• Patients with congenital Horner's syndrome (who seemed, on the basis of their clinical history and the distribution of anhidrosis, to have a preganglionic lesion) had partial mydriatic failure with hydroxyamphetamine hydrobromide and a supersensitivity to phenylephrine hydrochloride. This apparent paradox can be readily explained by postulating an orthograde transsynaptic dysgenesis of the postganglionic neuron, such as has been demonstrated in the sympathetic nervous system of newborn animals. The failure of hydroxyamphetamine to cause mydriasis indicates damage to the postganglionic sympathetic neuron, but in the neonate this damage may be secondary to a preganglionic lesion.
Weinstein JM, Zweifel TJ, Thompson HS. Congenital Horner's Syndrome. Arch Ophthalmol. 1980;98(6):1074-1078. doi:10.1001/archopht.1980.01020031064011