Endocrine Abstracts

Introduction: Hyponatremia from SIADH secondary to undiagnosed Myelomas are extremely rare and we present one such case.

Clinical case: 63 year old woman with background of MGUS under regular haematology review/ILD/NAFLD/IHD presented with severe constipation on top of weakness, weight loss and tingling in the peripheral extremities. She was euvolemic on admission with Serum Sodium 124mmol/l, Potassium 4.3 mmol/l, Urea 3.1 mmol/l, Creatinine 53 micromol/l & eGFR >90 and normal Sodium 2 months before admission. 9 am cortisol > 400 nmol/l & TFTs normal. Plasma osmolality 265 mOsm/kg, Urine sodium 138 mmol & urine osmolality 631 mOsm/kg consistent with SIADH. Pseudohyponatremia excluded using Direct ISE. CTTAP showed stable but advanced ILD but no malignancy and CT/MRI Brain/Pituitary unremarkable. Symptoms worsened after fluid restriction, so Tolvaptan 15 mg OD started. Diarrhoea replaced constipation along with ascending distal sensorimotor symptoms & Autonomic neuropathy/Postural Hypotension. Investigated in depth by Neurology with LP showing oligoclonal bands, nerve Biopsy showing possible Vasculitic neuropathy, with mildly positive MPO antibody, normal CRP and VEGF levels & TTR gene negative. PET showed some increased activity in the left femur, liver & spleen. Neurology started her on Methylprednisolone with minimal improvement in symptoms, while Sodium levels hovered around 131 on 15 mg of Tolvaptan daily. IgG/FreeKappa raised but MGUS deemed stable by Haematology, however we pushed for Bone Marrow biopsy, as SIADH remained unexplained. BM biopsy suggested IgG Kappa myeloma and Haematology added Lenalidomide, alongside the Prednisolone which Neurology had continued for her Vasculitic/paraproteinemic neuropathy. Sodium levels maintained above 135 mmol/l on Lenalidomide, patient fully mobile back to baseline and completely off Tolvaptan.

Discussion : Literature evidence for Myelomas as a cause of SIADH is very limited, apart from the odd case report and possible mechanisms include effect of IL-6 on AVP.