Specifically, their brain. I had a lesson on the brain today in biology, and it got me thinking; "What were the long-term effects of 2D's initial car accident?" I had a chat with my biology teacher, made some diagrams, read up on the brain, and now I think I've got an answer
So, we all know poor Stu got a hyphema from this accident and some brain damage. But what I wanted to know was what type of damage specifically. What in their behaviour would change? Would their motor coordination be affected? Would they have a speech impediment? We can only start to get an understanding of this once we look at the type of car accident it was.
"On 15 August 1997, when Stuart was 19, his life changed forever after hooligan Murdoc Niccals, age 31, purposefully crashed his Vauxhall Astra through the wall of Uncle Norm's Organ Emporium, where Stuart had been working, in an attempt to ram-raid the store and steal the synthesizers. Murdoc's bumper crashed into Stuart's face, landing him with an 8-ball fracture also known as hyphema, although he could still see through his injured eye."
While this is the exact description of the event on the fandom wiki, I deemed it inaccurate ever so slightly as being hit by the bumper would've killed 2D. So, I prefer to interpret it as the wheel hitting Stu directly in the eye, so the bumper would just graze the top of their head.
To determine the exact initial damage that could've been done, we must first know the kind of car. Here, it says Vauxhall Astra, so we'll go with that. Murdoc was canonically not-very-well-off at the time, so I'm assuming it was a smaller, older model. What came up was the 1992 Vauxhall Astra, with the smallest wheel size being 15 inches (roughly 35 cm).
For the bumper to have just grazed the top of 2D's head, a maximum of 7.5 inches (roughly 18.5 cm) of wheel would've hit their face. Given that the average face length (from crown to chin) for a 6'4 person (2D's canonical height) is 11 inches, that leaves everything from the nose up to be damaged.
Now, the fun part. To analyse what parts of the brain would be affected + what each part does. Outside of PTSD, what mental changes would there be? Outside of a hyphema and probably the need for a nose and brow bone reconstruction, what physical changes would there be? So, I found a detailed image of the brain, with labelled areas and functions, to determine the area of damage.
As you can see, it's pretty obvious what areas would take damage here. I zoomed in on the text in the second image, and there is a list of what each numbered+colour coded section does.
As I highlighted in the full version of this image, sections 2, 3, 4, 6, 8, 9, 12, and 13 would take damage. Sections 2, 6, 8, and 13 would be most affected, while 3, 4, 9, and 12 would simply go through some knock-on pressure. Meaning that their Association Area, Emotional Area, Olfactory Area, and Higher Mental Functions would be most damaged.
Association Area: This area deals with short-term memory, equilibrium, and emotion. This means that a hit that hard would seriously effect 2D's ability to remember things (like where they left their keys, the names of people they just met, etc), balance (so they'd probably fall over a lot [pretty sure this is canon???], and emotional regulation skills (so things like not knowing how they're feeling, not crying when appropriate, not laughing when appropriate, anger issues, etc).
Emotional Area: Surprisingly, this area deals more with primitive emotions, like pain, hunger, and FFF responses. Meaning that 2D would be left not feeling much pain, not knowing when their hungry, and having strange or even no panic responses. This could lead to them potentially starving themselves on accident, pitting themselves in danger, and not realising they've potentially injured themselves.
Olfactory Area: This is literally just smell. Like okay, 2D has a shit sense of smell, big deal😭😭😭 I'm not writing about ts I lwk dgaf😭😭😭
Higher Mental Functions: Okay, here's the bit I really wanted to talk about. This area controls concentration, planning, judgement, emotional expression, creativity, and inhibitions. This is the area with the biggest amount of damage, so a lot's going to shit for our poor Stu. They've practically lost the ability to concentrate (could explain random behaviour in interviews), plan for events, and judge things. This could explain why they're rarely in charge of stuff in the canon or why they still hang around Murdoc despite all the obvious reasons not to. They've also practically lost the ability to express their emotions. So, on top of not knowing how they're feeling, they can't explain that to others either! I don't care much for the creativity bit of this, but it's the inhibitions that caught my attention. They now have no impulse control. They say what's on their mind when it comes to mind. They act recklessly. They don't think things through. Why? Their inhibitions control practically doesn't exist!
The other areas I don't care much about as they were probably just squished a bit upon impact of the wheel. They'd be less sensitive to pressure and heat against their skin, have limited hand-eye coordination, have a speech impediment, and would have some trouble doing basic things like wrapping a door handle or walking, probably making them clumsy and injury prone.
Summary of my whole yap (ty for making it this far!!!): 2D is cooked, basically. No emotional regulation or expression control, lack of panic response, no concentration, no inhibitions, speech impediments, and clumsiness. All from one car accident that they miraculously survived. Oh also potentially legal blindess from the severe hyphema.