Wow....I thought this was a joke at first.

So, apparently there is this disease that cripples its victims' ability to talk. People suffering from --- can talk only in a pinched, altered tone of voice, or when reciting rhymes. No kidding.

Check it out:
Ailing Dilbert Cartoonist Talks Again

Need more info?
Wikipedia article on spasmodic dysphonia


Absolutely fascinating.