Since Alice in Wonderland is in the news right now, I'm putting up 2 history posts in one day.
The Mad Hatter is well known in English literature. He was created by Lewis Carroll (real name Charles Lutwidge Dodgson) for the story Alice's Adventures in Wonderland. An interesting side note is that the character was most likely based on a furniture dealer, and not a hatter.
The phrase "mad as a hatter" actually predated the story, and has an interesting neurological history.
Mercury is a metal with multiple human toxicities. It can affect many organ systems, and in sufficient amounts can cause brain damage. When this occurs common symptoms are memory loss, confusion, and behavioral changes.
Mercury poisoning is uncommon in modern medicine, but before it had been identified as a toxin it was commonly used in the cloth industry, in the manufacture of felt.
A hatter, obviously, is someone who makes hats. And in 18th & 19th century England, felt was commonly used in hats. So hatters had a fairly high level of exposure to mercury, and after several years of plying their trade they sometimes developed brain damage, and went "mad". And that's where the phrase came from.
Alice in Wonderland has other neurological trivia. As many of my migraine patients will tell you, their headaches can be preceded by all kinds of visual changes. Typically these are flashing or sparking lights, dark spots, colors, or squiggly or zigzag lines. But some patients will see visual distortions, where things suddenly seem to grow or shrink in front of them. This perception change is now called Alice in Wonderland Syndrome.
It's known from his personal diaries that Lewis Carroll suffered from migraines with visual changes. It's unknown if he had the perception changes of things growing and shrinking, but who knows? Maybe one of the most famous books ever written was partially inspired by a migraine.
It's something to think about.