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

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

Intraventricular hemorrhage (IVH) is the most likely etiology in this 3-month-old infant with hydrocephalus, particularly if the infant was born prematurely. 1

Clinical Reasoning

The presentation of vomiting, poor feeding, lethargy, bulging fontanelle, and high-pitched cry represents classic signs of symptomatic hydrocephalus with elevated intracranial pressure in an infant. 2 The key to determining etiology lies in the infant's birth history and timing of symptom onset.

Posthemorrhagic Hydrocephalus (PHH) - Most Likely if Premature

• Approximately 15-20% of preterm infants weighing less than 1500g at birth who develop IVH subsequently develop posthemorrhagic hydrocephalus. 3
• About one-third of infants with IVH develop posthemorrhagic ventricular dilation (PHVD), with 76% of infants with Grade III IVH developing PHVD. 1
• PHH typically presents with progressive splaying of sagittal sutures, fontanel fullness, worsening apnea and bradycardia episodes, lethargy, and feeding intolerance—matching this clinical picture. 1
• The pathophysiology involves fibrosis of arachnoid granulations, meningeal fibrosis, and subependymal gliosis that impair CSF resorption. 1

Postinfectious Hydrocephalus - Consider if Febrile Illness History

• Postinfectious hydrocephalus requires either a history of febrile illness (with or without seizures) before hydrocephalus onset, or imaging/endoscopic findings suggesting previous ventriculitis. 1
• The absence of hydrocephalus at birth with subsequent development after a febrile illness suggests acquired postinfectious etiology. 3
• This diagnosis cannot be made without documented infection history or imaging evidence of prior ventriculitis. 1

Congenital Causes - Less Likely at 3 Months

• Chiari malformation and aqueductal stenosis typically present earlier, often detected prenatally or in the immediate newborn period with progressive macrocephaly. 4, 5
• Congenital hydrocephalus is usually present at or near birth and linked to gene mutations disrupting brain morphogenesis. 5
• Approximately 80% of children with spina bifida (myelomeningocele) develop hydrocephalus, but this would typically be evident at birth. 3

Arachnoid Cyst - Least Likely

• Arachnoid cysts can cause obstructive hydrocephalus but represent a less common etiology compared to PHH in this age group. 1
• These are typically congenital lesions that would more likely present earlier or be detected on prenatal imaging. 5

Critical Diagnostic Approach

The CT scan findings should be carefully reviewed for:

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

Essential history to obtain:

• Gestational age at birth and birth weight (prematurity strongly suggests PHH). 1, 3
• Any documented IVH on neonatal cranial ultrasounds. 1
• History of febrile illness, seizures, or suspected meningitis. 1
• Prenatal imaging findings if available. 4

Common Pitfall

Do not assume congenital aqueductal stenosis without excluding acquired causes. Infections can cause aqueduct stenosis through inflammation, so apparent stenosis on imaging could represent postinfectious hydrocephalus rather than a congenital malformation. 1 The clinical history and timing of symptom onset are essential to distinguish these etiologies.