A 54-year-old woman is evaluated in the neurosurgical intensive care unit for worsening so… — USMLE Step 2 CK

A 54-year-old woman is evaluated in the neurosurgical intensive care unit for worsening somnolence over the past 12 hours. She was admitted 6 days ago with a subarachnoid hemorrhage secondary to a ruptured left middle cerebral artery aneurysm, which was successfully treated with surgical clipping. Over the last 24 hours, her urine output has been 4.5 liters.

Vitals:
- Temperature: $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ )
- Blood pressure: $88/54 \text{ mm Hg}$
- Heart rate: $122/\text{min}$
- Respirations: $16/\text{min}$

Physical examination shows dry mucous membranes and poor skin turgor. Neurologic examination reveals a Glasgow Coma Scale of 13 (E3, V4, M6). She has a right-sided pronator drift and mild right facial droop, which are unchanged from her immediate postoperative baseline.

Laboratory results:

Parameter	Value	Reference Range
Serum sodium	$125 \text{ mEq/L}$	$136-145 \text{ mEq/L}$
Serum potassium	$3.9 \text{ mEq/L}$	$3.5-5.0 \text{ mEq/L}$
Blood urea nitrogen	$42 \text{ mg/dL}$	$8-20 \text{ mg/dL}$
Serum creatinine	$1.3 \text{ mg/dL}$	$0.6-1.2 \text{ mg/dL}$
Serum osmolality	$258 \text{ mOsm/kg H}_2\text{O}$	$275-295 \text{ mOsm/kg H}_2\text{O}$
Urine sodium	$110 \text{ mEq/L}$	$>20 \text{ mEq/L}$
Urine osmolality	$680 \text{ mOsm/kg H}_2\text{O}$	Varies with hydration

Which of the following is the most appropriate next step in management?

Intravenous infusion of 0.9% sodium chlorideAnswer
B
Strict fluid restriction to less than 1,000 mL daily
C
Intravenous administration of desmopressin
D
Intravenous administration of conivaptan
E
Intravenous administration of mannitol

Answer

Intravenous infusion of 0.9% sodium chloride

The correct intervention is the intravenous infusion of 0.9% sodium chloride. This patient's clinical picture is classic for cerebral salt wasting (CSW) syndrome, a frequent and potentially dangerous complication following a subarachnoid hemorrhage (SAH). CSW is thought to be driven by the release of brain natriuretic peptide (BNP) and other natriuretic factors in response to neurological injury. These factors induce primary renal sodium loss with subsequent water loss, leading to a state of hypovolemic hyponatremia.

The most critical differential diagnosis in this scenario is the syndrome of inappropriate antidiuretic hormone (SIADH). Both CSW and SIADH manifest with similar laboratory derangements: hyponatremia, high urine osmolality (concentrated urine), and high urine sodium (typically

>40 \text{ mEq/L}

). The key to differentiating them lies entirely in the patient's volume status. Patients with SIADH are characteristically euvolemic or slightly hypervolemic. In contrast, patients with CSW are distinctly hypovolemic. This patient demonstrates clear signs of severe hypovolemia, including hypotension (

88/54 \text{ mm Hg}

), tachycardia (

122/\text{min}

), high urine output (

4.5 \text{ L/day}

), dry mucous membranes, and a markedly elevated BUN-to-creatinine ratio (

>20:1

).

The immediate goal of treatment in CSW is aggressive volume resuscitation. This is particularly crucial in the setting of a recent SAH, as patients are at an exceedingly high risk for vasospasm-induced delayed cerebral ischemia between days 3 and 14 post-hemorrhage. Hypovolemia drastically reduces cerebral perfusion pressure, magnifying this ischemic risk. Therefore, restoring intravascular volume with isotonic saline (0.9% NaCl) or hypertonic saline is the most appropriate next step.

Step-by-Step Solution

Analyze the patient's volume status based on clinical and vital signs.

The patient exhibits hypotension, tachycardia, poor skin turgor, dry mucous membranes, and an elevated BUN-to-creatinine ratio, indicating profound hypovolemia.

Establishing accurate volume status is the essential first step in differentiating the causes of hyponatremia in neurocritical care patients.

Evaluate the laboratory findings in the context of the established volume status.

The combination of severe hyponatremia, high urine sodium, high urine osmolality, and clinical hypovolemia points definitively to cerebral salt wasting (CSW) rather than SIADH.

Both CSW and SIADH present with hyponatremia and concentrated, sodium-rich urine. The differentiating factor is that SIADH is a euvolemic state, whereas CSW is a hypovolemic state driven by primary renal sodium loss.

Determine the appropriate management for cerebral salt wasting.

Volume resuscitation with an intravenous isotonic crystalloid (0.9% sodium chloride) is required.

Restoring intravascular volume is critical not only to correct the hypovolemia but also to maintain adequate cerebral perfusion pressure, mitigating the high risk of vasospasm and delayed cerebral ischemia that peaks 3 to 14 days following a subarachnoid hemorrhage.

Key Concept

Cerebral salt wasting (CSW) is a common complication following subarachnoid hemorrhage. It causes hypovolemic hyponatremia due to renal sodium loss and must be differentiated from SIADH (euvolemic hyponatremia). CSW is managed with volume repletion (isotonic or hypertonic saline), whereas SIADH is managed with fluid restriction.

Estimated Time:1m 30s

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