HYPONATREMIA MANAGEMENT

Explore the fundamentals of hyponatremia management in this concise guide designed for medical students. Understand serum osmolarity, volume status, and urine sodium levels to effectively diagnose and treat this common electrolyte imbalance.

HYPONATREMIA MANAGEMENT

Hyponatremia Basic ApproachStep 1: Serum Osmolarity

• Hypotonic (Osm < 285): Requires further workup and treatment.
• Normotonic (Osm = 285 - 295): Indicates pseudohyponatremia.
• Hypertonic (Osm > 295): Caused by contrast dye, hypertriglyceridemia, etc.

Note: Clinically relevant hyponatremia is hypotonic (Osm < 285).

Step 2: Volume Status

• Hypovolemic: Cerebral Salt Wasting, Vomiting, Diarrhea, Third Space Losses.
• Euvolemic: SIADH, Polydipsia.
• Hypervolemic: CHF, Cirrhosis, Nephrotic Syndrome, Renal Failure.

Step 3: Urine Sodium Level

• Hypovolemic: High urine sodium suggests CSW; low/normal indicates diarrhea, vomiting, or third space losses.
• Euvolemic: High urine sodium suggests SIADH; low/normal indicates polydipsia.
• Hypervolemic: High urine sodium suggests renal failure.

Diagnosing HypovolemiaHistory

• Poor intake, vomiting, diarrhea, diuretics, bleeding.

Clinical Signs

• Dry underside of tongue, dry axilla, low JVP.

Labs

• Urine creatinine, urea, and sodium (random).

Fractional Excretion

• FE Na (Fractional Excretion of Sodium) < 1% indicates hypovolemia.
• [ FE Na = \frac{(urine sodium / plasma sodium) \times 100}{(urine creatinine / plasma creatinine)} ]
• FE Urea (Fractional Excretion of Urea) < 35% indicates hypovolemia.
• [ FE Urea = \frac{(Urine urea / Blood urea) \times 100}{(Urine creatinine / Blood Creatinine)} ]

Hyponatremia with Hypovolemia

• Indicates dehydration due to renal losses, third space losses, vomiting, or diarrhea.
• Urine sodium > 20: renal loss.
• Urine sodium < 10: third space loss, vomiting, diarrhea.
• Treatment: Normal Saline (see dosing regimen).

Consider Cerebral Salt Wasting (CSW)

• Excess renal loss of sodium and water; sodium loss is disproportionately greater.
• Urine Osm > 100, Urine sodium > 40.

Hyponatremia in Isovolemic Patient

• Urine sodium < 10 & Urine Osm < 100: Water intoxication / primary polydipsia / poor solute intake.
• Urine sodium > 20 & Urine Osm > 100: SIADH / Hypothyroidism / Addison's Disease.

Management of SIADH

• Fluid restriction: 500-750 ml/day.
• Demeclocycline: 300 mg po bid.
• Lasix: 80 mg IV.
• Use normal saline as per protocol.

Key Point: SIADH patients are euvolemic due to ADH-induced reabsorption of water in the distal tubule.

Hyponatremia and Hypervolemia (Edematous Patient)

• Urine sodium < 20: CHF, Cirrhosis, Nephrotic syndrome.
• Urine sodium > 20: Renal failure.

Hyperosmolar Hyponatremia (Osm > 295)Formula to Calculate Osmolarity

[ 2 \times sodium + \frac{glucose}{18} + \frac{BUN}{2.8} ]

Osmolar Gap

• Osm gap = Osm measured - Osm calculated.
• < 10 is normal.
• 
  

10: Can indicate endogenous (acetones, renal failure, lactate) or exogenous (methanol, ethylene glycol, ethanol, glycine, mannitol) causes.