A 65-year-old man with a recent diagnosis of small cell lung cancer is admitted to the hos… — USMLE Step 2 CK

A $65$ -year-old man with a recent diagnosis of small cell lung cancer is admitted to the hospital due to progressive confusion, lethargy, and nausea over the past week. He has a $40$ -pack-year smoking history. His temperature is $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), blood pressure is $128/82$ mmHg, pulse is $78$ /min, and respirations are $14$ /min. Physical examination reveals moist mucous membranes, normal skin turgor, no jugular venous distention, and clear lungs. There is no peripheral edema.

Laboratory results are as follows:

Laboratory Test	Result	Reference Range
Serum sodium	$114$ mEq/L	$136$ - $145$ mEq/L
Serum potassium	$4.1$ mEq/L	$3.5$ - $5.0$ mEq/L
Serum chloride	$85$ mEq/L	$98$ - $106$ mEq/L
Serum bicarbonate	$24$ mEq/L	$22$ - $28$ mEq/L
Blood urea nitrogen	$7$ mg/dL	$8$ - $20$ mg/dL
Serum creatinine	$0.6$ mg/dL	$0.7$ - $1.3$ mg/dL
Serum osmolality	$245$ mOsm/kg H₂O	$275$ - $295$ mOsm/kg H₂O
Urine osmolality	$600$ mOsm/kg H₂O	$50$ - $1200$ mOsm/kg H₂O
Urine sodium	$65$ mEq/L	$> 20$ mEq/L
TSH	$2.1$ $\mu$ U/mL	$0.5$ - $5.0$ $\mu$ U/mL
Morning cortisol	$18$ $\mu$ g/dL	$5$ - $23$ $\mu$ g/dL

While in the emergency department, the patient develops a generalized tonic-clonic seizure that lasts for $2$ minutes and resolves spontaneously. He remains postictal. Which of the following is the most appropriate next step in management?

Administer a bolus of 3% hypertonic salineAnswer
B
Administer a continuous infusion of 0.9% normal saline
C
Administer intravenous broad-spectrum antibiotics and perform a lumbar puncture
D
Administer intravenous furosemide and initiate noninvasive positive-pressure ventilation
E
Transport the patient for an urgent magnetic resonance imaging of the brain

Answer

Administer a bolus of 3% hypertonic saline

The correct answer is the administration of 3% hypertonic saline. This patient is presenting with a classic paraneoplastic syndrome secondary to small cell lung cancer (SCLC). His clinical euvolemia combined with hypotonic hyponatremia, high urine osmolality, and elevated urine sodium is highly characteristic of the Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH). While asymptomatic or mildly symptomatic SIADH is typically managed with fluid restriction, this patient has developed a generalized tonic-clonic seizure. Severe, symptomatic hyponatremia (e.g., seizures, coma) is a life-threatening oncologic and metabolic emergency that requires immediate treatment with 3% hypertonic saline. This intervention rapidly raises the serum sodium level just enough to reduce cerebral edema and arrest the seizures, after which the rate of correction is slowed to prevent osmotic demyelination syndrome.

Step-by-Step Solution

Analyze the patient's volume status and laboratory results.

The patient is clinically euvolemic with profound hyponatremia (114 mEq/L), low serum osmolality, inappropriately concentrated urine, and elevated urine sodium.

To identify the etiology of the hyponatremia as the Syndrome of Inappropriate Antidiuretic Hormone (SIADH), a common paraneoplastic syndrome of small cell lung cancer.

Assess the clinical severity of the presentation.

The patient is experiencing progressive confusion leading to a generalized tonic-clonic seizure.

The presence of severe neurologic symptoms dictates the urgency and modality of treatment, shifting from conservative fluid restriction to immediate active correction.

Select the appropriate acute intervention for symptomatic SIADH.

Administer 3% hypertonic saline.

Severe symptomatic hyponatremia is an oncologic/metabolic emergency that requires rapid correction of serum sodium to reduce cerebral edema and halt seizures.

Key Concept

Management of severe, symptomatic paraneoplastic SIADH

Estimated Time:1m 30s

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