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 of hydrocephalus in this 3-month-old infant, particularly if there is any history of prematurity or low birth weight. 1

Clinical Reasoning

The clinical presentation described—vomiting, poor feeding, lethargy, bulging fontanelle, and high-pitched cry—represents classic signs of symptomatic hydrocephalus with increased intracranial pressure in an infant. 2 The key to determining etiology lies in understanding the epidemiology and typical presentations of hydrocephalus in this age group.

Why Intraventricular Hemorrhage is Most Likely

• IVH is the most common cause of acquired hydrocephalus in infants, affecting approximately 15-20% of preterm infants weighing less than 1500g at birth who develop intraventricular hemorrhage. 3

• Approximately one-third of infants with IVH develop posthemorrhagic ventricular dilation (PHVD), with 76% of infants with Grade III IVH progressing to PHVD. 1

• Posthemorrhagic hydrocephalus (PHH) typically presents at 3 months of age with progressive symptoms including splaying of sagittal sutures, fontanel fullness, worsening apnea and bradycardia episodes, lethargy, and feeding intolerance—matching this patient's presentation exactly. 1

• The pathophysiology involves fibrosis of arachnoid granulations, meningeal fibrosis, and subependymal gliosis that impair CSF resorption following the hemorrhagic insult. 3, 1

Critical History to Obtain

Essential information that would confirm IVH as the etiology includes:

• Gestational age at birth and birth weight—prematurity (especially <32 weeks) and low birth weight (<1500g) are major risk factors. 3, 1

• Any documented IVH on neonatal cranial ultrasounds performed during the initial hospitalization. 1

• History of hemodynamic instability, hypotension, or respiratory distress in the first 3 days of life. 3

• History of chorioamnionitis, thrombocytopenia, or need for red blood cell transfusions during the neonatal period. 3

Why Other Options Are Less Likely

Meningitis (Option A):

• Postinfectious hydrocephalus requires either a documented history of febrile illness or imaging/endoscopic findings suggesting previous ventriculitis. 1
• Without a clear history of CNS infection, fever, or sepsis, this is less likely than PHH. 1
• Postinfectious hydrocephalus is more prevalent in certain geographic regions (South Asia, Africa) where CNS infections are more common. 1

Chiari Malformation (Option C):

• Chiari malformations typically present with congenital hydrocephalus evident at or shortly after birth, not with acute decompensation at 3 months. 4
• The acute presentation with bulging fontanelle and high-pitched cry suggests acquired rather than congenital pathology. 2

Arachnoid Cyst (Option D):

• Arachnoid cysts can cause obstructive hydrocephalus but represent a less common etiology compared to PHH in this age group. 1
• Arachnoid cysts typically cause more gradual progression rather than the acute symptomatic presentation described. 4

Diagnostic Approach

The CT scan should be carefully reviewed for:

• Blood products or evidence of prior hemorrhage in the ventricles, which would strongly suggest PHH. 1

• Periventricular changes suggesting prior hemorrhagic infarction or white matter injury. 1

• Aqueduct patency status—stenosis can be postinfectious rather than congenital if inflammation occurred. 1

Clinical Implications and Urgency

• This infant requires urgent neurosurgical evaluation given the signs of increased intracranial pressure (bulging fontanelle, high-pitched cry, lethargy). 2

• Approximately 26% of preterm infants with IVH develop PHH requiring ventriculoperitoneal shunt placement. 5

• Untreated symptomatic hydrocephalus causes white matter damage through compression and ischemia from increased intracranial pressure, leading to irreversible developmental delays. 6

• The severity of IVH (Grade III or IV) is the most important risk factor for requiring permanent CSF diversion. 5

Common Pitfalls to Avoid

• Do not assume congenital etiology without obtaining detailed birth history—many cases of "hydrocephalus in infants" are actually acquired PHH from unrecognized or underappreciated neonatal IVH. 1

• Do not delay treatment while pursuing extensive workup—symptomatic hydrocephalus with signs of increased intracranial pressure requires urgent intervention regardless of etiology. 2

• Approximately 20% of hydrocephalus cases have unclear etiology, but in a 3-month-old with acute presentation, PHH from prematurity remains most likely until proven otherwise. 1