USMLE/COMLEX - Step 2 - Hyponatremia

USMLE/COMLEX - Step 2 - Hyponatremia Mild hyponatremia: 130-136 mEq/L Moderate hyponatremia: 120-129 mEq/L Severe hyponatremia: <120 mEq/L Acute (less than 48 hours) Chronic (greater than 48 hours or unknown duration) To confirm hyperglycemia as the cause of hyponatremia, calculate corrected serum sodium. Add 2 mEq/L to serum sodium for every 100 mg/dL increase in glucose levels above the normal range. If corrected serum sodium falls within normal range then hyperglycemia-induced hyponatremia is determined to be the cause. Treatment varies depending on duration, severity and the underlying cause. General measures include fluid restriction and avoiding hypotonic fluids. Acute hyponatremia

• Symptomatic: 3% hypertonic saline
• Asymptomatic: monitor, 3% hypertonic saline if no autocorrection or autocorrection stops or serum sodium decreases

Chronic hyponatremia

• Mild/moderate hyponatremia: general measures
• Severe hyponatremia/severe symptoms/ intracranial pathology: 3% hypertonic saline

Hypervolemic and euvolemic etiologies can be treated with water restriction with or without diuretics. Hypovolemic etiologies need intravenous normal saline. Osmotic demyelination syndrome (formerly central pontine myelinolysis) is caused by rapid correction of hyponatremia. Clinical manifestations include lethargy, dysarthria, dysphagia, quadriparesis, behavioral disturbances, seizures, and coma. The maximum rate of hyponatremia correction should not exceed 8 mEq/L in 24 hours. Serum sodium less than 136 mEq is defined as hyponatremia. Etiologies of hyponatremia can be divided by serum osmolality into:

• Hypotonic hyponatremia (<280 mEq/L): renal and extrarenal causes
• Hypertonic hyponatremia (>295 mEq/L): hyperglycemia, IV mannitol, IV radiocontrast agents
• Isotonic hyponatremia (280-295 mEq/L): TURP syndrome, pseudohyponatremia (hyperlipidemia, hyperproteinemia)

Etiologies of hypotonic hyponatremia can be divided by extracellular volume status into:

• Hypovolemic hyponatremia, which can be further divided by fractional excretion of sodium
• FENa < 1%: vomiting, diarrhea, burns, third-spacing (ascites, peritonitis)
• FENa >2%: diuretics (thiazide), acute or chronic renal failure with polyuria, addison disease (hypoaldosteronism)

• Euvolemic hyponatremia, which can be further divided by urine osmolality
• >100 mOsm/kg: SIADH, hypothyroidism, glucocorticoid deficiency
• <100 mOsm/kg: water intoxication (primary polydipsia, beer potomania)

• Hypervolemic hyponatremia, which can be further divided by fractional excretion of sodium
• FENa < 1%: congestive heart failure, liver cirrhosis, nephrotic syndrome
• FENa > 2%: acute or chronic renal failure with low urine output

Serum osmolality can be calculated by the formula:

• Serum osmolality = (2 x serum sodium) + (BUN/2.8) + (Glucose/18)

Patients can be asymptomatic or symptomatic depending upon the severity and duration of hyponatremia. Potential symptoms include:

• Mild/moderate symptoms: headache, nausea, vomiting, muscle cramps, lethargy, forgetfulness, gait disturbances, confusion
• Severe symptoms: seizures, coma and respiratory arrest

Severe and life-threatening hyponatremia that can occur following prolonged exercise (e.g., marathons, triathlons). Ingestion of large amounts of fluids (e.g., water, sports drink), decreased water excretion due to persistent secretion of ADH, and loss of sodium through sweat plays a role in its pathogenesis.