Vomiting, Diarrhea, and Dehydration in Infants and Children

1. THE 2-MINUTE PHYSIOLOGY (Rapid Pathophysiology)

• Fluid Deficit Mechanics: Dehydration in pediatric patients develops from excess fluid loss (e.g., vomiting, diarrhea, evaporative losses from fever or tachypnea), insufficient fluid intake, or a combination of both.
• The Hemodynamic Cascade: As intravascular volume depletes, pediatric patients heavily rely on compensatory mechanisms—specifically, a profound increase in chronotropy (tachycardia) and peripheral vasoconstriction (manifesting as delayed capillary refill, cool extremities, and mottled skin) to maintain cardiac output.
• Acid-Base Derangements: Severe dehydration and poor tissue perfusion lead to anaerobic metabolism and lactic acidosis. Furthermore, excessive bicarbonate loss via diarrhea can worsen the metabolic acidosis, which clinically manifests as compensatory hyperpnea or Kussmaul respirations.
• Hypotension is Late: Unlike adults, young children and infants can maintain a normal blood pressure despite severe volume loss. Hypotension indicates an impending cardiovascular collapse and failure of compensatory mechanisms.

2. THE BEDSIDE ACTION PLAN (Rapid ER Management)

• Immediate Stabilization: Assess airway, breathing, and circulation. Focus on rapid identification of shock (altered mental status, tachycardia, delayed capillary refill, oliguria). Check a point-of-care blood glucose immediately.
• Mild to Moderate Dehydration: Oral Rehydration Therapy (ORT) is the primary intervention. Do not start IV fluids without first attempting a trial of PO fluids and antiemetics. If oral access is difficult but the gut works, consider administering Oral Rehydration Solution (ORS) via a nasogastric (NG) tube at 50 mL/kg over 3 hours.
• Severe Dehydration & Shock: Administer a rapid IV infusion of isotonic crystalloid (Normal Saline or Ringer's Lactate) as a 20 mL/kg bolus. Repeat boluses as needed based on clinical response and reassessment of perfusion.
• Hypoglycemia Management: Infants and children who are symptomatically hypoglycemic require IV dextrose. Administer 0.5–1 g/kg/dose. This equates to 5–10 mL/kg/dose of D10W, or 2–4 mL/kg/dose of D25W (max 25 g/dose).
• Contraindicated Therapies: Antidiarrheal treatment should be strictly avoided in pediatric patients as the risks drastically outweigh the benefits.

3. THE DIAGNOSTIC GRID (Differential Diagnosis & Workup)

• Top "Can't-Miss" Mimics:

• Malrotation with Volvulus: Must be suspected in any neonate or infant presenting with bilious (green/yellow) emesis.

• Intussusception: Consider in children 6 to 36 months old with sudden, intermittent, severe, crampy abdominal pain or unexplained altered mental status/sepsis.

• Necrotizing Enterocolitis (NEC): Consider in neonates presenting with feeding intolerance, abdominal distension, and bloody/bilious emesis.

• Diabetic Ketoacidosis (DKA): Can masquerade as simple vomiting/dehydration; always check glucose and evaluate for Kussmaul respirations.

• Hemolytic Uremic Syndrome (HUS) / Toxic Megacolon / Appendicitis: Lethal etiologies that can present primarily with vomiting and diarrhea.

• Prioritized Diagnostic Workup:

• Point-of-Care Testing: Blood glucose is mandatory for all lethargic or vomiting children.

• Laboratory Panels: In patients with severe dehydration or those requiring IV fluids, obtain a basic metabolic panel (electrolytes, BUN, creatinine) to assess for acute kidney injury and electrolyte derangements (e.g., hyper/hyponatremia), and a blood gas to evaluate acid-base status.

• Stool Studies: Routine testing is discouraged. Consider stool studies only if there are specific risk factors for non-benign illness, dysentery (blood/mucus), or persistent toxic appearance.

