A 60-year-old man is brought to the emergency department due to altered mental status. His wife says that he has a nagging cough that seemed to worsen over the past several months and that he also began experiencing occasional dizziness earlier this week. Over the last couple of days, the patient has become increasingly confused, and today he "does not make any sense at all." He has no significant past medical history but has smoked a pack of cigarettes daily for the last 40 years. On physical examination, the patient is incoherent but has an otherwise normal neurologic examination.
Laboratory results are as follows:
| Serum chemistry | |
| Sodium | 120 mEq/L |
| Potassium | 4.0 mEq/L |
| Blood urea nitrogen | 12 mg/dL |
| Creatinine | 0.8 mg/dL |
| Calcium | 9.0 mg/dL |
| Glucose | 98 mg/dL |
| Plasma osmolality | 250 mOsm/kg |
| Urine drug screen | negative |
Chest x-ray reveals a mass in the right lung. Which of the following additional findings is most likely to be present in this patient?
Show Explanatory Sources
This patient's combination of hyponatremia and a lung mass is suggestive of the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Antidiuretic hormone (ADH) is normally produced in the hypothalamus and secreted from the posterior pituitary in response to changes in plasma osmolality and intravascular volume. However, small cell lung carcinomas, which are tumors of neuroendocrine origin, often release ADH independent of feedback inhibition (paraneoplastic effect). This inappropriate secretion of ADH leads to hyponatremia, decreased plasma osmolality, and elevated urine osmolality (which normally should be <100 mOsm/kg when hyponatremia is present). The profound hyponatremia that occurs in SIADH can cause headache, weakness, altered mental status, and seizures.
In SIADH, increased ADH causes excessive water absorption by the kidneys, leading to a transient, subclinical hypervolemia. This mild increase in extracellular fluid volume suppresses the renin-aldosterone axis and stimulates the production of natriuretic peptides, leading to excretion of sodium in the urine (natriuresis). As a result, patients with SIADH have a clinically normal extracellular fluid volume and low plasma osmolality (euvolemic hyponatremia). Features of volume overload (eg, peripheral edema, pulmonary crackles, elevated jugular venous pressure) are not seen (Choices A, B, and E).
(Choice C) Patients with SIADH have inappropriately concentrated urine (generally >100 mOsm/kg) for their degree of hyponatremia. Hyponatremia with a urine osmolality of <100 mOsm/kg (maximally dilute urine) indicates appropriate suppression of ADH secretion (as would be seen in primary polydipsia).
Educational objective:
The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is characterized by low plasma sodium and osmolality, inappropriately concentrated urine, and clinically normal volume status (euvolemic hyponatremia). An important cause of SIADH is a paraneoplastic effect secondary to small cell carcinoma of the lung.