A 65-year-old woman is brought to the emergency department due to altered mental status. Her family notes that she awoke in the morning with a headache and became progressively incoherent over the course of the day. The day before, she had nausea and vomited several times during the day. Three weeks ago, the patient was diagnosed with depression and started on sertraline. Her only other medical problem is rheumatoid arthritis, which was diagnosed a year ago and is well controlled with adalimumab. She takes no other medications, supplements, or herbs. She is afebrile. Blood pressure is 110/70 mm Hg without an orthostatic drop, pulse is 75/min, and respirations are 15/min. She is disoriented and irritable. Neck is supple. Neurologic examination is otherwise unremarkable. There is no papilledema. Mucous membranes are moist. There is no jugular venous distension. Lungs are clear to auscultation. Abdomen is soft, nontender, and nondistended. There is no peripheral edema. Laboratory results are as follows: Serum osmolality is 260 mOsm/kg H₂O, urine osmolality is 500 mOsm/kg H₂O, and urine sodium is 56 mmol/L. What is the most likely cause of this patient's hyponatremia?

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11ec25c8_d4fe_1229_bc81_e1551d069411_MD0003_00 This patient has symptomatic hypotonic (serum osmolality <275 mOsm/kg H₂O) hyponatremia due to the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Normally, the kidneys are able to excrete free water at a sufficient rate to prevent blood hypotonicity. However, in SIADH, inappropriately high levels of antidiuretic hormone (ADH) prevent the kidneys from excreting dilute urine (leading to urine osmolality >100 mOsm/kg H₂O), and hypotonicity and hyponatremia develop. Patients with SIADH are typically euvolemic; therefore, urine sodium concentration is typically elevated (>40 mEq/L), unlike in patients with hypovolemia. In addition, in SIADH, serum uric acid levels are characteristically low, serum potassium level is normal, and acid-base status is normal. Selective serotonin reuptake inhibitors are commonly associated with SIADH and elderly patients are especially at risk. Nausea can further exacerbate SIADH. Symptoms of SIADH vary depending on the severity and rate of development of hyponatremia; patients can be asymptomatic, mildly symptomatic (eg, nausea, lethargy) or, in severe cases, present with profound confusion, seizures, or coma. (Choices B and F) Both severely decreased solute intake (in the setting of ongoing free water intake) and polydipsia can lead to hyponatremia. However, in these situations, ADH levels are normal and patients are able to excrete dilute urine; therefore, urine osmolality would be appropriately low (<100 mOsm/kg H₂O). (Choice C) Intracranial neoplasms may lead to hyponatremia due to SIADH or cerebral salt wasting (caused by increased intracranial pressure). However, the absence of papilledema and focal neurologic findings (eg, focal weakness) in this patient makes intracranial neoplasm less likely. (Choice D) Intravascular volume depletion occurs in dehydration and can lead to hyponatremia due to decreased renal perfusion and excess free water retention. The urine sodium concentration should be low (<40 mEq/L) as the kidneys attempt to retain solute in an effort to increase blood volume. (Choice E) Mineralocorticoid deficiency typically presents with hyperkalemia and metabolic acidosis, which are not present in this patient. (Choice G) Renal resistance to ADH occurs in nephrogenic diabetes insipidus. The result is uncontrolled loss of free water by the kidneys and consequent hypernatremia. Educational objective: The syndrome of inappropriate antidiuretic hormone secretion is characterized by hypotonic hyponatremia and euvolemia. Low serum osmolality (<275 mOsm/kg), high urine osmolality (>100 mOsm/kg), and an elevated urine sodium concentration (>40 mEq/L) strongly suggest the diagnosis. References: Hyponatremia and the syndrome of inappropriate secretion of antidiuretic hormone associated with the use of selective serotonin reuptake inhibitors: a review of spontaneous reports. (http://www.ncbi.nlm.nih.gov/pubmed/8804257)

A 65-Year-Old Woman Is Brought to the Emergency Department Due