A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 68-year-old male is brought to the emergency department by his wife. An hour earlier, he dropped to the floor and began to violently shake his extremities. He urinated on the carpet and seemed confused for several minutes after. He is now feeling better. He has never experienced an episode like this before, nor does he think anyone in his family has. He and his wife are concerned that he has unintentionally lost 22.6 kg (50 lb) in the past 6 months. He has also been experiencing chest pain and has coughed up blood on a few occasions. He has a 50-pack-year smoking history and quit 2 years ago. His temperature is 36.8°C (98.2°F), heart rate is 98/min, respiratory rate is 15/min, blood pressure is 100/75 mm Hg, and he is O2 saturation is 100% on room air. The physical exam, including a full neurologic and cardiac assessment, demonstrates no abnormal findings. Edema, ascites, and skin tenting are notably absent. A brain MRI does not indicate areas of infarction or metastatic lesions. ECG is normal. Urine toxicology screen is negative. EEG is pending. Laboratory findings are shown below:
BUN 15 mg/dL N: 7 to 20 mg/dL
pCO2 40 mm Hg N: 35-45 mm Hg
Creatinine 0.8 mg/dL N: 0.8 to 1.4 mg/dL
Glucose 95 mg/dL N: 64 to 128 mg/dL
Serum chloride 103 mmol/L N: 101 to 111 mmol/L
Serum potassium 3.9 mEq/L N: 3.7 to 5.2 mEq/L
Serum sodium 115 mEq/L N: 136 to 144 mEq/L
Total calcium 2.3 mmol/L N: 2-2.6 mmol/L
Magnesium 1.7 mEq/L N: 1.5-2 mEq/L
Phosphate 0.9 mmol/L N: 0.8-1.5 mmol/L
Hemoglobin 14 g/dL N: 13-17 g/dL (men), 12-15 g/dL (women)
Glycosylated hemoglobin 5.5% N: 4%-6%
Total cholesterol 4 mmol/L N: 3-5.5 mmol/L
Bicarbonate (HCO3) 19 mmol/L N: 18-22 mmol/L
What is indicated first?
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A
PhenytoinIncorrect. Phenytoin treats the seizure symptom but won't work in hyponatremia-induced seizures, where the underlying problem is cerebral edema from low osmolality.
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B
Valproic acidIncorrect. Valproic acid is for seizure prophylaxis but, like phenytoin, fails to address the underlying severe hyponatremia driving the seizure.
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C
DiazepamIncorrect. Diazepam aborts active seizure activity but is only temporizing; the seizure will recur unless the sodium is raised.
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D
Hypertonic salineCorrect. Symptomatic hyponatremia with seizure requires hypertonic (3%) saline to acutely raise serum sodium by 4-6 mEq/L and halt cerebral edema.
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E
DemeclocyclineIncorrect. Demeclocycline is used for chronic SIADH management by inducing nephrogenic diabetes insipidus; it has no role in acute symptomatic hyponatremia with seizure.
↑ Tap an answer to reveal the reasoning
Answer: D. This patient had a witnessed generalized tonic-clonic seizure in the setting of severe hyponatremia (Na 115 mEq/L) and signs of malignancy (50-pack-year smoking, 50-lb weight loss, hemoptysis, chest pain). The unifying diagnosis is small cell lung carcinoma producing SIADH with severe euvolemic hyponatremia. The absence of edema, ascites, and skin tenting confirms euvolemia, distinguishing SIADH from hypovolemic (orthostasis, dry mucosa, elevated BUN/Cr) and hypervolemic (CHF, cirrhosis) causes.
Symptomatic hyponatremia with seizure is a neurologic emergency. The treatment of choice is hypertonic (3%) saline, typically as a 100-150 mL bolus, repeated as needed to raise serum sodium by 4-6 mEq/L acutely to halt cerebral edema and seizures. Anticonvulsants (phenytoin, valproic acid, diazepam) treat the symptom but do not address the underlying osmotic insult and will not resolve seizures driven by hyponatremia.
Critical pearl: correct sodium no faster than ~8-10 mEq/L in 24 hours overall to avoid osmotic demyelination syndrome, but for symptomatic hyponatremia (seizure, coma), the initial 4-6 mEq/L correction with hypertonic saline can and should be rapid.
**Why each option:**
**A.** Phenytoin treats the seizure symptom but won't work in hyponatremia-induced seizures, where the underlying problem is cerebral edema from low osmolality.
**B.** Valproic acid is for seizure prophylaxis but, like phenytoin, fails to address the underlying severe hyponatremia driving the seizure.
**C.** Diazepam aborts active seizure activity but is only temporizing; the seizure will recur unless the sodium is raised.
**D.** Correct. Symptomatic hyponatremia with seizure requires hypertonic (3%) saline to acutely raise serum sodium by 4-6 mEq/L and halt cerebral edema.