Hypernatremia in the Emergency Department

1. How to Spot It at the Bedside - "See It at a Glance"

• Elderly or debilitated patient who cannot get up to drink
• Infant (especially if formula is improperly mixed)
• Altered/unconscious patient or anyone intubated and sedated
• Diabetic patient with poor control
• Patient with known diabetes insipidus
• Anyone with a history of prolonged vomiting, diarrhea, burns, or high fever

2. The Three Types - Classification by Volume Status

3. Diagnostic Work-Up

1. Serum electrolytes + serum osmolality - confirms hypernatremia and hyperosmolar state
2. Urine osmolality - the key discriminator:
   • Urine osmolality > 800 mOsm/kg = concentrated urine = extrarenal loss (kidneys working normally - vomiting, diarrhea, burns)
   • Urine osmolality < 300 mOsm/kg = dilute urine = central DI (kidneys cannot concentrate)
   • Urine osmolality 300-800 mOsm/kg = partial DI
   • Urine osmolality > 100, urine Na > 20 = renal causes (osmotic diuresis, loop diuretics)
3. Urine sodium - helps distinguish renal vs. extrarenal:
   • Urine Na < 10 mEq/L = extrarenal hypovolemic (kidneys retaining Na appropriately)
   • Urine Na > 20 mEq/L = renal loss or Na overload
4. BUN/creatinine ratio - ratio > 40 indicates hyperosmolar dehydration
5. Blood glucose - osmotic diuresis from hyperglycemia is a common cause
6. Cortisol level if Cushing's is suspected

TBW (L) = body weight (kg) × correction factor (0.6 for young male; 0.5 for elderly male or young female; 0.45 for elderly female)

Free water deficit (L) = TBW × [(Measured Na - 140) / 140]

• TBW = 0.6 × 70 = 42 L
• Deficit = 42 × [(165-140)/140] = 7.5 L

4. Emergency Treatment - Step by Step

Step 1: Stabilize Hemodynamics First

Step 2: Treat the Underlying Cause

• Vomiting → antiemetics
• Fever → antipyretics
• Central DI → desmopressin (DDAVP)
• Nephrogenic DI → remove offending drug (lithium, amphotericin), low-sodium diet, hydrochlorothiazide
• Hyperglycemia → insulin + appropriate fluids

Step 3: Correct the Free Water Deficit - SLOWLY

Step 4: Monitor

• Urine output closely (risk of acute tubular necrosis)
• Neurological status (watch for signs of cerebral edema - headache, confusion worsening, seizures)
• Repeat serum sodium frequently

5. Special Warnings

• Severity: Serum Na > 150-160 mEq/L carries ~75% mortality if untreated. Na > 160 mEq/L in children requires immediate attention - risk of permanent neurologic damage.
• Do not over-correct: Brain cells adapt to hypernatremia by accumulating organic osmolytes. Rapid correction causes osmotic swelling and cerebral edema - potentially causing central pontine myelinolysis.
• Acute vs chronic matters: If you do not know when it started, assume chronic and correct slowly.
• Acute lethal salt ingestion (< 6h): Rare exception - can correct fast without harm; consider hemodialysis.

Quick Mental Algorithm at the Bedside

Patient with AMS / dehydration / polyuria / high-risk profile
        ↓
Check serum Na → > 145? → Hypernatremia confirmed
        ↓
Assess volume status (exam: skin turgor, JVD, BP, HR, edema)
        ↓
  Hypovolemic?         Normovolemic?         Hypervolemic?
  (dehydrated)         (DI likely)           (Na overloaded)
       ↓                    ↓                     ↓
  Uosm > 800          Uosm < 300             Uosm > 100
  → GI/sweat loss     → Central DI           → Cushing's/
  Uosm > 100          Uosm 300-800           → iatrogenic
  → Renal loss        → Partial DI
        ↓
  Calculate free water deficit
  Step 1: NS if shock
  Step 2: Treat cause
  Step 3: Correct deficit over 48-72h (0.5 mEq/L/h max)

The Basics: Body Fluids, Osmolality, and Tonicity

1. Where Is the Water in Your Body?

Guyton and Hall Textbook of Medical Physiology

• Cell membrane - separates ICF from ECF (highly selective, water moves freely but ions like Na+ mostly cannot)
• Capillary membrane - separates plasma from interstitial fluid (allows most things through except large proteins)

Women average 50% body water (more fat, less muscle). Newborns are 70-75% water.

2. What Is Osmolality?

• Normal serum osmolality = 275-295 mOsm/kg
• > 295 mOsm/kg = hyperosmolar (too many particles in the blood)

• Osmolarity = osmoles per liter of solution (mOsm/L)
• Osmolality = osmoles per kilogram of water (mOsm/kg)
• In clinical practice, these numbers are nearly identical, and the terms are used interchangeably.

3. What Makes Up Serum Osmolality?

Posm (mOsm/L) = 2 × [Na⁺] + Glucose/18 + BUN/2.8

• Sodium (×2) - Na⁺ is the dominant extracellular cation; you multiply by 2 to account for its accompanying anion (Cl⁻, HCO₃⁻). This is the biggest contributor.
• Glucose/18 - glucose is normally a minor contributor, but in diabetic patients with high glucose, it matters a lot
• BUN/2.8 - urea contributes to osmolality but not tonicity (explained below)

• Estimated Posm = (2 × 145) + 90/18 + 14/2.8
• = 290 + 5 + 5 = 300 mOsm/kg - slightly elevated

4. Osmolality vs. Tonicity - The Crucial Difference

• Sodium (Na⁺) - cannot cross freely → affects BOTH osmolality AND tonicity → causes cells to shrink/swell
• Urea (BUN) - crosses cell membranes freely → contributes to osmolality but NOT tonicity → does NOT shift water between compartments
• Glucose - normally impermeable (needs insulin/transporters) → affects both → but with insulin it becomes permeable

• A patient with a very high BUN (renal failure) may have high osmolality but NOT hypertonicity - their cells are fine
• A patient with high Na⁺ has both high osmolality AND high tonicity - water is being pulled OUT of cells, which is what causes all the brain symptoms in hypernatremia

5. Osmosis - Why Water Moves

6. Isotonic, Hypotonic, Hypertonic - What They Do to Cells

Guyton and Hall Textbook of Medical Physiology

7. Now Connect It Back to Hypernatremia

• Confusion, restlessness (mild)
• Lethargy, stupor (moderate)
• Seizures, coma, intracranial hemorrhage (severe/rapid)

Summary in One Mental Map

Body water → 3 compartments (ICF 28L | Interstitial 10.5L | Plasma 3.5L)
         ↓
Osmolality = particle concentration in plasma (normal 275-295 mOsm/kg)
         ↓
Estimated by: 2×Na + Glucose/18 + BUN/2.8
         ↓
Tonicity = only the impermeable particles (mainly Na⁺)
         ↓
High tonicity → water leaves cells → cells shrink
Low tonicity → water enters cells → cells swell
         ↓
Hypernatremia = ↑ Na⁺ = ↑ tonicity = brain cells shrink = neurological symptoms