writer's cramp

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A cramp or spasm of the muscles of the fingers, hand, and forearm during writing.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun muscular spasms of thumb and forefinger while writing with a pen or pencil