What is the appropriate emergency evaluation and management for a 3‑month‑old infant with a pulsating anterior fontanelle and recurrent vomiting without fever?

Emergency Evaluation and Management of a 3-Month-Old with Pulsating Fontanelle and Recurrent Vomiting

This infant requires immediate assessment for meningitis, increased intracranial pressure, and surgical emergencies—particularly if any vomiting is bilious—followed by rapid evaluation for dehydration and systemic infection.

Immediate Red-Flag Assessment

Life-Threatening Causes to Rule Out First

• Bacterial meningitis must be excluded urgently in any infant with a bulging or pulsating fontanelle, especially when accompanied by fever, lethargy, poor perfusion, or toxic appearance 1, 2.
• Increased intracranial pressure from hydrocephalus, intracranial hemorrhage (consider shaken baby syndrome), or mass lesion can present with a bulging fontanelle and vomiting 2, 3, 4.
• Bilious (green) vomiting constitutes a surgical emergency until proven otherwise, as it indicates obstruction distal to the ampulla of Vater and may represent midgut volvulus requiring immediate surgical intervention 2, 5, 3.
• Sepsis can present with vomiting, altered mental status, and a bulging fontanelle due to meningeal inflammation or cerebral edema; look for fever, hypothermia, poor perfusion, cool extremities, and lethargy 1, 2, 6.

Critical Physical Examination Findings

• Document the fontanelle characteristics: A truly bulging fontanelle is tense and elevated above the skull when the infant is upright and calm; a pulsating fontanelle can be normal, but when combined with vomiting warrants concern 4, 7.
• Assess mental status: Lethargy, inability to localize pain, altered consciousness, or inconsolable irritability are red flags for meningitis or increased intracranial pressure 1, 2.
• Evaluate for signs of shock or severe dehydration: Prolonged capillary refill >2 seconds, cool extremities, prolonged skin tenting, dry mucous membranes, and decreased urine output (<4 wet diapers/24 hours) 2, 6.
• Characterize the vomitus: Bilious (green), bloody, or "coffee-ground" appearance versus non-bilious; projectile versus non-projectile 2, 3.
• Examine the abdomen: Distension, absent bowel sounds, tenderness, or a palpable mass suggest obstruction, intussusception, or necrotizing enterocolitis 2, 6, 3.
• Check for neck stiffness (though less reliable in infants) and other signs of meningeal irritation 1.

Diagnostic Algorithm

If Bilious Vomiting Is Present

1. Obtain an abdominal radiograph immediately as the first imaging study to look for obstruction patterns such as dilated bowel loops, "double bubble" (duodenal obstruction), or "triple bubble" (jejunal obstruction) 1, 2, 5.
2. Proceed urgently to upper GI contrast series if clinical suspicion for malrotation/volvulus remains high, even if the plain radiograph appears normal, because plain films miss up to 7% of malrotation cases 1, 2, 5.
3. Place the infant nil per os (NPO) and insert a nasogastric tube for gastric decompression 6, 3.
4. Obtain immediate surgical consultation because midgut volvulus can cause intestinal necrosis within hours 2, 5.

If Non-Bilious Vomiting with Bulging/Pulsating Fontanelle

1. Assess for toxic appearance, fever, or altered mental status: If present, this infant requires lumbar puncture to rule out bacterial meningitis 1, 2.
   • In febrile infants ≤90 days with concerning signs (lethargy, poor perfusion, full fontanelle), cerebrospinal fluid analysis is mandatory 1.
   • One study described a 2.5-month-old with bacterial meningitis who presented as "ill appearing, lethargic, with mottled skin, poor perfusion, and full anterior fontanelle" 1.
2. Consider head CT or ultrasound (if fontanelle is open) before lumbar puncture if there are focal neurologic signs, papilledema, or concern for mass effect or hydrocephalus 1, 2, 4.
3. Obtain blood cultures, complete blood count, and electrolytes to evaluate for sepsis and metabolic derangements 1, 2, 3.

If Well-Appearing Infant Without Fever

• Recent case reports describe infants with COVID-19 presenting with fever, bulging fontanelle, and benign course without meningitis; consider SARS-CoV-2 testing in well-appearing infants with isolated bulging fontanelle and fever 8, 7.
• Benign intracranial hypertension (pseudotumor cerebri) can mimic meningitis with bulging fontanelle and fever but has normal CSF cell counts and elevated opening pressure; this is a diagnosis of exclusion 9.
• Severe dehydration from gastroenteritis can cause a sunken fontanelle, but paradoxically, some infants may have a tense fontanelle from crying or increased venous pressure 4.

Management Priorities

Immediate Interventions

• Establish IV access and initiate fluid resuscitation if severe dehydration is present (≥10% deficit with prolonged skin tenting >2 seconds, cool extremities, lethargy) 2.
• Administer empiric antibiotics immediately (before lumbar puncture if necessary) if bacterial meningitis or sepsis is suspected; do not delay antibiotics for imaging 1, 2.
• NPO and nasogastric decompression if bilious vomiting or bowel obstruction is suspected 6, 3.

Hydration Management

• For mild-to-moderate dehydration without red flags, initiate oral rehydration therapy with small frequent volumes (approximately 5 mL every minute) 2, 6.
• Monitor urine output (goal ≥4 wet diapers/24 hours), capillary refill, and mucous membrane moisture 6.

Antiemetic Use

• Ondansetron (0.2 mg/kg oral or 0.15 mg/kg IV, maximum 4 mg) may be used for persistent vomiting preventing oral intake, but should not replace proper fluid and electrolyte management 2, 3.
• Antiemetics are generally contraindicated in preterm infants because they may mask clinical deterioration 6.

Common Pitfalls and Caveats

• Never dismiss bilious vomiting as "just gastroenteritis"; it requires urgent surgical evaluation regardless of how well the infant appears 2, 5, 3.
• A pulsating fontanelle alone can be normal, but when combined with vomiting, fever, or altered mental status, it warrants thorough evaluation for increased intracranial pressure or meningitis 4, 7.
• Normal abdominal radiographs do not exclude malrotation or volvulus; proceed to upper GI series if clinical suspicion remains high 2, 5.
• Do not rely on antidiarrheal agents (loperamide, kaolin-pectin) in infants, as they are ineffective and can cause severe complications including ileus and death 2.
• Infants have higher risk of rapid dehydration due to larger body-surface-to-weight ratio and higher metabolic rate; reassess hydration status frequently 2.
• Consider non-gastrointestinal causes: Urinary tract infection, pneumonia, otitis media, metabolic disorders, and toxic ingestions can all present with vomiting 2, 3.