4. THE VISUAL BOARD (ECG / POCUS / Imaging)

• Point-of-Care Ultrasound (POCUS): The definitive diagnostic modality for intussusception. Look for the classic "target" sign or "doughnut" sign in the right quadrants. POCUS may also reveal a "cork-screw" sign or "whirlpool" sign suggestive of malrotation with volvulus.
• Abdominal X-Ray (AXR): In the neonate with feeding intolerance and bloody stools, look for pneumatosis intestinalis, pneumoperitoneum, or hepatobiliary gas, which confirm Necrotizing Enterocolitis (NEC).
• Upper GI Series with Oral Contrast: The gold-standard imaging study to diagnose malrotation with volvulus.

5. THE SCORING MATRIX (Risk Stratification & Guidelines)

• Clinical Predictors of Significant Dehydration: The most useful physical exam predictors of clinically significant dehydration (>5% loss of body weight) in children are: abnormal capillary refill, abnormal skin turgor, absent tears, and abnormal respiratory pattern.
• The Clinical Dehydration Scale: A validated tool that is fairly reliable to assess the severity of dehydration in children aged 1 to 36 months presenting with gastroenteritis.
• Severe Dehydration/Shock Criteria: Defined clinically by the presence of shock markers: profound changes in mental status, severe tachycardia, delayed capillary refill (>2 seconds), and oliguria.

6. THE DANGER ZONE (Pitfalls & Critical Actions)

• The Hypotension Trap: Pitfall: Waiting for the blood pressure to drop before diagnosing shock. Critical Action: A normal, age-appropriate blood pressure is NOT reassuring. Hypotension is a late, catastrophic finding of shock in children. Rely on heart rate, mental status, and capillary refill.
• The IV Fluid Reflex: Pitfall: Immediately placing an IV for fluids in a mildly dehydrated, crying child. Critical Action: Do not start IV fluids in children without first attempting a dedicated trial of PO fluids alongside an antiemetic.
• The Bilious Emesis Dismissal: Pitfall: Assuming yellow/green vomit is just severe gastroenteritis. Critical Action: Any infant with bilious emesis has malrotation with volvulus until definitively proven otherwise by an upper GI series.
• The Antidiarrheal Error: Critical Action: Never prescribe antidiarrheal medications (e.g., loperamide) for pediatric acute gastroenteritis due to severe risks including paralytic ileus and toxic megacolon.

7. MCQ MASTERCLASS (Written Exam Tips)

• Buzzword: “Sausage-shaped abdominal mass,” “Currant jelly colored bloody stool,” and “Drawing legs up to the abdomen.”
• Diagnosis: Intussusception.
• Distractor Warning: Up to 20% of intussusception patients will NOT have these classic abdominal signs and will present solely with severe, unexplained altered mental status or sepsis. Do not let the absence of "currant jelly stool" trick you into ruling it out.
• Buzzword: “Neonate,” “Feeding intolerance,” and “Bilious vomiting.”
• Diagnosis: Malrotation with volvulus.
• Buzzword: “Unconscious infant with doughy skin and dry mucous membranes.”
• Diagnosis/Complication: Hypernatremic dehydration. This specific presentation carries a high risk of lethal cerebral edema and seizures during rehydration.
• Distractor: A question describes an infant with a 3-day history of vomiting/diarrhea and a heart rate of 180 bpm that fluctuates with crying. One of the answer choices is Supraventricular Tachycardia (SVT). Reality: Fluctuation with crying (beat-to-beat variability) defines Sinus Tachycardia secondary to dehydration, not SVT.

8. THE BOARDROOM SCRIPT (OSCE & Oral Board Tips)

• The Opening Salvo: "This is a pediatric patient presenting with acute vomiting and diarrhea. My immediate priorities are to assess the airway, evaluate for signs of compensated or uncompensated shock, and obtain a point-of-care blood glucose."
• The Dehydration Assessment: "I am evaluating the severity of dehydration based on objective clinical features: capillary refill, skin turgor, presence of tears, and respiratory pattern. Because the child is mildly dehydrated, I will initiate an oral rehydration trial and administer an antiemetic rather than immediately placing an IV."
• Ruling Out the 'Must Not Miss': "Given the history of emesis, I am specifically asking the parents about the color of the vomit to rule out bilious emesis, which would indicate malrotation with volvulus. I am also assessing for episodic irritability, lethargy, or bloody stools that would prompt me to order an abdominal ultrasound to evaluate for intussusception."