micrographia

noun

mi·​cro·​graph·​ia ˌmī-krō-ˈgraf-ē-ə 

: abnormally small handwriting or handwriting that becomes progressively smaller that is characteristic especially of Parkinson's disease

An almost diagnostic sign of parkinsonism is micrographia—minute, and often diminishingly small, handwriting.

—Oliver Sacks, The New Yorker