The Correct Answer is C
Hyponatremia is defined as a serum sodium concentration below 135 mmol/L, NOT below 140 mmol/L 1. This makes option C the statement that does NOT apply to hyponatremia.
Why Each Other Statement is TRUE:
A) Hypotonic fluids can cause hyponatremia in hospitalized patients - TRUE
Iatrogenic hyponatremia from hypotonic fluid administration is a well-recognized cause in hospitalized patients, particularly when combined with non-osmotic ADH release from pain, nausea, or stress.
B) Serum osmolality and assessment of intravascular status are essential - TRUE
The diagnostic approach requires both serum osmolality measurement and volume status assessment 1. The guidelines explicitly recommend:
- Initial workup should include serum and urine osmolality, urine electrolytes, and uric acid
- Volume status determination is critical to distinguish between hypovolemic, euvolemic, and hypervolemic hyponatremia
- This differentiation is essential for distinguishing SIADH from cerebral salt wasting (CSW), where extracellular fluid status is the key distinguishing feature
Physical examination alone has poor sensitivity (41.1%) for determining volume status, so invasive monitoring like central venous pressure may be needed in complex cases 1.
D) Most common electrolyte disorder in hospitalized patients - TRUE
Hyponatremia affects approximately 5% of adults and 35% of hospitalized patients, making it the most common electrolyte disorder 2.
E) Can lead to neurological complications - TRUE
Hyponatremia causes significant neurological morbidity and mortality 1:
- Severe acute hyponatremia causes cerebral edema leading to seizures, coma, and death
- Patients with sodium <130 mmol/L had a 60-fold increase in mortality (11.2% vs 0.19%)
- Sodium <120 mmol/L carried 25% mortality vs 9.3% for levels >120 mmol/L
- Hyponatremic patients were 7 times more likely to die in hospital
- Associated with increased falls (21% vs 5% in normonatremic patients)
- In SAH patients, hyponatremia increases cerebral ischemia risk and poor outcomes at 3 months (OR 2.7)
The Critical Threshold
The classic definition is sodium <135 mmol/L 1, though clinically significant hyponatremia requiring intervention typically occurs at <131 mmol/L according to the neurosurgical guidelines 1. A sodium of 140 mmol/L is within the normal range (typically 135-145 mmol/L), making option C factually incorrect.