Horner Syndrome With a Positive Harlequin Sign in an Adolescent Male

An 11-year-old boy presented to the emergency department with a history of uneven facial flushing. The boy's parents described that the left side of his face became flushed with exercise while the right side remained pale. On clinical examination, the boy had right-sided ptosis and miosis, with pupils measuring 5.0 mm on the right eye and 8.0 mm on the left eye (Fig. 1).