Words are important things, especially for writers. Not exactly an earth-shattering kaboom of an idea, I know. What few readers I have (assuming any of them have stuck with me) know I listen to Radiolab, a radio science program. Back in may they did a program on a study someone did on the use of vocabulary in the works of Agatha Christie. I'm not sure if it was done intentionally to try and diagnose her, but by studying the words she used, and the frequency with which she used them, they were able to determine that Christie suffered from some form of mental degeneration, beyond what might be expected from simple old age. Alzheimers or a similar form of dementia were mentioned as likely candidates.
Now, while such a disease is scary in its own right, and I speak here as one who's seen too many close family members afflicted with it, as a writer this story carries a particular horror with it. The disease took away Christie's use of words. Her later novels, especially those towards the end, are far less complex and varied in word choice. She relies on more generic terms, less descriptive language.
Moreover, as in many cases of these kind of diseases, she seems to have been all too aware of it. In her last novel, it seems very likely one of her characters became a stand-in for herself, leading to the implication she knew what was happening to her. Knew, and was powerless to do anything about it. As a writer, I cannot imaging being forced to watch as my ability to express myself clearly and vividly was slowly taken from me, word by word.
The actual decline was somewhat less dramatic than that, of course. It's not as though one day she woke up and was able to use one word less. The study talked in frequencies of occurrences, and complexity of ideas, and how those could be charted to demonstrate Christie's failing mental faculties. But it's the idea of such a thing happening, of sitting down to write each day and finding, day by day, that your skills were a little less, and that there was nothing you could do about it, that is at once sad and terrifying.