The evaluation of patient demographics, etiologies and apraclonidine test results in adult Horner's syndrome.

Despite detailed investigations, in a significant number of patients with Horner's syndrome an underlying cause may not be detected. Among the identifiable lesions, central and preganglionic involvements are still the first leading causes of Horner's syndrome. In addition, apraclonidine test may not be positive in all patients and a negative response does not exclude Horner's syndrome.