Key Facts

Dehydration is a common and potentially life-threatening complication of gastroenteritis, febrile illness, and other conditions causing fluid loss in children. Accurate clinical assessment of dehydration severity guides appropriate rehydration strategy and prevents complications.

Epidemiology

Dehydration requiring medical attention occurs in approximately 5-10% of children with gastroenteritis. It is the leading cause of childhood morbidity and mortality from diarrhoeal illness worldwide (~525,000 deaths per year in under-5s globally). In the UK, deaths from dehydration are rare but near-misses occur, particularly in neonates and young infants.

Aetiology

Causes of dehydration in children:

• Increased losses: Gastroenteritis (most common), DKA, burns, polyuria, fever
• Reduced intake: Oral ulcers, pharyngitis, vomiting, reduced consciousness, feeding difficulties
• Third-space losses: Sepsis, bowel obstruction, pancreatitis

Types of dehydration by serum sodium:

• Isonatraemic (Na 135-145 mmol/L): Most common (~80%)
• Hyponatraemic (Na <135 mmol/L): Fluid loss replaced with water/hypotonic fluids
• Hypernatraemic (Na >150 mmol/L): Commonly associated with breast-feeding difficulties, high insensible losses; requires slow rehydration

Pathophysiology

Fluid loss leads to reduction in extracellular fluid volume → decreased circulating volume → compensatory tachycardia → decreased tissue perfusion → metabolic acidosis → organ dysfunction. In isonatraemic dehydration, fluid is lost proportionally from intracellular and extracellular compartments. In hypernatraemic dehydration, water moves intracellularly → preserved intravascular volume → clinical signs underestimate severity. In hyponatraemic dehydration, water moves extracellularly → more rapid cardiovascular compromise.

NICE CG84 Assessment Framework

No Clinical Dehydration (<5%):

• Alert, responsive
• Normal skin colour
• Warm extremities
• Normal eyes
• Moist mucous membranes
• Normal heart rate, CRT, breathing
• Normal urine output

Clinical Dehydration (5-10%):

• Altered responsiveness (irritable, lethargic)
• Reduced skin turgor
• Sunken eyes ± sunken fontanelle (infants)
• Dry mucous membranes
• Tachycardia
• Tachypnoea
• Reduced urine output

Clinical Shock (>10%):

• Decreased level of consciousness
• Pale, mottled, cold skin
• Prolonged CRT (>3 seconds)
• Tachycardia, weak pulses
• Hypotension (late sign)
• Anuria

Red Flags

• Altered consciousness — immediate IV access and fluid resuscitation
• CRT >3 seconds or hypotension — circulatory shock
• Sunken fontanelle in infant with tachycardia — significant dehydration
• Serum Na >150 mmol/L — hypernatraemic dehydration; requires careful slow rehydration
• High stool output (>8 stools/day) or persistent vomiting — may not tolerate ORS

Bedside

• Weight: Compare to recent known weight — most accurate dehydration measure
• Clinical assessment: Use NICE CG84 framework — responsiveness, skin turgor, eyes, mucous membranes, CRT, HR, urine output
• Capillary blood gas: pH, bicarbonate, lactate — metabolic acidosis in moderate-severe dehydration
• Blood glucose: Hypoglycaemia risk in young/malnourished children
• Urine output: Wet nappies; urine dipstick (specific gravity >1.020 suggests dehydration)

Bloods

• U&Es: Sodium (assess dehydration type), potassium, urea, creatinine
• Serum bicarbonate: Low in metabolic acidosis
• FBC: Haemoconcentration (raised haematocrit)
• Blood gas (venous): Assess acid-base status, lactate

Imaging

• Not routinely required
• Abdominal USS: If pyloric stenosis suspected

Special Tests

• Paired urine and serum osmolality: If diabetes insipidus suspected
• Short synacthen test: If adrenal crisis suspected

Non-pharmacological

• No clinical dehydration: Continue normal fluids and feeds; encourage extra fluid intake; ORS supplementation after each loose stool (NICE CG84)
• Clinical dehydration: ORS 50ml/kg over 4 hours, given frequently in small volumes (5ml every 1-2 minutes initially); continue breastfeeding; reassess after 4 hours
• If ORS not tolerated: NG tube for ORS delivery before resorting to IV fluids

Pharmacological

• Clinical shock: 0.9% NaCl 20ml/kg IV bolus over <10 minutes; reassess; repeat if needed (max 40-60ml/kg before senior review/inotropes)
• IV rehydration (if ORS fails): Replace deficit + maintenance over 24 hours using 0.9% NaCl with 5% glucose
  • Deficit (ml) = % dehydration × body weight (kg) × 10
  • Maintenance: 100ml/kg for first 10kg + 50ml/kg for next 10kg + 20ml/kg thereafter (Holliday-Segar)
• Hypernatraemic dehydration (Na >150): Replace deficit slowly over 48 hours — aim Na drop ≤0.5 mmol/L/hour to avoid cerebral oedema
• Potassium replacement: Add 20-40 mmol/L KCl to IV fluids once urine output established
• Ondansetron: 0.15mg/kg single dose if vomiting prevents ORS tolerance

Surgical/Interventional

• Intraosseous access if IV access unobtainable in shocked child

Referral Criteria

• Clinical shock — immediate emergency department
• Clinical dehydration not responding to ORS — hospital admission for IV fluids
• Hypernatraemic dehydration — hospital admission; senior paediatric input
• Age <3 months — lower threshold for hospital assessment

• Mild-moderate dehydration: Full recovery within 24-48 hours with appropriate oral rehydration
• Severe dehydration/shock: Excellent prognosis if recognised and treated promptly; mortality in UK is very low
• Hypernatraemic dehydration: Risk of cerebral oedema with rapid correction — careful management essential; neurological sequelae in ~5% of severe cases
• Hyponatraemic dehydration: Rapid correction can cause central pontine myelinolysis (rare in children)
• Global: Dehydration from diarrhoea remains a leading cause of child mortality worldwide (mainly low-income settings)

Fluid Calculation Summary

NICE CG84 Dehydration Management Pathway

Red Flag Electrolyte Values