Meningitis is the most likely etiology
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 diagnosis. This clinical constellation represents classic signs of increased intracranial pressure with infection in an infant 1, 2.
Clinical Reasoning
Key Diagnostic Features Present
The infant displays the hallmark signs of meningitis in this age group:
- Bulging fontanelle - indicates increased intracranial pressure and is specifically associated with meningitis in infants 1
- High-pitched cry - a classic neurological sign of CNS infection in infants 2
- Lethargy and poor feeding - nonspecific but consistent with systemic infection and increased ICP 2
- Vomiting - associated with raised intracranial pressure from multiple causes including meningitis 1
Why Meningitis Over Other Options
Meningitis causes hydrocephalus through a well-established mechanism: The infection produces granulomatous and suppurative inflammation that causes fibrosis of the subarachnoid space, obstructing CSF flow and absorption 3. This can occur acutely or develop as a complication.
Against Intraventricular Hemorrhage (IVH): While IVH is a common cause of hydrocephalus, it predominantly affects premature infants and typically presents in the immediate neonatal period, not at 3 months of age 4. The clinical presentation described—particularly the high-pitched cry and acute systemic symptoms—is more consistent with infection than hemorrhagic complications.
Against Chiari Malformation: This congenital anomaly typically presents with chronic progressive symptoms rather than acute deterioration with fever-like systemic signs. The acute presentation with lethargy and poor feeding suggests an infectious/inflammatory process.
Against Arachnoid Cyst: These are typically asymptomatic or present with chronic progressive symptoms, not acute deterioration with systemic signs of illness.
Critical Clinical Context
The ACR guidelines specifically list meningitis as a medical cause of raised intracranial pressure presenting with vomiting, bulging fontanelle, and neurologic signs in infants 1. The acute presentation at 3 months with the complete clinical picture—including the characteristic high-pitched cry—strongly points toward an infectious etiology rather than structural or hemorrhagic causes 2.
Immediate Management Implications
This diagnosis carries significant mortality risk if not treated urgently. The infant requires:
- Immediate empiric antibiotics (do not delay for lumbar puncture if meningitis is suspected) 2
- Blood cultures and other septic workup
- Lumbar puncture when safe (after addressing increased ICP concerns)
- Close monitoring for complications including worsening hydrocephalus
The combination of acute systemic illness, classic CNS infection signs in an infant, and secondary hydrocephalus makes meningitis (Option A) the most likely etiology requiring urgent intervention.