Background: Hyponatraemia is the commonest electrolyte abnormality in clinical practice, and may be a biochemical manifestation of different diseases including malignancy. Uterine neuroendocrine tumours causing hyponatraemia are rare and can cause a diagnostic challenge.
We present a case of 68 years widow who presented with 3 weeks history of nausea, occasional vomiting, confusion, increased urinary frequency, urinary incontinence, chronic constipation, and weight loss. Her past medical history included left knee arthroscopy, cochlea implant and sterilisation in 1976. On admission she was haemodynamically stable with normal observations. Blood test revealed profound hyponatraemia (Na+118 mmol/l), and serum osmolality of 247 mOsm/kg, urine osmolality of 701 mOsm/kg, urinary sodium of 110 mmol/l, random cortisol of 542 nmol/l, in keeping with SIADH. Tumour markers were negative. Staging CT revealed heterogeneous enhancing uterine mass. Flexible sigmoidoscopy was normal. Her medications included Paracetamol, Codeine Phosphate and laxatives. She was started on fluid restriction of 750 ml/24 h. She remained hyponatraemic (116117 mmol/l) despite this. Oral Demeclocycline 300 mg TDS was added, but she was unable to tolerate it and was started on tolvaptan (30 mg od). Discharged on tolvaptan (30 mg) and was reviewed by gynaecology team in outpatient clinic. Cervical biopsy histology showed a high grade neuroendocrine malignant tumour confirmed by positive immuno labelling for chromogranin, AE1/3 and CAM5.2. She is currently undergoing chemotherapy and still on tolvaptan. Her sodium remains low but at safer levels of 127130 mmol/l.
Discussion: Hyponatraemia secondary to uterine neuroendocrine tumours is rare and generally associated with chemotherapy treatment rather than being related to the uterine neuroendocrine tumour. Conventional treatment for hyponatraemia due to SIADH was ineffective or not tolerated. Tolvaptan treatment was more successful even though it is mainly used in patient with mild to moderate euvolaemic hyponatraemia. This case report demonstrates the possibility of using tolvalptan in severe hyponatraemia.