---

Hypernatremia - Viva Questions & Answers

---

🔵 BASICS

Simple memory trick: "Too much salt, too little water"

---

1. Hypernatremia is always a water problem, and sometimes also a salt problem.
2. For hypernatremia to persist, the patient must either have a defect in thirst or limited access to free water.
3. The serum Na+ level itself does NOT tell you anything about total body sodium or volume status.

---

• Elderly (impaired thirst sensation)
• Infants (cannot access water themselves)
• Comatose or intubated patients (no access to water)
• Patients with dementia or neurological disease
• Patients in the ICU (6-26% develop hypernatremia in the ICU)

---

🔵 CAUSES / CLASSIFICATION

---

• Renal causes: Diuretics, osmotic diuresis (glucose, mannitol), post-obstructive diuresis
• Extrarenal causes: GI losses (diarrhea, vomiting, NG tube drainage), skin losses (burns, excessive sweating in marathon runners)

Key fact: Sweat Na+ is ~40 mEq/L - always lower than serum, so sweat loss causes hypotonic fluid loss → hypernatremia

---

• Diabetes insipidus (DI) - most important cause
• Inadequate water intake (impaired thirst / no access)
• Insensible losses (fever, respiratory)

---

• Hypertonic saline IV
• IV sodium bicarbonate
• Salt tablet ingestion / seawater ingestion
• Hypertonic saline enemas
• Hypertonic tube feeds
• Primary hyperaldosteronism / Conn syndrome
• Cushing syndrome
• Hemodialysis errors

---

🔵 DIABETES INSIPIDUS (DI)

---

---

• If urine concentrates after DDAVP → Central DI (ADH deficient, responds to replacement)
• If urine does NOT concentrate after DDAVP → Nephrogenic DI (resistant to ADH)

---

🔵 PATHOPHYSIOLOGY

• High serum osmolality causes water to shift OUT of brain cells → brain shrinkage (cerebral dehydration)
• This can cause rupture of cerebral veins and subarachnoid hemorrhage
• In chronic hypernatremia, brain cells adapt by accumulating idiogenic osmoles (organic solutes) to pull water back in - this is why rapid correction is dangerous (causes cerebral edema)

---

• Acute (<24 hours): Brain has not adapted; severe neurological damage occurs rapidly. Na+ >160 mEq/L → 75% mortality in adults
• Chronic (>48-72 hours): Brain adapts by generating idiogenic osmoles; symptoms may be mild. Mortality ~10%
• This distinction drives how quickly we correct it

---

🔵 CLINICAL FEATURES

• Tachycardia
• Orthostatic hypotension
• Dry mucous membranes and axillae

• Thirst (earliest symptom)
• Muscle weakness, restlessness, lethargy, insomnia
• Depressed sensorium → delirium → coma
• Focal neurological deficits
• Seizures
• Intracranial hemorrhage (subarachnoid hemorrhage)

Symptoms are more severe when hypernatremia develops rapidly.

• Inhibits insulin release → hyperglycemia
• Increases insulin resistance
• Decreases hepatic gluconeogenesis and lactate clearance
• Decreases cardiac function

---

---

🔵 INVESTIGATION

• Serum Na+, K+, creatinine, BUN, glucose
• Serum osmolality
• Urine osmolality (key!)
• Urine sodium concentration
• Urine specific gravity

---

Free Water Deficit (L) = TBW × [(Serum Na+ / 140) - 1]

• TBW = 70 × 0.6 = 42 L
• Deficit = 42 × [(160/140) - 1] = 42 × 0.143 = 6 L

Important: This formula gives a snapshot only. It does NOT account for ongoing losses (urine, sweat, GI). Always recheck Na+ frequently and factor in ongoing losses.

---

🔵 MANAGEMENT

1. Correct underlying shock/hypovolemia - give isotonic (0.9%) saline FIRST if hemodynamically unstable
2. Treat the underlying cause (e.g., fever, DI, vomiting)
3. Carefully lower serum Na+ by replacing the free water deficit

---

Why? Rapid correction → brain cells (which have accumulated idiogenic osmoles) now face a hypo-osmolar environment → cerebral edema → seizures → permanent neurological damage → death

---

Oral water is ALWAYS preferred when feasible - it is the most physiological route.

---

• Hypovolemic hypernatremia: NS first to restore volume, then switch to hypotonic fluids for water deficit
• Central DI: DDAVP (desmopressin) 1-2 mcg IV/SC - acts as ADH analog
• Nephrogenic DI: Low Na+ diet + thiazide diuretics (induce mild volume depletion → enhanced proximal tubule reabsorption → reduced urinary free water loss)
• Hypervolemic hypernatremia (salt loading): Stop the sodium source; give D5W; loop diuretics if needed

---

🔵 COMPLICATIONS & PROGNOSIS

• Neurological: Seizures, coma, subarachnoid hemorrhage, permanent brain damage
• Metabolic: Hyperglycemia (inhibits insulin), impaired lactate clearance
• Cardiac dysfunction
• Increased mortality: Hospital-acquired hypernatremia with Na+ >150 mEq/L carries a severity-adjusted relative risk of death of 2.6x
• Increased length of hospital stay

---

• Fewer than 50% of patients with hospital-acquired hypernatremia receive free water replacement within 24 hours
• Patients whose hypernatremia is corrected within 72 hours have lower mortality than those corrected later
• It should NEVER be treated as incidental or negligible

---

🔵 HIGH-YIELD VIVA POINTS SUMMARY

---