A 60-year-old woman is brought to the hospital due to lethargy and confusion that have progressed over the last 2 days. This morning she was hard to arouse. The patient's husband says that she has lost 4.5 kg (10 lb) over the preceding 2-month period and has had a dry cough. She has no other medical problems and takes no medication. The patient has a 40-pack-year smoking history. She does not drink alcohol. Temperature is 37 C (98.6 F), blood pressure is 110/70 mm Hg, pulse is 75/min, and respirations are 16/min. She is not orthostatic. The patient is markedly confused. Her mucous membranes are moist. There is no jugular venous distension. Breath and heart sounds are normal. Neurologic examination shows no focal abnormalities. There is no ankle edema or ascites. Laboratory results are as follows: Serum osmolality is 250 mOsm/kg H₂O, and urine osmolality is 500 mOsm/kg H₂O. Chest x-ray reveals a mass in the right hilar region. CT scan of the head is unremarkable. Which of the following is the best next step in management of this patient?

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11ec25c8_d20f_cb0a_bc81_cb9efdeab5dc_MD0003_00 This patient's chest x-ray findings of a hilar mass in the setting of weight loss, cough, and a significant smoking history are highly suspicious for small cell lung cancer, which is a common cause of the syndrome of inappropriate antidiuretic hormone secretion (SIADH). The characteristic findings of SIADH are hyponatremia, serum osmolality <275 mOsm/kg H₂O, and urine osmolality >100 mOsm/kg H₂O (inappropriately elevated) in a euvolemic patient. The symptoms of SIADH depend on the degree of hyponatremia and the rate at which it occurred. Rapid decreases (over <48 hours) in serum sodium concentration lead to profound fluid shifts and severe symptoms. Patients with serum sodium 130-135 mEq/L are usually asymptomatic; those with chronic hyponatremia with serum sodium 120-130 mEq/L may be asymptomatic or display mild symptoms (lethargy, forgetfulness). These patients usually respond to fluid restriction (<800 mL/day), sometimes with the addition of salt tablets. Patients with serum sodium <120 mEq/L may have severe symptoms (eg, profound confusion, seizures, coma), which signal an increased risk of cerebral edema and brainstem herniation. These patients require an urgent increase in their serum sodium concentration that is best accomplished with administration of hypertonic (3%) saline. Extreme care must be taken with the rate of correction (<8 mEq/L over the first 24 hours) as rapid correction can lead to osmotic demyelination syndrome, a devastating and potentially fatal complication (eg, dysarthria, quadriparesis, coma). < (Choices B and F) Effective management of SIADH requires the infused fluid to have a higher electrolyte concentration than that of urine (500 mOsm/kg H₂O in this patient). Normal saline (0.9%) and half normal saline (0.45%) have electrolyte concentrations of approximately 300 and 150 mOsm/kg H₂O, respectively. Intravenous infusion of either of these fluids would cause a net increase in total body free water and worsen the hyponatremia. (Choice C) Desmopressin is an antidiuretic hormone analog used to treat diabetes insipidus. It would worsen hyponatremia in a patient with SIADH. (Choice D) Intravenous hydrocortisone is the appropriate treatment for hyponatremia due to adrenal insufficiency, which would present with hypovolemia (eg, dry mucous membranes), hypotension, and possibly orthostasis. (Choices E and G) Mannitol is effective for treating increased intracranial pressure and high intraocular pressure, and sodium bicarbonate can be used for treating severe metabolic acidosis and hyperkalemia. Educational objective: Patients with the syndrome of inappropriate antidiuretic hormone secretion who are asymptomatic or have mild symptoms usually respond to fluid restriction. Patients with severe symptoms require treatment with hypertonic (3%) saline. References: The treatment of hyponatremia. (http://www.ncbi.nlm.nih.gov/pubmed/19523575) Diagnosing and treating the syndrome of inappropriate antidiuretic hormone secretion. (http://www.ncbi.nlm.nih.gov/pubmed/26584969)

A 60-Year-Old Woman Is Brought to the Hospital Due to Lethargy