A study has been made of the response of the iris and the intraocular dynamics in three human subjects with unilateral Horner's syndrome that ranged in duration (since onset) from a few months to many years. It is shown that adrenergic denervation supersensitivity to epinephrine bitartrate may persist for over 20 years. Adrenergic supersensitivity was absent in one subject, and reasons are presented for believing that this is indicative of a preganglionic cervical sympathetic lesion. On the basis of these studies a means of differentiation between preganglionic and postganglionic cervical lesions is proposed. The method is believed to be more specific and less open to misinterpretation than that in current usage.
Langham ME, Weinstein GW. Horner's SyndromeOcular Supersensitivity to Adrenergic Amines. Arch Ophthalmol. 1967;78(4):462-469. doi:10.1001/archopht.1967.00980030464010