What is the most likely etiology of a 3-month-old infant presenting with vomiting, poor feeding, lethargy, bulging fontanelle, and high-pitched cry, with a CT scan showing hydrocephalus?

Most Likely Etiology of Hydrocephalus in a 3-Month-Old Infant

In a 3-month-old infant presenting with vomiting, poor feeding, lethargy, bulging fontanelle, high-pitched cry, and hydrocephalus on CT scan, meningitis (postinfectious hydrocephalus) is the most likely etiology among the options provided.

Clinical Reasoning

The clinical presentation strongly suggests acute symptomatic hydrocephalus with elevated intracranial pressure. The constellation of symptoms—bulging fontanelle, high-pitched cry (indicating meningeal irritation or increased ICP), lethargy, and poor feeding—points toward an acute infectious or inflammatory process rather than a chronic congenital condition 1.

Why Meningitis is Most Likely:

• Postinfectious hydrocephalus is one of the most common acquired forms of hydrocephalus in infants, alongside posthemorrhagic and tumor-related causes 2, 3
• The acute presentation with systemic symptoms (poor feeding, lethargy) combined with signs of meningeal irritation (bulging fontanelle, high-pitched cry) is classic for meningitis 1
• At 3 months of age, infants with intracranial infection present with nonspecific symptoms including fever, poor feeding, irritability, and lethargy—exactly matching this case 1
• Bacterial meningitis can cause hydrocephalus through inflammation-dependent dysregulation of CSF secretion and clearance, as well as blockage of CSF pathways by inflammatory debris 3, 4

Why Other Options Are Less Likely:

Intraventricular hemorrhage (IVH):

• IVH causing posthemorrhagic hydrocephalus is most common in preterm infants, particularly those <32 weeks gestation 1
• The typical presentation is in the neonatal period or first weeks of life, not at 3 months 1
• There is no mention of prematurity or perinatal complications in this case 1

Chiari malformation:

• This is a congenital structural anomaly that typically presents with chronic rather than acute symptoms 2, 4
• Would not explain the acute systemic symptoms (poor feeding, lethargy) or signs of infection/inflammation 4
• Usually diagnosed earlier or presents later with different symptomatology 3

Arachnoid cyst:

• These are congenital lesions that generally cause slowly progressive symptoms rather than acute presentation 2, 3
• Would not account for the systemic symptoms suggesting infection 3

Critical Clinical Pearls:

• Any infant with bulging fontanelle, lethargy, and poor feeding requires immediate evaluation for meningitis, including lumbar puncture (if safe after imaging) and blood cultures 1
• The high-pitched cry is a particularly concerning sign indicating meningeal irritation or significantly elevated ICP 1
• In infants aged 0-90 days with fever, the risk of bacterial meningitis is approximately 0.4%, but rises significantly with clinical signs of meningeal irritation 1
• Postinfectious hydrocephalus develops through inflammation-dependent mechanisms including blockage of CSF pathways and dysregulation of CSF dynamics 3, 4

Immediate Management Priorities:

• Obtain blood cultures and consider empiric antibiotics if meningitis is suspected, even before lumbar puncture if the infant is unstable 1
• MRI with and without contrast is indicated for suspected intracranial infection, as it is more sensitive than CT for detecting meningitis, encephalitis, and complications 1
• Neurosurgical consultation is essential given the presence of hydrocephalus, as CSF diversion may be needed if medical management fails 2, 3