Most dehydration in children is mild and correctable at home, and most parents manage it intuitively by offering plenty to drink when a child is unwell. But dehydration can progress rapidly in small children, particularly infants, and the signs that it has become clinically significant are specific enough that they are worth knowing in advance rather than trying to Google while managing a sick child.
The two things that make the most difference in practice are knowing which signs are serious and using the right fluid – oral rehydration solution rather than water or fruit juice.
Healthbooq (healthbooq.com/apps/healthbooq-kids) covers acute childhood illness and home management.
Why Children Dehydrate More Quickly Than Adults
Children have a higher ratio of body surface area to volume than adults, which means they lose proportionally more fluid through the skin with fever. Infants have higher baseline fluid requirements per kilogram of body weight. Their kidney function is less efficient at concentrating urine in response to dehydration. And young children cannot communicate thirst effectively or advocate for themselves.
Gastroenteritis is the most common cause of significant dehydration in children: rotavirus (before vaccine introduction, the most common cause of severe gastroenteritis in children; the UK introduced rotavirus vaccine in 2013), norovirus, and bacterial gastroenteritis cause losses through both vomiting and diarrhoea simultaneously. Fever increases insensible fluid losses. Poor oral intake due to any illness compounds these losses.
Clinical Signs of Dehydration
NICE CG84 (Diarrhoea and vomiting caused by gastroenteritis in under-5s) provides a useful clinical framework.
No dehydration signs: Normal behaviour, eyes, and tongue. Tears present when crying. Normal urine output. Skin returns to normal position quickly when briefly pinched (normal turgor). Normal fontanelle if visible.
Clinical dehydration (5-10% fluid deficit): Decreased activity, altered behaviour. Eyes sunken, with or without tears. Dry or sticky mouth. Reduced urine output. Mildly reduced skin turgor.
Shock (severe dehydration, >10% deficit): Decreased consciousness. Sunken eyes, no tears. Cold, pale, mottled extremities. Very fast heart rate. Weak pulse. Prolonged capillary refill time (press a fingernail for 5 seconds; colour should return within 2 seconds). Very reduced or no urine output. This is a medical emergency: call 999 or go to A&E immediately.
Urine output is one of the most reliable home indicators: a child who has had a wet nappy or gone to the toilet within 6 hours is unlikely to be severely dehydrated. No wet nappy for 8 or more hours in an infant, or no urine for 8 hours in an older child, warrants urgent assessment.
What to Give
Oral rehydration solution (ORS) is the correct fluid for a dehydrated child. It contains glucose and electrolytes (sodium, potassium) in concentrations that enable active glucose-coupled sodium absorption in the gut, maximising fluid absorption. Plain water is not equivalent – it dilutes blood sodium without providing the electrolytes needed for recovery. Sports drinks have the wrong glucose and sodium concentrations for rehydration. Fruit juice is high in fructose (poorly absorbed and worsens diarrhoea) and low in sodium. ORS sachets (Dioralyte, Electrolade) are available from pharmacies without prescription.
For an infant who is breastfed, breastfeeding should continue alongside ORS. Formula-fed infants can continue formula, using ORS as an additional fluid rather than a replacement. Regular food should be offered as soon as the child is willing to eat; there is no benefit to withholding food during gastroenteritis (NICE CG84).
NICE recommends against antidiarrhoeal medicines such as loperamide in children under 12. The diarrhoea is clearing the infection; slowing it pharmacologically without addressing the infection is not helpful.
When to Seek Help
Any infant under 3 months with fever and reduced feeding should be assessed urgently regardless of other signs. Go to A&E or call 999 for any child showing the signs of shock listed above. Seek urgent same-day assessment for: a child who cannot keep any fluid down for 8 hours; a child showing signs of clinical dehydration after home management; a child with bloody diarrhoea; a child under 6 months who is becoming dehydrated; a child with a known condition such as diabetes, heart disease, or renal disease.
Heat-Related Dehydration
In hot weather or during vigorous physical activity, children lose significant fluid through sweating. Young children do not self-regulate body temperature as efficiently as adults and may not recognise thirst reliably. Reminders and scheduled water breaks during outdoor activity in summer, cool clothing, shade, and access to water prevent heat-related dehydration in children who are otherwise healthy.
Key Takeaways
Dehydration in children occurs when fluid losses exceed intake, most commonly from gastroenteritis (vomiting and diarrhoea), fever, or reduced drinking during illness. Mild-to-moderate dehydration is managed at home with oral rehydration solution (ORS). Severe dehydration is a medical emergency requiring intravenous fluids. NICE CG84 defines clinical signs and provides an assessment framework for children with gastroenteritis. The most dangerous signs in children are: sunken fontanelle, no tears when crying, eyes sunken, dry mouth, reduced skin turgor, and significantly reduced or absent urine output. Giving only plain water or sports drinks to a dehydrated child worsens electrolyte imbalance – oral rehydration solution contains the correct balance of glucose and electrolytes.
