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.