Sodium Disorders I: Hyponatremia

Mechanism

Physiopathological Classification of Hyponatremia

The diagnostic algorithm requires unequivocal confirmation of the directly measured plasma osmolarity status:

• Isotonic Hyponatremia (Pseudohyponatremia): It is associated with normal plasma osmolarity (280-295 mOsm/kg). It occurs in the presence of extreme hyperlipidemias or hyperproteinemias, where the solid phase of lipids or proteins artificially displaces the volumetric measurement of the water fraction of the plasma of the traditional spectrophotometer.
• Hypertonic Hyponatremia (Translational): It presents a high serum osmolarity (> 295 mOsm/kg). It occurs secondary to the accumulation of osmotically effective solutes that do not penetrate the cell (such as glucose or mannitol), forcing the passage of water from the LIC to the ECF. For every 100 mg/dL increase in plasma glucose above a baseline value of 100 mg/dL, measured plasma sodium decreases approximately 1.6 mEq/L (or up to 2.4 mEq/L if blood glucose exceeds 400 mg/dL).
• True Hypotonic Hyponatremia: Presents decreased plasma osmolarity (< 275 mOsm/kg) and requires exhaustive evaluation of total body extracellular volume:

Hypovolemic (Sodium Loss)

There is concurrent sodium and water depletion, with saline loss predominating:

• Renal Losses (urinary Na⁺ > 20 mEq/L): Thiazide diuretics, salt-losing nephropathy, primary adrenal insufficiency.
• Extrarenal Losses (urinary Na⁺ < 20 mEq/L): Profuse vomiting, severe secretory diarrhea, third space or extensive burns.

Hypervolemic (Excess Free Water)

There is sodium and total body water retention, with a marked predominance of free water, usually secondary to cardiac output dysfunction or profound systemic vasodilation:

• Edematous States (urinary Na⁺ < 20 mEq/L): Advanced congestive heart failure, Child-Pugh C liver cirrhosis, severe nephrotic syndrome.
• Kidney Failure (urinary Na⁺ > 20 mEq/L): Acute kidney injury or end-stage chronic kidney disease.

Normovolemic Hyponatremia: It is characterized by a clinically normal extracellular fluid volume with subtle excess of free water. The main etiological exponent is Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH), characterized by inappropriately high urinary osmolarity (> 100 mOsm/kg), high urinary sodium (> 30 mEq/L) and rigorous exclusion of hypothyroidism or secondary adrenal insufficiency.

Quantitative Management and the Adrogué-Madias Equation

Correction of severe symptomatic hypotonic hyponatremia requires precise stimulation of sodium levels to avoid swelling of the brain parenchyma and consequent intracranial edema. The Adrogué-Madias equation estimates the expected impact of infusing one liter of any solution on serum sodium:

Δ [Na⁺]_serum = [Na⁺]_infused - [Na⁺]_serumACT + 1

Where total body water (TAW) is calculated based on the patient's body weight multiplied by a correction factor (0.6 in young men, 0.5 in young women and elderly men, 0.45 in elderly women).

If it is decided to coinfuse potassium together with the solution (since potassium also exerts an active osmotic effect and is introduced into the cell, displacing sodium to the extracellular space), the equation is modified as follows:

Δ [Na⁺]_serum = ([Na⁺]_infused + [K⁺]_infused) - [Na⁺]_serumACT + 1

Security

Critical Toxicity: Osmotic Demyelination Syndrome (ODS)

Excessively rapid correction of chronic hyponatremia (>48 hours of evolution) exposes the patient to a catastrophic and irreversible neurological injury called Osmotic Demyelination Syndrome (pontine and extrapontine myelinolysis). When the brain adapts to prolonged hyponatremia, astrocytes expel organic osmolytes (glutamate, taurine, myo-inositol) from their cytosol to prevent cellular brain edema.

If the ECF osmotic gradient is abruptly elevated by rapid infusion of sodium, free water rushes out of the brain cells before they can recover their intracytoplasmic osmolytes. This generates massive astrocytic dehydration with cellular apoptosis and physical destruction of the myelin sheath of the brainstem tracts (especially the pons).

Strict Safe Correction Limits:

• General limit: Do not exceed an increase in plasma sodium of 6 to 8 mEq/L in any continuous 24-hour period.
• Very high-risk patients (Alcoholism, extreme malnutrition or concomitant hypokalemia): Maintain an even more conservative correction target, limiting it to 4 to 6 mEq/L in 24 hours.

In case of accidental excessive correction of sodium, active therapy should be stopped immediately and therapeutic rescue infusions with 5% glucose serum should be initiated together with the administration of desmopressin (DDAVP) in doses of 1 to 2 µg subcutaneously every 8 hours to stop the clearance of free water by the kidney and re-lower sodium to safe levels.

Clinical

Perla Clínica: Treatment Protocol for Acute Symptomatic Hyponatremia

In a patient who presents severe neurological clinical manifestations of acute hyponatremia (de novo seizures, severe alteration of the state of consciousness or imminent suspicion of uncal herniation), initial rapid correction is mandatory and takes precedence over chronic speed restrictions, with the sole purpose of reversing critical acute cerebral edema:

1. Administer an intravenous bolus of 100 mL of 3% Hypertonic Saline Solution via peripheral or central venous route over a period of 10 minutes.
2. Evaluate the patient's neurological clinical response. If symptoms persist, administer a second bolus of 100 mL of 3% NaCl over the next 10 minutes (a third bolus may be repeated as an absolute limit).
3. The therapeutic objective of this emergency phase is to achieve a rapid increase in sodium from 4 to 6 mEq/L, which significantly reduces brain volume, relieving ICP immediately. Once this increase is reached or seizure symptoms subside, any active hypertonic infusion should be suspended and strict sodium analytical monitoring should be initiated every 4 hours.