This case report details a patient diagnosed with neuromyelitis optica (NMO) with an associated left Horner syndrome. The female patient presented with neck pain and right hemibody decreased pain and temperature sensation. She then went on to develop left ptosis and miosis over the next three days. Magnetic resonance imaging of the cervical spine showed a longitudinally extensive intramedullary lesion more prominent on the left, with post-contrast enhancement extending from C2 to C5. The Horner syndrome was confirmed with 0.5% apraclonidine. In subtle cases of Horner syndrome, cocaine or apraclonidine drops can be used to confirm diagnosis although the latter is used increasingly in the clinical setting. In this case, application of 0.5% apraclonidine drops brought about a reversal of the anisocoria and eliminated the ptosis, confirming the diagnosis. Treatment for NMO has not been proven although opinion for the acute phase recommends intravenous glucocorticoids for five or more days. In this case, such treatment brought about a reduction in symptoms. Long-term immunosuppression for at least five years is recommended in cases of established NMO due to the high risk of relapse.