Writer's cramp, also called mogigraphia and scrivener's palsy, is a disorder which involves cramps or spasms of muscles of the hand and/or forearm.
“Writer's cramp is a focal dystonia of the hand in which an individual, usually someone whose occupation requires a significant amount of writing, has paresis or even paralysis when attempting to write despite having no weakness in the hand on standard clinical testing, no musculoskeletal problems in the arm that could account for the inability to write, and no lesions of the central nervous system or peripheral nerves demonstrable by standard imaging and electrophysiology techniques such as CT, MRI, nerve conduction studies, or electromyography.”
Source: Ramazzini and writer's cramp The Lancet, Volume 365, issue 9463, P938, March 12, 2005.
Exactly how or why the disorder develops is a mystery.
A similar (very likely the same) disorder can affect musicians. In which case, it's known as fiddler's palsy (or similar). It's thought to affect around 1% of professional musicians.
Also see :Cramp