Autism

From what I understand, both are caused by overload, either sensory, psychological, and so on, and that result in the person having a sudden shift in their behaviors, but with the capacity of understanding one's surroundings and to act rationally getting greatly diminished. With meltdowns, the person can display a sudden shift to a more aggressive and/or anxious behavior, maybe similar to a tantrum externally, but with the mind having gone blank. Meanwhile, shutdowns make the person go far more quiet, or unresponsive altogether.

For me, a shutdown is when I “burn out”. I can’t be around people, do my basic tasks like laundry or eating. I stop answering my phone, sometimes even placing it on “do not disturb”. I tend to be on the couch a lot if not the bed. It can last 1-2 days or sometimes weeks. On the rare occasion, I can be out for a month or more. During these times, I wither wither away, become really skinny, or someone comes to take care of me.

Meltdowns are when I’ve reached my limit of external stimuli or social bullshit. I either go to my room and lay in bed or try to distract myself. If I’m somewhere else, I try to go to the bathroom for like 30 mins if I feel one coming on. If a person doesn’t let me escape, then I become antagonistic and start “telling the truth” as I like to call it. I call them out on their bullshit very sternly and directly. I don’t cushion it. It tends to be hurtful to the person, but in the moment, I don’t care since they are being hurtful towards me, so I see it as defending myself.

My abusive ex was a therapist and realized I was autistic before I even did. She would trigger meltdowns on purpose to then gaslight me into thinking I was abusive, garner pity from other people, and make others hate me. I ended up losing the majority of my friends. So, it’s really important to be aware of your meltdowns, triggers, and when they are coming. It’s also just as important to protect yourself from abusive people by being highly selective of who you choose to be around. I will never ever let someone like that in my life again.

I can't give you how they look in general terms but I can give you my experiences. I have had shutdowns in overwhelming situations many times and generally it results in a few behaviours. I become mostly silent, certainly not able to articulate complex thoughts, and my remaining speech is often limited to a few phrases, most commonly "I don't know". I would have more trouble looking at a person than usual, sometimes staring off into the void and occasionally I would start to fall asleep or actually pass out. I have been in the middle of an argument with my parents and when they started yelling I just passed out in my chair lent over the kitchen table.

In other situations where it more sensory I have had different experience. On a loud train I found myself having trouble moving and missed my stop trying to get up. On a loud coach (long distance bus) I slept the entire trip and woke up desperate to urinate.

When my children have meltdowns, it's all out anger, everything is wrong and you can't really reason with them in that state. You have to let them calm down in their own way before you can help make the situation better.

It differs from person to person. They're basically akin to being overwhelmed - which is something that happens in NTS as well. Some burst into tears, some get angry, some just freeze. And you can do all three at any time. Same with NDs.

I couldn't tell you what they look like since I'm the one shutting or melting down, but I certainly know how it feels.

Shutdown is just detaching from the situation altogether. Totally non verbal; and not by choice. It's like I physically can not speak and sometimes can't even move.

Meltdown is the exact opposite. Emotions erupt, usually anger, frustration, fear or sadness. Think of a child throwing a tantrum, but it's an adult trying their damnedest to be calm and civil but failing.

I know this is a serious topic (and thanks for responding upper commenters) but before I saw which sub this was I was thinking of a different kind of meltdown.

For me, a shutdown is when I stop cataloging the complex bits of what I'm perceiving and just go 'listening to this is now unpleasant and I don't want to subject myself to any more of this'. I might smile mechanically or emit encouraging 'hmm' noises, but I am on autopilot, you're not going to be getting my higher brain functions applied to whatever you're talking about, if I remember what you said it's going to be as factual 'this person said such and such' memories that I have to actively recall instead of actual opinions on what you said that will already be integrated into our next conversation, and I will dither on the far end of human interaction range trying to figure out if I can flee 'right now' without being impolite.

A meltdown... I get those, but I am experienced at calming myself down, so they're basically 'flashes'. Every so often, I will have a flash of... absolute, raging frustration at something. Like (as an example that actually happened to me recently) failing to pick up my water bottle twice after dropping it by accident. The first fumbling was an accident, they happen, they're natural, I'm totally fine with that, I don't even give it thought, I just go to pick it back up on reflex. The second fumble/first failure, I'm like, oops, better actually give this attention, I'll be more careful the third time. Third fumble/second failure, my reaction is "REEEE I CAN'T PICK THIS THING UP! WHAT I JUST DID SHOULD HAVE PICKED IT UP! THIS SHOULD NOT BE!!!" (In internal monologue, not audible screaming. If it's audible, they're closer to the severe end of the spectrum.) And then I have to take in a deep breath through my nose and let it out slowly (and yes, I have surprised myself by actually making the 'reeee' sound effect while doing this) so that I can be PRECISE and DELIBERATE about picking the water bottle back up. This will then be followed by a temporary shutdown because I have just blown a fuse on my emotional centers.

The common point is, I feel, that they're both reactions to something anathema, sort of. A shutdown is passive avoidance, a meltdown is active rejection.

I only have shut downs. From the outside It doesn't look like much. I get gradually more silent, less active and less expressive. When it comes to a complete shut down, which is luckily rare, I become completely passive.

Pretty similar experience to what others have mentioned, so mostly echoing what they said.

With a shutdown I "hit the wall" so to speak. I could have been fine when things started out but I have reached the point where whatever it is has built up and I have reached my limit of overstimulation. I will stop participating in conversation, if it's really bad I won't even move much or be paying attention.

Because I grew up with "you're not autistic you're just high maintenance" parents I learned to "save" my meltdowns. I may have short responses or act annoyed when I'm around others. Inside I am raging. And I know I'm raging for no reason aside from the environment, but I can't help it. Which makes me more mad lol. Overstimulation is not just "wow it's loud in here" it's physically painful to me to continue to exist in that environment. Once I get home I rage cry.

Another thing that can make me shutdown or have meltdowns is plans changing, not only overstimulation. When I have plans it is like my entire life leading up to that point is built on the fact that I will be doing X at Y time. I have spent the time to mentally prepare myself to do X at Y time. If those things change, everything feels fucked up.

Recognizing when I am becoming overstimulated helps me a lot now that I actually understand what is happening. If I get to take a break and go for a walk or go somewhere quiet it usually prevents me from having a full on meltdown later. Also just... Admitting what is happening. Like "I am angry because the time changed from 3pm to 5pm and I was not mentally prepared for that." But some situations are just too much and it ends up happening anyway.

My meltdowns typically happen in high stress situations when something is out of my control, especially when it affects something I am responsible for. I get very physically hot, as I can feel the adrenaline start pumping. I get very angry and loud, and can't really filter or mask.

Growing up, meltdowns for me were all out tantrums. Screaming, crying, tearing up the place. I remember it being described as a tasmanian devil came through the place. Now that I'm in my 20s, I rarely have meltdowns, but they have happened. Mostly due to too much stimulation of my emotions, I just lose control of them. At this point in my life, it's mainly attached to romantic situations because they are probably the most alien and difficult situations for me to navigate. I would say that there is definitely an element of depression/anxiety included in them, however.

Shutdowns though... I'm not sure to be honest. There are days or even weeks where sometimes I'm just lost for motivation and I don't want to interact with anyone. I'd rather be alone. But I'm not sure if that's more of a bout of depression than a shutdown to be honest.