Functional Olfactory Neuroblastoma Inducing Symptomatic SIADH.

Olfactory neuroblastoma (ONB) is a rare malignancy of the sinonasal cavity, originating from neuroepithelial olfactory cells. Olfactory neuroblastoma can be difficult to diagnose due to its anatomic position and variable symptomatic presentation, leading to diagnosis at a more advanced stage. Here, we present the case of a 35-year-old man with no previous medical history who had a bicycle accident secondary to syncope. He was found to be hyponatremic and suspected to have syndrome of inappropriate antidiuretic hormone secretion (SIADH). In the workup of SIADH, a brain magnetic resonance imaging revealed a mass in the left middle meatus. The lesion secreted inappropriate amounts of ADH, resulting in symptomatic paraneoplastic SIADH. This ultimately led to the early recognition and successful resection of this rare ONB. The patient has remained disease-free for over 5 years. This case is a fortunate example of a functional malignancy of the sinonasal tract that was discovered early and successfully treated as a result of symptomatic SIADH.