Likely Etiology: Meningitis
In a 3-month-old infant presenting with acute symptoms of bulging fontanelle, high-pitched cry, lethargy, and poor feeding alongside hydrocephalus on CT, meningitis is the most likely etiology and requires immediate evaluation and treatment. 1
Clinical Reasoning
The clinical presentation is the critical distinguishing factor here:
- Acute symptomatic presentation with bulging fontanelle, high-pitched cry, lethargy, and poor feeding strongly indicates an acute infectious or inflammatory process with elevated intracranial pressure 1
- These systemic symptoms—particularly the combination of poor feeding, lethargy, and signs of meningeal irritation (bulging fontanelle and high-pitched cry)—are characteristic of meningitis in this age group 1
- Infants at 3 months with intracranial infection typically present with these nonspecific symptoms including irritability and lethargy 1
Why Not the Other Options?
Intraventricular Hemorrhage (IVH)
- IVH-related posthemorrhagic hydrocephalus is most common in preterm infants (particularly those <32 weeks gestation and <1500g birth weight), and typically presents in the neonatal period or first weeks of life, not at 3 months 2, 3, 1
- Approximately 15-20% of very low birth weight infants develop IVH, with about one-third progressing to posthemorrhagic ventricular dilation 2, 3
- The clinical presentation of posthemorrhagic hydrocephalus is more gradual, with progressive splaying of sagittal sutures and worsening apnea/bradycardia episodes—not the acute presentation described here 2, 3
- Critical history point: You would expect documented prematurity, low birth weight, respiratory distress, and prior cranial ultrasounds showing IVH 2
Chiari Malformation
- This is a congenital structural abnormality that would typically present earlier or be detected on prenatal imaging 4
- The acute symptomatic presentation with signs of infection makes this unlikely 1
Arachnoid Cyst
- While arachnoid cysts can cause obstructive hydrocephalus, they represent a less common etiology compared to infectious causes in this clinical scenario 2
- Arachnoid cysts typically present more gradually without the acute systemic symptoms of infection 4
Immediate Management Required
- Obtain blood cultures and consider empiric antibiotics immediately, even before lumbar puncture if the infant is unstable 1
- Lumbar puncture and blood cultures are essential for definitive diagnosis 1
- MRI with and without contrast is more sensitive than CT for detecting meningitis, encephalitis, and complications 1
Key Clinical Pitfall
Do not delay treatment while awaiting imaging or lumbar puncture results. The acute presentation with signs of elevated intracranial pressure and systemic illness requires immediate empiric antibiotic coverage for bacterial meningitis in this age group 1.
Answer: A. Meningitis