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

In a 3-month-old infant presenting with hydrocephalus, vomiting, poor feeding, lethargy, bulging fontanelle, and high-pitched cry, the most likely etiology is posthemorrhagic hydrocephalus (PHH) from intraventricular hemorrhage (IVH), particularly if the infant was born prematurely. 1

Primary Diagnostic Consideration: Posthemorrhagic Hydrocephalus

Intraventricular hemorrhage is the most common cause of hydrocephalus in this age group and clinical presentation. 1 The pathophysiology involves:

• Fibrosis of arachnoid granulations, meningeal fibrosis, and subependymal gliosis that impair CSF resorption 1
• Approximately one-third of infants with IVH develop posthemorrhagic ventricular dilation (PHVD), with 76% of infants with Grade III IVH progressing to PHVD 1
• PHH typically presents with progressive splaying of sagittal sutures, fontanel fullness, worsening apnea and bradycardia episodes, lethargy, and feeding intolerance—matching this infant's presentation 1

Critical History to Obtain

Essential information includes gestational age at birth, birth weight, and any documented IVH on neonatal cranial ultrasounds. 1 Specifically:

• Prematurity is the strongest risk factor, with approximately 15-20% of infants weighing less than 1500g at birth who develop IVH subsequently developing hydrocephalus 1
• Review any neonatal intensive care records for documented hemorrhage 1

Secondary Diagnostic Considerations

Postinfectious Hydrocephalus

This requires either a history of febrile illness (with or without accompanying seizures) before the onset of clinically evident hydrocephalus, or imaging/endoscopic findings suggesting previous ventriculitis. 1 Key features include:

• Absence of known hydrocephalus at birth 1
• History of febrile illness, seizures, or suspected meningitis should be assessed 1
• Postinfectious hydrocephalus is particularly prevalent in certain geographic regions (South Asia, Africa) where CNS infections are more common 2

Congenital Causes

Congenital etiologies are less likely given the 3-month presentation but must be considered:

• Aqueductal stenosis can present in infancy, though typically earlier 3, 4
• Spina bifida (myelomeningocele) is one of the most common congenital causes, with approximately 80% of children with spina bifida developing hydrocephalus 1
• Posterior fossa malformations represent another congenital etiology 3

Critical Imaging Review

The CT scan findings must be carefully reviewed for specific features that establish etiology:

• Look for blood products or evidence of prior hemorrhage in the ventricles, suggesting PHH 1
• Assess aqueduct patency status, as stenosis can be postinfectious rather than congenital if inflammation occurred 1
• Evaluate for periventricular changes suggesting prior infection or hemorrhagic infarction 1
• Assess for structural abnormalities such as posterior fossa malformations or neural tube defects 3

Clinical Pitfalls to Avoid

Do not assume aqueduct stenosis is congenital—infections can cause aqueduct stenosis due to inflammation, and some of these children could have had postinfectious hydrocephalus. 2 This is a critical diagnostic error that affects treatment planning.

Approximately 20% of hydrocephalus cases have unclear etiology, making classification difficult. 2 This highlights the importance of:

• Obtaining complete birth and neonatal history 1
• Reviewing all available imaging from birth onward 1
• Considering advanced imaging (MRI when available) to distinguish congenital structural abnormalities from acquired pathology 1

Geographic and Demographic Considerations

The etiology distribution varies by geographic region and economic factors:

• Postinfectious hydrocephalus is most common in South Asia (23.2%) 2
• Non-postinfectious causes are most common in East Asia (38.3%) 2
• In high-income countries, congenital causes represent 36.7% of cases 2

Early diagnosis and surgical treatment are critical to reduce morbidity and mortality from hydrocephalus, as longevity and severity of neurological deficiencies depend on the etiology. 2, 5