What is the earliest clinical sign of ataxia-telangiectasia?

Progressive cerebellar ataxia due to degeneration of Purkinje cells, beginning at age 12 to 18 months. Choreoathetoid movements, hypotonia, dysarthria, and abnormal eye movements gradually develop, and intelligence frequently declines.

Smith SL, Conerly SL: Ataxia-telangiectasia or Louis-Bar syndrome, J Am Acad Dermatol 12:681–696, 1985.