writer's cramp

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A cramp or spasm of the muscles of the fingers, hand, and forearm during writing.

Wiktionary

1. n. idiomatic An debilitating pain preventing easy movement of the wrist, hand, or fingers, resulting from prolonged use of a pen or pencil.

GNU Webster's 1913

1. n. (Med.) a painful spasmodic affection of the muscles of the fingers, brought on by excessive use, as in writing, violin playing, telegraphing, etc. Called also scrivener's palsy.

WordNet 3.0

1. n. muscular spasms of thumb and forefinger while writing with a pen or pencil