Most Likely Etiology: Meningitis
In a 3-month-old infant presenting with vomiting, poor feeding, lethargy, bulging fontanelle, high-pitched cry, and hydrocephalus on CT scan, meningitis is the most likely etiology. 1
Clinical Reasoning
Acute Symptomatic Presentation Points to Infection
The constellation of symptoms—bulging fontanelle, high-pitched cry, lethargy, and poor feeding—strongly indicates acute symptomatic hydrocephalus with elevated intracranial pressure from an infectious or inflammatory process. 1 These systemic symptoms with signs of meningeal irritation are characteristic of meningitis in infants at 3 months of age. 1
Infants with meningitis commonly display less specific features including poor feeding, irritability, high-pitched cry, and full fontanelle, rather than classic meningeal signs seen in older children. 2
Why Not Intraventricular Hemorrhage?
While intraventricular hemorrhage (IVH) is indeed the most common cause of hydrocephalus in preterm infants, it typically presents in the neonatal period or first weeks of life—not at 3 months. 3, 1
Key distinguishing factors against IVH:
- IVH occurs in approximately 15-20% of infants weighing less than 1500g at birth, predominantly affecting preterm infants less than 32 weeks gestation. 3, 1
- Posthemorrhagic hydrocephalus develops progressively with splaying of sagittal sutures and fontanel fullness, but the acute symptomatic presentation described here is more consistent with infection. 3
- The question provides no history of prematurity, low birth weight, respiratory distress, or neonatal intensive care—all critical risk factors for IVH. 3
Why Not Chiari Malformation?
Chiari malformation causing hydrocephalus would not present with acute systemic symptoms like poor feeding, lethargy, and high-pitched cry. 4 This is a congenital structural anomaly that would more likely present with progressive head enlargement without the acute inflammatory signs seen here. 5
Why Not Arachnoid Cyst?
Arachnoid cysts can cause obstructive hydrocephalus but represent a less common etiology compared to infectious causes in this age group and clinical context. 3 They would not explain the acute systemic symptoms and signs of meningeal irritation. 1
Diagnostic Approach
Essential Immediate Evaluation
Do not delay treatment pending further investigations when symptoms are highly suggestive of meningococcal disease or bacterial meningitis. 2
Critical next steps include:
- Obtain blood cultures and consider empiric antibiotics immediately, even before lumbar puncture if the infant is unstable. 1
- MRI with and without contrast is more sensitive than CT for detecting meningitis, encephalitis, and complications. 1
- Review CT scan for periventricular changes suggesting infection or inflammation. 3
Key Historical Elements to Obtain
- Any documented febrile illness or seizures. 3
- Birth history including gestational age and birth weight (to assess for missed neonatal IVH). 3
- Any neonatal cranial ultrasounds showing IVH. 3
Common Pitfall
The most critical error would be attributing the hydrocephalus to a chronic congenital cause (like Chiari malformation or arachnoid cyst) and missing acute bacterial meningitis, which requires immediate antibiotic therapy to prevent mortality and severe morbidity. 2 Most deaths from meningococcal disease occur in the first 24 hours, often before specialist care begins. 2