Recurrent Nausea and Hyponatraemia: Unmasking SIADH in a 50-Year-Old Woman

Syndrome of inappropriate antidiuretic hormone (SIADH) is a common cause of hyponatraemia that requires systematic investigation to identify underlying aetiologies. Central nervous system disorders represent an important but often underrecognised cause, particularly when patients present with non-specific gastrointestinal symptoms.

A 50-year-old woman presented with a 3-year history of intermittent nausea and vomiting that worsened over 20 days. Laboratory investigations revealed persistent hyponatraemia (serum sodium 125.4 mmol/L), low plasma osmolality (268.58 mOsm/kg H₂O) and inappropriately elevated urine sodium (>40 mmol/L). Brain magnetic resonance imaging (MRI) demonstrated obstructive hydrocephalus with cerebellar tonsillar herniation, while magnetic resonance venography (MRV) showed stenosis of the right transverse sinus, sigmoid sinus and internal jugular vein.

The patient was diagnosed with SIADH secondary to intracranial pathology. Treatment with fluid restriction (500-1000 mL/day) and oral salt supplementation (gradually increased to 2.5 g/day) successfully increased serum sodium to 130 mmol/L. Neurosurgical consultation recommended conservative management given the likely congenital developmental abnormalities. After 3 years of follow-up, the patient remains clinically stable with ongoing monitoring. Recent urine osmolality testing (611 mOsm/kg H₂O) confirmed the SIADH diagnosis.

This case demonstrates the importance of neuroimaging in unexplained SIADH and highlights practical diagnostic strategies when standard laboratory parameters like urine osmolality are unavailable. Central nervous system abnormalities should be systematically investigated in patients with persistent hyponatraemia and non-specific symptoms. Conservative management with fluid restriction and salt supplementation can be effective for mild-to-moderate SIADH cases.