Endocrine Abstracts

Introduction: The Syndrome of Inappropriate Antidiuretic Hormone (SIADH) is considered to be the most common cause of euvolemic hyponatremia. The most common malignancy associated with SIADH is small cell lung cancer with multiple myeloma only observed in few cases worldwide of SIADH. The first line of treatment used in multiple myeloma chemotherapy currently is a proteasome inhibitor, Bortezomib, which is considered significantly more tolerable compared with traditional chemotherapeutic drugs. Bortezomib has been reported to have a wide-ranging side effect profile affecting different systems. However, the endocrine system is rarely affected in patients receiving bortezomib. Eelctrolyte disturbance has been reported in 1–10% of patients treated with Bortezomib with sever hyponatraemia reported in 2.6%. We present a case of Bortezomib induced SIADH with sever hyponatraemia in a newly diagnosed multiple myeloma, with other potential causes of SIADH excluded. A tumour-related cause was deemed very unlikely as hyponatremia was only observed after treatment with Bortezomib and was not present at the time of diagnosis of multiple myeloma.

Case: A 71-year-old lady who was recently diagnosed with IgG kappa multiple myeloma with multiple lytic lesions (skull, cervical spine, thoracic spine) presented to the haematology clinic with progressively worsening fatigue, shortness of breath and dizziness following day 22 of her chemotherapy cycle. She was clinically euovolaemic with no neurological compromise. A blood test showed severe hyponatremia (107 mmol/l) and she was referred to the acute medical unit for admission. A diagnosis of SIADH was made according to Bartter-Schwartz criteria. Her serum osmolality was 226 mOsm/kg [275–295 mosm/kg], urine osmolality was 119 mOsm/Kg [300–900 mOsm/kg] and, her urine Na was 49 mEq/L/day. her Cortisol level and TSH were normal. A chest X ray and CT head were normal. Her sodium level gradually improved in a progressive manner following fluid restriction to 750 ml/day with close monitoring.Her Sodium level on discharge was 135 mmol/L. Bortezomib injection were stopped and her symptoms have improved dramatically.

Conclusion: Health care practitioners should be aware of the possibility of Bortezomib-induced hyponatremia. Close clinical and laboratory monitoring for electrolyte disturbance, neurological disturbances or confusion is essential after initiation of treatment.