A 48-year-old man is brought to the emergency department from a group home due to fever, cough, and lethargy. The group home supervisor says that the patient has had a productive cough for the past several days and since yesterday has become progressively lethargic. The patient has a history of hypertension and bipolar disorder. He takes amlodipine and valproic acid. He does not use tobacco, alcohol, or illicit drugs. Temperature is 38.3 C (101 F), blood pressure is 118/72 mm Hg, pulse is 102/min, and respirations are 20/min. Pulse oximetry shows 96% on ambient air. The patient is somnolent but awakens to touch and follows instructions. He has decreased breath sounds and crackles in the right lower hemithorax. Cardiac examination is normal with the exception of tachycardia. There is no extremity edema. Leukocytes are 17,200/mm³ and chest radiography shows right lower lobe opacity with air bronchograms. Other laboratory results are as follows: Which of the following is the most likely cause of this patient's observed electrolyte abnormalities?

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Explanation: 11ec2b69_c1c6_94e0_8f63_533188056180_MD0005_00 This patient's laboratory studies are consistent with hyponatremia due to the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Pneumonia, evidenced by fever, cough, leukocytosis, and chest x-ray findings of opacification with air bronchograms, is a common precipitant of SIADH through a poorly understood mechanism. In addition, a number of psychiatric medications (eg, selective serotonin reuptake inhibitors, carbamazepine, valproic acid) have been associated with SIADH. Hyponatremia due to SIADH is characterized by serum hypotonicity (<275 mOsm/kg H₂O), inappropriately high urine osmolality (>100 mOsm/kg H₂O), and clinical euvolemia; there is high urine sodium concentration (>40 mEq/L) as the kidneys do not aggressively retain salt in the setting of euvolemia. Although normal values for urine osmolality and sodium can vary (eg, based on sodium intake), urine osmolality >100 mOsm/kg H₂O and urine sodium >40 mEq/L are typically considered high and classic for SIADH. (Choice A) Pseudohyponatremia describes an artifactual laboratory result that occurs in patients with an increased fraction of the nonaqueous component of serum (eg, paraproteinemia, hypertriglyceridemia). The measured serum osmolality is normal in patients with pseudohyponatremia. (Choice B) Decreased solute intake can lead to hypovolemic hyponatremia; however, low urine sodium (<40 mEq/L) is expected as the kidneys attempt to retain as much salt as possible. (Choice C) Adrenal insufficiency (low cortisol) is a common cause of hyponatremia; excessive cortisol release (Cushing syndrome) only rarely leads to hyponatremia. This patient's pneumonia and valproic acid use suggest SIADH is the more likely cause of hyponatremia. (Choice D) Excessive water ingestion, often seen in patients with underlying psychiatric disease (psychogenic polydipsia), can overwhelm the capacity of the kidneys to excrete free water and lead to euvolemic hyponatremia. However, the urine osmolality is low (<100 mOsm/kg H₂O) as the kidneys attempt to excrete large amounts of free water. Educational objective: Hyponatremia due to the syndrome of inappropriate antidiuretic hormone secretion (SIADH) is characterized by low serum osmolality (<275 mOsm/kg H₂O), inappropriately high urine osmolality (>100 mOsm/kg H₂O), and high urine sodium concentration (>40 mEq/L) in the setting of clinical euvolemia. Pneumonia and a number of psychiatric medications (eg, selective serotonin reuptake inhibitors, carbamazepine, valproic acid) are common precipitants of SIADH. References: Diagnosing and treating the syndrome of inappropriate antidiuretic hormone secretion. (http://www.ncbi.nlm.nih.gov/pubmed/26584969)

A 48-Year-Old Man Is Brought to the Emergency Department from a Group