What is the most likely cause of hydrocephalus in a 3‑month‑old infant with vomiting, poor feeding, lethargy, bulging fontanelle, high‑pitched cry and CT showing hydrocephalus: meningitis, intraventricular hemorrhage, Chiari malformation, or arachnoid cyst?

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

Intraventricular hemorrhage (IVH) is the most likely etiology in this 3-month-old infant presenting with hydrocephalus, particularly if there is a history of prematurity. 1

Primary Diagnostic Consideration: Posthemorrhagic Hydrocephalus

The clinical presentation—bulging fontanelle, high-pitched cry, lethargy, poor feeding, and vomiting—is classic for posthemorrhagic hydrocephalus (PHH), which typically manifests with progressive splaying of sagittal sutures, fontanel fullness, worsening apnea and bradycardia episodes, lethargy, and feeding intolerance. 1

Critical Historical Information Required

The diagnosis hinges on obtaining specific perinatal history:

• Gestational age at birth and birth weight are essential, as approximately 15-20% of infants weighing less than 1500g at birth who develop IVH will progress to PHH 1
• History of respiratory distress, hypotension, or need for mechanical ventilation in the neonatal period strongly supports IVH as the underlying cause 1
• Documentation of IVH on neonatal cranial ultrasounds is critical, as approximately 76% of infants with Grade III IVH develop posthemorrhagic ventricular dilation (PHVD) 1
• About 15% of preterm infants with severe IVH will require permanent shunt placement for symptomatic PHH 1

Imaging Analysis

The CT scan should be carefully reviewed for blood products or evidence of prior hemorrhage in the ventricles, which would confirm PHH as the diagnosis. 1 Additionally:

• Periventricular changes suggesting prior hemorrhagic infarction should be evaluated 1
• Aqueduct patency status must be assessed, as stenosis can develop secondary to inflammation from prior hemorrhage 1

Alternative Diagnoses and Why They Are Less Likely

Meningitis (Postinfectious Hydrocephalus)

While postinfectious hydrocephalus is the single most common cause of pediatric hydrocephalus globally (28% of cases across Africa) 2, it requires either a documented history of febrile illness or imaging/endoscopic findings suggesting previous ventriculitis 1. The question does not mention:

• History of febrile illness, seizures, or suspected meningitis 1
• Absence of hydrocephalus at birth with subsequent development after a febrile illness or infection 1

Postinfectious hydrocephalus is particularly prevalent in resource-limited settings and specific geographic regions (South Asia 23.2%, Africa 28%) 2, but without documented infection history, this becomes a less likely diagnosis in this clinical scenario.

Chiari Malformation

Chiari malformation typically presents as part of spina bifida (myelomeningocele), which causes obstructive hydrocephalus in approximately 80% of affected children 3. However:

• The question provides no mention of spinal dysraphism or myelomeningocele
• Chiari malformation would typically be identified earlier or present with additional neurological findings beyond isolated hydrocephalus

Arachnoid Cyst

Arachnoid cysts can cause obstructive hydrocephalus but represent a less common etiology compared to PHH in this age group 1. While arachnoid cysts do occur in infants:

• Patients younger than 2 years with arachnoid cysts commonly present with macrocephaly (50% of cases) 4
• Only 14% of infants with arachnoid cysts present with hydrocephalus 4
• Cyst size ≥68 cm³ is the strongest predictor of requiring surgical intervention (100% sensitivity, 75% specificity) 5
• Arachnoid cysts would typically be visible as a distinct cystic structure on CT, not just generalized ventriculomegaly

Pathophysiology Supporting IVH as Primary Diagnosis

The mechanism of PHH involves fibrosis of arachnoid granulations, meningeal fibrosis, and subependymal gliosis that impair CSF resorption 1. This process:

• Results from elevated TGF-β2 and extracellular matrix proteins in CSF that stimulate deposition in perivascular spaces 1
• Causes white matter damage from compression and ischemia due to increased intracranial pressure 1
• Explains the progressive nature of symptoms in this 3-month-old infant

Clinical Decision Algorithm

1. Obtain detailed birth history: gestational age, birth weight, neonatal complications 1
2. Review neonatal cranial ultrasounds for documented IVH 1, 6
3. Examine CT scan for blood products in ventricles or periventricular changes 1
4. If prematurity + documented IVH → Diagnosis: Posthemorrhagic hydrocephalus
5. If no prematurity history, assess for infection history (fever, meningitis) 1
6. If infection history present → Consider postinfectious hydrocephalus
7. If CT shows distinct cystic structure → Consider arachnoid cyst 4, 5

Answer: B. Intraventricular hemorrhage is the most likely etiology given the age, clinical presentation, and epidemiological data showing IVH affects 15-20% of very low birth weight infants and is the leading cause of acquired hydrocephalus in this age group. 1