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"Happens a lot in Georgette Heyer's period romance"? I don't think so. I'd like to see a specific example, because I can't remember a single one!
It's my understanding that 'brain fever' was the Victorian word for something they didn't understand very well, thus, like "psychotic" it got all kinds of misused by novelists. Specifically, 'brain fever' was the period word for what in the early 20th century was called a [http://en.wikipedia.org/wiki/Mental_breakdown | "nervous breakdown"] and these days doesn't have a specific DSM-IV tag, but seems to be a stress-induced issue related to PTSD or acute stress disorder.
I wish people wouldn't change the names of disorders and diseases. Makes it hard to figure out what the heck is really going on in older novels, where people have fits of "ague" (malaria), suffer from "brain fever" (noted above), or "shell shock" (PTSD).
I've been wondering for some time whether Rodya in Crime And Punishment should count. His fever starts before he commits the murders, and he's able to walk around for some time before he is incapacitated, rather than the illness coming on suddenly as is more typical of brain-fever. It's certainly true that Rodya's physical state is linked to his mental state throughout the novel, but as far as I can remember the narration never quite says that the illness was caused by the chaos of his mind.
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How well does it match the trope?