SIADH as an uncommon presentation of olfactory neuroblastoma: a case-based overview.

Background: Olfactory neuroblastoma (ONB) is a rare malignant neuroectodermal tumour that usually arises in the nasal cavity, typically presenting with unilateral nasal obstruction and epistaxis. In rare instances, ONB can manifest as a paraneoplastic syndrome of inappropriate antidiuretic hormone (SIADH) secretion, leading to hyponatraemia.

Case presentation: We describe a 42-year-old woman with a 4-year history of cyclical, symptomatic hyponatraemia characterised by intermittent episodes of dizziness, severe headaches and marked fatigue, initially without overt nasal or otolaryngological symptoms. Investigations confirmed SIADH, yet repeated imaging (including thoracic CT and FDG PET-CT) failed to identify a source. Eventually, a ⁶⁸Ga-DOTANOC PET-CT revealed an isolated lesion in the left ethmoid region. Surgical resection via an endoscopic approach confirmed the diagnosis of a Hyams grade 1 ONB. Following complete tumour removal, the patient's hyponatraemia resolved.

Literature review: ONB accounts for <3% of nasal tumours and can present with non-specific symptoms, delaying diagnosis. Most patients have a typical combination of nasal obstruction and epistaxis, but paraneoplastic SIADH is reported in about 2% of cases. Complete surgical excision is the cornerstone of therapy, often accompanied by radiotherapy or chemotherapy for higher-grade lesions. Recent advances suggest that somatostatin receptor imaging and targeted radionuclide therapy might benefit select patients, particularly those with unresectable or recurrent disease.

Conclusion: This case highlights the importance of considering ONB in patients with unexplained SIADH, especially when initial investigations are inconclusive. It also underscores the utility of ⁶⁸Ga-DOTANOC PET-CT in detecting occult neuroendocrine tumours, and reinforces the value of prompt surgical intervention for definitive treatment.