Likely Etiology: Intraventricular Hemorrhage (IVH)
The most likely etiology of hydrocephalus in this 3-month-old infant is intraventricular hemorrhage (IVH), particularly if the infant was born prematurely. 1
Clinical Reasoning
Why IVH is Most Likely
- IVH is the most commonly diagnosed brain lesion in preterm infants, affecting approximately 15-20% of infants weighing less than 1500g at birth 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
- The clinical presentation matches posthemorrhagic hydrocephalus (PHH) precisely: bulging fontanelle, progressive splaying of sagittal sutures, lethargy, feeding intolerance (poor feeding), and high-pitched cry are classic signs of increased intracranial pressure from PHH 1
- About 15% of preterm infants with severe IVH will require permanent shunt placement for symptomatic PHH 1
Critical History to Obtain
You must immediately determine the infant's gestational age at birth and birth weight 1:
- Premature birth (especially <34 weeks) dramatically increases IVH likelihood 3
- Birth weight <1500g carries 15-20% IVH risk 2
- History of respiratory distress, hypotension, or mechanical ventilation in the neonatal period 1
- Any documented IVH on neonatal cranial ultrasounds 1
- History of red blood cell transfusions (independent risk factor for IVH) 3
CT Scan Review Strategy
Carefully examine the CT scan for these specific findings 1:
- Blood products or evidence of prior hemorrhage in the ventricles
- Periventricular changes suggesting prior hemorrhagic infarction
- Aqueduct patency status (stenosis can be postinfectious rather than congenital)
Why Other Options Are Less Likely
Meningitis (Option A) would require 1:
- History of febrile illness or seizures
- Imaging/endoscopic findings suggesting previous ventriculitis
- Postinfectious hydrocephalus is more common in certain geographic regions (South Asia, Africa) but requires documented infection 1
Chiari malformation (Option C) 4:
- Typically presents with posterior fossa abnormalities visible on imaging
- Would be a congenital structural malformation, not acquired at 3 months
Arachnoid cyst (Option D) 1:
- Can cause obstructive hydrocephalus but represents a less common etiology compared to PHH in this age group
- Would appear as a distinct cystic structure on CT
Pathophysiology of PHH
The mechanism involves fibrosis of arachnoid granulations, meningeal fibrosis, and subependymal gliosis that impair CSF resorption 3:
- Elevated TGF-β2 and extracellular matrix proteins stimulate deposition in perivascular spaces 3
- White matter damage occurs from compression and ischemia due to increased intracranial pressure 3
Common Pitfall to Avoid
Do not assume congenital causes without first ruling out acquired PHH 1. The timing (3 months old) and acute presentation with signs of increased intracranial pressure strongly suggest an acquired process, most commonly PHH from prematurity. Many IVHs are clinically silent initially, so lack of earlier symptoms does not exclude this diagnosis 3.