Differential Diagnosis for Fever and Emesis in an Infant
The differential diagnosis for fever and vomiting in an infant must be approached systematically, prioritizing life-threatening conditions first: serious bacterial infections (meningitis, sepsis, urinary tract infection) in young infants, followed by gastrointestinal obstruction (particularly malrotation with volvulus if bilious emesis), and then common benign causes like viral gastroenteritis.
Age-Dependent Risk Stratification
The infant's age fundamentally changes your diagnostic approach and urgency:
Neonates (0-28 days)
- Serious bacterial infection (SBI) risk: ~10% with mortality around 10% if untreated 1
- Must-exclude diagnoses:
- Bacterial meningitis
- Sepsis/bacteremia
- Urinary tract infection
- Congenital GI abnormalities (if presenting in first week): malrotation with volvulus, intestinal atresia, annular pancreas, Hirschsprung disease, meconium ileus 2
Young Infants (29-60 days)
- SBI risk: ~5% 1
- Same infectious concerns as neonates but slightly lower risk
- Hypertrophic pyloric stenosis becomes more common (typically 3-12 weeks)
Older Infants (>60 days)
- SBI risk: 0.5-1% 1
- Broader differential with less immediate life-threat
Primary Diagnostic Categories
1. Infectious Etiologies (Most Common Overall)
Serious Bacterial Infections:
- Meningitis
- Bacteremia/sepsis
- Urinary tract infection
- Pneumonia
- Otitis media
- Enteritis/gastroenteritis (bacterial)
Viral Infections:
- Viral gastroenteritis (most common benign cause) - presents with sudden onset, mild fever, diarrhea, short duration, often epidemic pattern 2
- Systemic viral illness
2. Gastrointestinal Obstruction (Critical to Identify)
Character of emesis guides urgency:
Bilious vomiting = surgical emergency until proven otherwise 2
- Malrotation with volvulus (can present at ANY age, not just neonates)
- Intestinal atresia/stenosis
- Annular pancreas
Nonbilious forceful vomiting:
- Hypertrophic pyloric stenosis (classic "olive" mass, typically 3-12 weeks old)
- Pylorospasm
- Gastric volvulus
Less common GI causes:
- Intussusception (unusual <3 months; presents with crampy pain, bloody stools, lethargy)
- Neonatal appendicitis
- Foreign body/lactobezoar
3. Neurologic Causes
Look for specific warning signs:
- Raised intracranial pressure: enlarging head circumference, bulging fontanelle, neurologic deficits 2
- Causes: tumor, trauma, hydrocephalus, meningitis
4. Metabolic Disorders
Consider especially if:
- Recurrent episodes
- Poor feeding/failure to thrive
- Altered mental status
- Family history
Specific disorders: 2
- Phenylketonuria
- Hyperammonemia
- Maple syrup urine disease
- Galactosemia
- Adrenocortical hyperplasia
- Methylmalonic acidemia
- Kernicterus
5. Benign Causes
- Gastroesophageal reflux (GER) - most common cause of nonbilious vomiting, especially first weeks of life, often from overfeeding 2
- Formula intolerance
- Overfeeding
Key distinguishing feature: Infants with normal weight gain and no other symptoms are unlikely to have obstruction 2
Critical Clinical Warning Signs for Serious Disease
Physical examination findings demanding immediate action: 1
- Cyanosis
- Tachypnea
- Poor peripheral perfusion
- Petechiae
- Rectal temperature >40°C
Additional red flags:
- Bilious emesis
- Repeated forceful vomiting
- Bloody stools
- Lethargy/altered mental status
- Bulging fontanelle
- Difficulty feeding with decreased urine output
Common Pitfalls to Avoid
Never assume a well-appearing febrile neonate is safe - neonates with life-threatening infections often appear only mildly ill initially 1
Don't dismiss bilious vomiting - this is malrotation with volvulus until proven otherwise, regardless of age 2
Don't rely on parental or physician "gestalt" alone - while both parental concern and physician concern are important warning signs for SBI, they cannot exclude serious disease 1
Don't forget malrotation in older infants - although most common in newborns, malrotation can present at any age with decreasing frequency 2
Don't overlook metabolic disease - consider in recurrent presentations or when clinical course doesn't fit infectious pattern 2