Ominous Hyponatremia

Introduction: Hyponatremia is the most common electrolyte disorder in hospitalized patients and a signal for an underlying disorder. The differential diagnosis is broad and often times multifactorial in etiology. It is important to note that some less common causes may be overlooked in the initial evaluation. Case Description: A 52-year-old man presents to the emergency room for right upper abdominal pain and found to have severe asymptomatic hyponatremia of 112 mmol/L. He had poor po intake, 30 lb. weight loss over the last months and drinks about 1.5L water daily. He has no known significant medical problems except vitreous hemorrhage, 30 pack year tobacco use and is originally from El Salvador. Initially he was thought to have hyponatremia due to hypovolemia vs. low solute state and initially treated with NS. His initial Uosm 495 and urine Na 89 mmol/L were consistent with hypovolemia. Na corrected to124 mmol/L, but then plateaued and relowered to 119 mmol/L. He was thought to have underlying SIADH from pain, lung lesion found on CXR and new diagnosis of TB. Despite fluid restricted and salt tabs, Na did not improve and renal was consulted. Repeat Uosm was 588 and urine Na 98 remained elevated and patient was noted to have hyperkalemia of K 6.1 mmol/L on admit without evidence of AKI.Patient was not hypotensive or orthostatic; AM Cortisol was low normal 5.4 mcg/dL however cosyntropin stimulation test was abnormal suggesting adrenal insufficiency. Further work up of lung lesion with CT revealed that patient had primary lung cancer with diffuse metastasis to brain, peritoneum, pericardium, liver, kidneys and adrenal glands. Patient was treated with hydrocortisone and fludrocortisone with improvement in Na to 129 mmol/L on discharge. Discussion: Hyponatremia can be the first sign of an underlying malignancy. Often in malignancy patients, the etiology can be multifactorial including hypovolemia, low solute intake, SIADH, renal salt wasting or adrenal insufficiency. A thorough and detailed evaluation should be done to consider all possible etiologies and provide the optimal treatment.