Pupillary "dilatation lag" in Horner's syndrome.

1. Clinical examination will often fail to identify the presence of a unilateral Horner's syndrome. 2. Confirmation and localization of Horner's syndrome is of clinical prognostic value. 3. Cocaine testing provides confirmation of a Horner's syndrome, but it takes 45 min, is sometimes equivocal, and always delays the localizing hydroxyamphetamine test. 4. "Dilatation lag" is a sensitive and physiological sign of Horner's syndrome, and can be demonstrated by simple Polaroid flash photographs.