Define Hypernatraemic Dehydration. Describe its causes, pathophysiology, management and investigations.

Definition

• Hypernatraemic dehydration is dehydration with a serum sodium > 150 mmol/L, resulting when water loss exceeds sodium loss.
• It is dangerous because of the osmotic shifts it produces in the brain.

Causes

• Diarrhoea with high fever / high insensible water loss; inadequate water intake.
• Feeding with high-solute (over-concentrated) formula or giving salt-rich fluids.
• Diabetes insipidus; osmotic diuresis.

Pathophysiology & Clinical Features

• High ECF osmolality draws water out of the brain cells → the ECF is relatively preserved, so skin turgor and circulation are maintained longer than expected (dehydration is under-estimated).
• The skin has a characteristic 'doughy' feel; the child is very irritable with a high-pitched cry, lethargy, hypertonia and, if severe, seizures.
• Brain shrinkage can tear bridging vessels → intracranial haemorrhage.

Management

• Rehydrate slowly over ~48 hours so that serum sodium falls by no more than 0.5 mmol/L per hour (≤ 10–12 mmol/L/day).
• Use isotonic fluids initially for shock; then correct the free-water deficit gradually.
• Monitor sodium and neurological status closely.

Investigations

• Serum sodium > 150 mmol/L confirms it; also check glucose (hyperglycaemia common), calcium, and renal function.
• Assess the underlying cause (feeding history, diabetes insipidus).

Key Principle

Because circulatory signs are misleadingly mild, hypernatraemic dehydration is easily under-estimated — maintain a high index of suspicion in a diarrhoeal child who is very irritable with doughy skin, and correct the sodium slowly over ~48 hours.

Causes — Quick List

• Water loss > sodium loss: high fever, hot climate, tachypnoea.
• Excess sodium intake: over-concentrated feeds, incorrectly mixed ORS/home fluids.
• Reduced water intake: a sick child who refuses feeds; diabetes insipidus.

Reduce sodium by no more than 10–12 mEq/L per day.