Case

A 65-year-old white woman is brought to the emergency room by her family for increasing confusion and lethargy over the past week. She was recently diagnosed with small cell cancer of the lung. She has not been febrile or had any other recent illnesses. She is not taking any medications. Her blood pressure is 136/82 mmHg. heart rate 84 bpm, and respiratory rate 14 breaths per minute and unlabored. She is afebrile. On examination, she is an elderly appearing woman who is difficult to arouse and reacts only to painful stimuli. She is able to move her extremities without apparent motor deficits, and her deep tendon reflexes are decreased symmetrically. The remainder of her examination is normal, with a normal jugular venous pressure and no extremity edema. You order some laboratory tests, which reveal the serum sodium level is 108 mmol/L, potassium 3.8 mmol/L, bicarbonate 24 mEq/L, blood urea nitrogen 5 mg/dL. and creatinine 0.5 mg/dL. Serum osmolality is 220 mOsm/kg, and urine osmolality is 400 mOsm/kg. A CT scan of the brain shows no masses or hydrocephalus.

♦ What is the most likely diagnosis? ^ What is your next step in therapy? ^ What are the complications of therapy?

ANSWERS TO CASE 5: Hyponatremia, Syndrome of Inappropriate Antidiuretic Hormone Secretion

Summary: A 65-year-old white woman with small cell lung cancer has increasing confusion and lethargy over the past week. She is afebrile and normoten-sive. and she has no edema or jugular venous distention. She is lethargic but is able to move her extremities without apparent motor deficits, and her deep tendon reflexes are decreased symmetrically. Her serum sodium level is 108 mmol/L. potassium 3.8 mmol/L. bicarbonate 24 mEq/L. blood urea nitrogen (BUN) 5 mg/dL. and creatinine 0.5 mg/dL; serum osmolality is 220 mOsm/kg, and urine osmolality is 400 mOsm/kg. A CT scan of the brain shows no masses or hydrocephalus.

^ Most likely diagnosis: Coma/lethargy secondary to severe hyponatremia, which is most likely caused by a tumor-related syndrome of inappropriate antidiuretic hormone secretion (SIADH).

^ Next therapeutic step: Treat the hyponatremia with hypertonic saline.

^ Most serious complication of this therapy: Osmotic cerebral demyelination. also referred to as central pontine myelinolysis.

Analysis

Objectives

1. Learn the causes of hyponatremia.

2. Understand the use of laboratory testing in the diagnosis of hyponatremia.

3. Know how to treat hyponatremia, and some of the potential complications of therapy.

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