Severe Dehydration

A significant deficit of total body water and electrolytes, most commonly from gastrointestinal losses, but also from inadequate intake, excessive sweating, burns, diabetic emergencies, or renal losses. Severe dehydration is a medical emergency given the risk of hypovolemic shock, electrolyte derangement, and end-organ hypoperfusion.

Classified by degree (WHO clinical assessment):

• No dehydration: <5% body weight fluid loss

• Some/moderate dehydration: 5-10% fluid loss

• Severe dehydration: >10% fluid loss

Also classified by tonicity, which is relevant to correction rate: isotonic (proportionate water and sodium loss, most common), hypotonic (sodium loss exceeds water loss; greater risk of shock at a given deficit), hypertonic (water loss exceeds sodium loss ; risk of cerebral injury if corrected too rapidly).

• Severe dehydration: lethargy or reduced consciousness, markedly sunken eyes, inability to drink or drinking poorly, very slow skin pinch return (>2 seconds), dry mucous membranes, absent tears, tachycardia progressing to bradycardia (pre-terminal), weak or absent peripheral pulses, prolonged capillary refill (>3 seconds), hypotension (a late sign ; significant volume is already lost by the time BP drops), cool or mottled extremities, minimal or absent urine output

• Infants: sunken fontanelle, absence of tears when crying, marked irritability or lethargy

• Assess underlying cause: fever and diarrhea (infectious), polyuria and polydipsia preceding (diabetic emergency), burns history, poor oral intake (elderly, disability, neglect)

• Complications: acute kidney injury (pre-renal), hypovolemic shock, electrolyte derangement, seizures (especially with hypernatremic or hyponatremic dehydration), metabolic acidosis

• Primarily clinical assessment of severity: do not delay treatment awaiting lab results in clinically severe cases

• U&E/creatinine: electrolyte derangement, renal function (urea rising disproportionately to creatinine in a pre-renal pattern)

• Glucose: exclude or identify a diabetic emergency as the cause

• Venous blood gas: metabolic acidosis, lactate as a tissue hypoperfusion marker

• FBC: hemoconcentration

• Urinalysis: specific gravity, ketones if relevant

• Identify the underlying cause: stool studies if diarrheal, blood cultures if septic, medication review (diuretics)

Differentials for the underlying presentation: sepsis or septic shock (which frequently coexists with and compounds dehydration rather than being mutually exclusive), diabetic ketoacidosis or HHS, adrenal insufficiency, heat illness.

Severe dehydration is an emergency, obtain IV access immediately, considering intraosseous access if IV cannot be obtained rapidly in a critically unwell child.

IV fluid resuscitation:

• Adults: initial bolus 500mL-1L isotonic crystalloid (Ringer's lactate or 0.9% saline) over 15-30 minutes if shocked, reassess and repeat as needed; then calculate ongoing deficit replacement (weight × %deficit) plus maintenance plus ongoing losses, typically over 24 hours

• Children (WHO severe dehydration protocol, if not shocked): 100mL/kg total: infants <12 months, 30mL/kg over 1 hour then 70mL/kg over 5 hours; older children, 30mL/kg over 30 min then 70mL/kg over 2.5 hours; reassess frequently and switch to oral or NG ORS as soon as able to drink

• If shocked (any age): rapid bolus 10-20mL/kg crystalloid, reassess, repeat as needed, escalate to senior or ICU review if not responding after 40-60mL/kg

Tonicity-specific correction:

• Hypernatremic dehydration (Na >150mmol/L): correct SLOWLY ; rapid correction risks cerebral edema. Aim to reduce serum sodium by no more than 0.5mmol/L/hour (10-12mmol/L per 24 hours), replacing the deficit over 48 rather than 24 hours

• Hyponatremic dehydration: correct cautiously if severe or symptomatic ; risk of osmotic demyelination syndrome with overly rapid correction; general limit <8-10mmol/L rise per 24 hours

Other:

• Potassium replacement once urine output is established and hyperkalemia excluded ; dehydration with acidosis causes a transcellular shift that masks a total body deficit

• Monitor: vital signs, urine output (target ≥0.5mL/kg/hr adults, ≥1mL/kg/hr children), mental status, repeat electrolytes every 4-6 hours during aggressive correction, fluid balance chart

• Treat the underlying cause concurrently

• Transition to oral rehydration as soon as tolerated, continuing to replace ongoing losses

Admission/escalation: any severe dehydration, hemodynamic instability, altered consciousness, significant electrolyte derangement, inability to tolerate oral rehydration, complicating comorbidity, or extremes of age.