him awake, Leo tried throwing water on his face, Donnie tried to shake him gently, but then Casey and Raph just freaked out and picked him up, Casey tried to slap Mikey in the face and Raphael just began shaking him like if he was trying to get a soda bottle to explode with gas pressure, April and Leo had to grab him back and pull Raph away so he wouldn't accidentally injure Michelangelo during his panic.
They placed Mikey back in the bed and Donnie put a pillow under his head to keep it still, Mikey was already knocked out asleep at that point and was very much limp, Donatello was getting frustrated and distracted by everyone's panicking and screaming at eachother so he ends up yelling for everyone to get the hell out of his lab before they make him go insane, even threatening that he might hurt Michelangelo if he can't focus properly on what he's doing, guilting them into leaving the room so he can calm down and watch his work closely to make sure that his brother was going to be alright.
He can't quite figure out what Michelangelo was feeling since he was unable to speak and passed out soon after, so there was not much he knew what to do in that sittuation exept make sure his little bro was comfortable and check his vitals in case anything concerning happens, Mikey's breath was a hissing a little bit but it was getting better after a few hours, he ran a few tests and Michelangelo seemed to be healing slowly, he just needed to rest, his collapse was a result of his pain and feeling of ill, but nothing that couldn't be taken care of or recovered from, and his body was already healing on it's own so all that was left for Donnie to do was to keep track of the process and make sure he didn't have any relapse.
Mikey stayed asleep for almost a whole day, 22 hours of rest until he woke up, his mind was fuzzy and he took a few minutes to fully wake up, Donnie helped him sit up and let Mikey wrap his arm over his shoulder for support, helping him stay up so he could get his thoughts back in order, Donnie held him up and assisted his brother as he woke up, it was a bit weird for Mikey, suddenly, his vision was better, one of his eye came back to normal but the other one was now almost completely blind, some spots of his body were back to being his original green tone, the big cool tail he had shrinked into a small one, it was a bit bigger than his hand, it was cute but very different of what he had grown accustomed to.
Mikey tried talking, forgetting his speech disorder, but much to their surprise, actual words came out of his mouth, they still sounded weird, his voice was still sounding like a bunch of other voices combined, but now, they were all syncronized and speaking the same things at the same time, and the slight echo effect he made while talking was kinda funny, Donnie jumped back in surprise after hearing his brother actually speaking after quite a long time of listening to him making nothing but animalistic sounds, Mikey was still waking up mentally so he barely noticed, but after Donnie shook his in excitement he became quite aware of what was going on and jumps on his big borther to celebrate his... relative cure, Donnie hugged Mikey and lifted him off his feet and they spinned around in joy.

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Double mutant Mikey (2012 AU)
FanfictionMikey is injected a refined version of the mutagen by the Kraang and is made prisoner for 3 weeks in dimension X, after the rescue, his family have to deal with the effects of the drastic changes on their little brother