BACKGROUND: Heterochromia iridis, asymmetry of iris pigmentation, has been well 
described with congenital Horner syndrome. Acquired heterochromia associated 
with lesions in the ocular sympathetic pathways in adulthood, however, is rare.
METHODS: Two cases are reported in which sympathectomy in adults resulted in 
ipsilateral Horner syndrome with heterochromia. In each case, pharmacologic 
testing with cocaine and hydroxyamphetamine was performed.
RESULTS: In both cases, sympathectomy occurred at the level of the second order 
neuron, but hydroxyamphetamine testing suggested at least partial third order 
neuron involvement.
CONCLUSION: Acquired heterochromia can occur in adults. The partial response to 
hydroxyamphetamine in the two cases presented may reflect trans-synaptic 
degeneration of the postganglionic neuron. A reduction in trophic influences on 
iris melanocytes may have contributed to the observed heterochromia.