Conditions · Nephrology and urology
Dehydration (fluid volume deficit) !
Dehydration is a loss of body water and salts larger than what is taken in. It is common with diarrhoea and fever, and severe cases can cause shock.
Updated 2026-11-01
Structures involved
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In embryology
In the clinical exam
Why it happens
- Shock: types, stages and the organs it damages
- Dehydration and fluid overload
- Body fluid compartments
- ADH, thirst and water balance
- The renin–angiotensin–aldosterone system
- IV fluids: tonicity and where they go
What it is
A deficit of body water, usually with loss of sodium and other electrolytes, that reduces the volume of blood and tissue fluid. In nursing terms it is a fluid volume deficit; when the extracellular volume is depleted it is also called hypovolaemia.
Who gets it
Dehydration from acute diarrhoeal disease remains a leading cause of illness and death in children under five worldwide; in Colombia, «enfermedad diarreica aguda» is a public health priority, especially in rural and dispersed areas. Older adults are another high-risk group because of reduced thirst, illness and medicines.
Causes
- Gastrointestinal losses: diarrhoea and vomiting (the commonest cause, especially in children)
- Increased insensible losses: fever, heat exposure, heavy sweating, burns
- Renal losses: diuretics, osmotic diuresis in uncontrolled diabetes, diabetes insipidus, adrenal insufficiency
- Poor intake: inability to drink, dysphagia, confusion, lack of access to water
- Third-space losses: pancreatitis, bowel obstruction, plasma leak in severe dengue
Risk factors
- Infants and young children (higher body water turnover, depend on others to drink)
- Older adults, with reduced thirst, cognitive impairment or diuretic use
- Acute diarrhoeal disease, especially without access to safe water
- Uncontrolled diabetes
- Hot climates and intense physical work or exercise
- Malnutrition
Pathophysiology
Losing water and sodium shrinks the extracellular and intravascular volume, so venous return, cardiac output and kidney perfusion fall. The body responds with thirst, sympathetic activation (tachycardia, vasoconstriction), renin–angiotensin–aldosterone and ADH release to retain sodium and water, which concentrates the urine and lowers its volume. If losses continue, compensation fails, leading to hypotension, prerenal kidney injury and hypovolaemic shock. The type of fluid lost determines the sodium: isotonic, hypernatraemic or hyponatraemic dehydration.
Classification
- By severity (WHO for children): no dehydration, some dehydration, severe dehydration
- By percentage of weight lost in children: mild (under about 5%), moderate (about 5–10%), severe (over about 10%)
- By serum sodium: isotonic, hypertonic (hypernatraemic) and hypotonic (hyponatraemic)
Symptoms and signs
- Thirst, dry mouth and dry mucous membranes
- Reduced urine output and dark urine; fewer wet nappies in infants
- Fatigue, headache, dizziness on standing
- Tachycardia and orthostatic hypotension
- Reduced skin turgor (slow skin pinch recoil), sunken eyes, no tears; sunken fontanelle in infants
- Prolonged capillary refill and cool extremities as it worsens
Diagnosis
- Clinical
- Mainly clinical: history of losses and intake, weight compared with a recent weight, vital signs including orthostatic changes, mental status, mucous membranes, eyes, skin turgor and capillary refill; WHO signs classify severity in children.
- Tests
- Not needed in mild cases. In moderate or severe cases: serum sodium, potassium, chloride and bicarbonate, glucose, urea and creatinine (BUN-to-creatinine ratio over about 20 suggests a prerenal state), and urine specific gravity or osmolality (concentrated urine, low urinary sodium).
- Imaging
- Not routine. Point-of-care ultrasound of the inferior vena cava (small, collapsing) can help assess volume status in critical care.
Differential diagnosis
- Sepsis or septic shock
- Diabetic ketoacidosis or hyperosmolar hyperglycaemic state
- Adrenal crisis
- Haemorrhage (blood loss rather than fluid loss)
- Heat stroke
Goal of treatment
Restore circulating volume and correct water and electrolyte deficits safely, replace ongoing losses and treat the cause.
Treatment
- Conservative
- Oral rehydration solution (ORS) is first-line for mild to moderate dehydration, with continued feeding and breastfeeding; zinc supplementation in children with diarrhoea.
- Acute
- Intravenous isotonic crystalloids (normal saline or a balanced solution such as Ringer's lactate) for severe dehydration, shock, altered consciousness or failure of oral rehydration.
- Medical
- Correction of sodium and potassium disorders at a controlled rate, and treatment of the underlying cause (infection, hyperglycaemia, stopping diuretics).
- Follow-up
- Nursing care: daily weight, strict intake and output, urine output and specific gravity, vital signs with orthostatic checks, mental status, skin and mucous membranes, and fall prevention when dizzy.
Complications
- Hypovolaemic shock
- Acute kidney injury (prerenal)
- Hyponatraemia or hypernatraemia, and seizures or cerebral oedema from correcting sodium too fast
- Hypokalaemia and metabolic acidosis (diarrhoea) or alkalosis (vomiting)
- Falls, delirium and thrombosis in older adults
- Heat injury and rhabdomyolysis with exertion
Prognosis
With timely oral or intravenous rehydration, recovery is usually complete. Untreated severe dehydration can cause shock, kidney failure, brain injury and death, especially in infants and frail older adults.
Prevention
- Early use of oral rehydration solution at the first episodes of diarrhoea
- Safe drinking water, sanitation, hand washing and rotavirus vaccination
- Extra fluids in hot weather, during exercise and during fever, with closer attention to older adults
Test yourself
Think of the answer, then tap the question to see it.
What is the first-line treatment for mild to moderate dehydration from diarrhoea?
Oral rehydration solution with continued feeding (plus zinc in children).
Which WHO signs define severe dehydration in a child?
Two or more of: lethargy or unconsciousness, sunken eyes, inability to drink or drinking poorly, and a skin pinch that goes back very slowly.
What is the most reliable bedside measure to track fluid balance in a dehydrated patient?
Daily weight on the same scale at the same time, together with intake and output.
Which BUN-to-creatinine ratio suggests a prerenal state from dehydration?
A ratio above about 20:1, with concentrated urine and low urinary sodium.
Which fluid is given intravenously for severe dehydration with shock?
An isotonic crystalloid such as normal saline or Ringer's lactate, given as a bolus and reassessed.
Related conditions
- Acute (viral) gastroenteritis
- Hypovolaemic shock
- Acute kidney injury
- Hypernatraemia
- Hyponatraemia
- Hypokalaemia
- Metabolic alkalosis
- Diabetic ketoacidosis
- Dengue
- Sepsis and septic shock
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Dehydration: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2024); Diarrhoeal disease (World Health Organization (WHO), 2024).
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