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

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

The most likely etiology is intraventricular hemorrhage (IVH), particularly if this infant was born prematurely, as posthemorrhagic hydrocephalus (PHH) represents the most common acquired cause of hydrocephalus in this age group. 1

Clinical Reasoning

Why IVH/PHH is Most Likely

• IVH is the most commonly diagnosed brain lesion in preterm infants, with approximately 15-20% of infants weighing less than 1500g at birth developing IVH 2
• Approximately one-third of infants with IVH develop posthemorrhagic ventricular dilation (PHVD), with 76% of infants with Grade III IVH progressing to PHVD 1
• About 15% of preterm infants with severe IVH will require permanent shunt placement for symptomatic PHH 2
• The clinical presentation described—bulging fontanelle, high-pitched cry, lethargy, vomiting, and poor feeding—is classic for PHH in infants 1, 3

Critical History to Obtain

You must immediately determine:

• Gestational age at birth and birth weight (prematurity is the key risk factor) 1
• Any documented IVH on neonatal cranial ultrasounds performed in the NICU 1
• History of respiratory distress, hypotension, or need for mechanical ventilation in the neonatal period 2

Why Other Options Are Less Likely

Meningitis (Option A):

• Would require documented history of febrile illness, seizures, or suspected CNS infection 1
• Postinfectious hydrocephalus typically shows periventricular changes or imaging findings suggesting prior ventriculitis 1
• More common in certain geographic regions (South Asia, Africa) where CNS infections are prevalent 1

Chiari Malformation (Option C):

• Would typically present with posterior fossa abnormalities visible on CT/MRI
• Less common as isolated cause of hydrocephalus at 3 months
• Usually associated with myelomeningocele (80% of spina bifida patients develop hydrocephalus) 1

Arachnoid Cyst (Option D):

• Can cause obstructive hydrocephalus but represents a less common etiology compared to PHH in this age group 1
• Would show characteristic cystic lesion on imaging

Pathophysiology of PHH

The mechanism involves:

• Fibrosis of arachnoid granulations, meningeal fibrosis, and subependymal gliosis that impair CSF resorption 2, 1
• Elevated TGF-β2 and extracellular matrix proteins in CSF that stimulate deposition in perivascular spaces 2
• White matter damage from compression and ischemia due to increased intracranial pressure 2

Critical Imaging Review

On the CT scan, specifically look for:

• Blood products or evidence of prior hemorrhage in the ventricles suggesting PHH 1
• Periventricular changes suggesting prior hemorrhagic infarction 1
• Aqueduct patency status (stenosis can be postinfectious rather than congenital) 1

Common Pitfall

The major pitfall is assuming all infant hydrocephalus is congenital. In reality, acquired causes—particularly PHH from prematurity—are extremely common in the first few months of life 2, 4. The timing (3 months old) and acute presentation with signs of increased ICP strongly favor an acquired etiology, with IVH being the leading cause in this population 1, 5.