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For further information, see CMDT Part 21–02: Hyponatremia
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Essentials of Diagnosis
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Thorough history and volume status examination are essential to determining etiology
Hyponatremia reflects excess water relative to sodium; total body sodium may be appropriate, low, or high
Hyponatremia in hospitalized patients commonly caused by administration of hypotonic fluids
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General Considerations
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Most common electrolyte abnormality in hospitalized patients
Defined as a serum sodium concentration < 135 mEq/L (135 mmol/L)
Whether hyponatremia is symptomatic depends on both severity and acuity
Severity can be subclassified as
Mild (130–134 mEq/L)
Moderate (125–129 mEq/L)
Severe (< 125 mEq/L)
Acuity
A diagnostic algorithm separates the causes of hyponatremia using serum osmolality and volume status (Figure 21–1)
SIADH is a clinical diagnosis characterized by
Hyponatremia
Hypoosmolality (< 280 mOsm/kg)
Absence of heart, lung, or kidney disease
Normal thyroid and adrenal function
Urine Na usually > 20 mEq/L
BUN may be < 10 mg/dL [3.6 mmol/L] and serum uric acid < 4 mg/dL [238 mcmol/L]
Low BUN and low serum uric acid are not only due to dilution but to increased urea and uric acid clearances in response to volume expansion
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History and physical examination
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Obtain history, including
New medications
Changes in fluid and solute intake (polydipsia, anorexia, intravenous fluid rates, and composition)
Fluid output (vomiting, diarrhea, ostomy output, polyuria, oliguria, insensible losses)
The physical examination should help categorize the patient's volume status into
Hypovolemia
Euvolemia
Hypervolemia
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Mild hyponatremia (sodium concentrations of 130–135 mEq/L)
Nausea and malaise may progress to headache, lethargy, and disorientation as the sodium concentration drops
Most serious symptoms are seizure and, very rarely, coma, brainstem herniation, and death
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Documented acute hyponatremia < 48 hours
Most hyponatremia is chronic > 48 hours
Pseudohyponatremia from hypertriglyceridemia or hyperproteinemia
Translocational hyponatremia