What is the most appropriate initial imaging study for a 1‑month‑old infant presenting with a bulging fontanelle?

Best Imaging for Bulging Fontanelle in 1-Month-Old

Cranial ultrasound through the anterior fontanelle is the most appropriate initial imaging study for a 1-month-old infant presenting with a bulging fontanelle.

Rationale for Ultrasound as First-Line Imaging

Ultrasound is the optimal initial modality because it is non-invasive, requires no sedation, produces no ionizing radiation, can be performed at bedside, and provides excellent visualization of intracranial structures through the open anterior fontanelle in this age group. 1, 2, 3

Key Advantages in This Clinical Context

• Immediate accessibility: Can be performed portably on unstable infants without transport, preserving thermal stability 4
• Rapid acquisition: Examination completed in 5-10 minutes 4
• No sedation required: Critical advantage in a 1-month-old 2
• Excellent diagnostic capability: Detects the most common causes of bulging fontanelle including:
  • Intracranial hemorrhage (intraventricular and parenchymal) 3, 4
  • Hydrocephalus and ventricular enlargement 5, 3
  • Meningitis/infection-related changes 5, 2
  • Mass lesions and vascular malformations 2, 4

Clinical Assessment Before Imaging

Before proceeding with imaging, rapidly assess for:

• Signs of increased intracranial pressure: Altered mental status, vomiting, lethargy, rapidly enlarging head circumference 5
• Infection indicators: Fever, irritability, poor feeding (consider viral testing for RSV, influenza, COVID-19, enterovirus) 5
• Hydration status: While bulging suggests increased ICP, also check mucous membranes, skin turgor, capillary refill to exclude paradoxical dehydration assessment errors 5

When to Escalate Beyond Ultrasound

MRI Indications

Consider MRI as next step if:

• Ultrasound findings are equivocal or require further characterization 6
• Suspected posterior fossa pathology (ultrasound has limited posterior fossa visualization) 6
• Need for detailed parenchymal assessment or small hemorrhage detection 6
• Vascular malformation suspected requiring detailed anatomic delineation 6

CT Indications

CT is not first-line due to radiation concerns in infants 6, but consider if:

• Ultrasound unavailable and clinical urgency exists 6
• Trauma suspected (though ultrasound can detect skull fractures with near-CT sensitivity) 6
• Need to confirm calcifications (though SWI MRI can often accomplish this) 6

Important Caveats

Ultrasound limitations: While excellent for most pathology, ultrasound has reduced sensitivity for small subdural hematomas, particularly in the posterior fossa, and small extra-axial hemorrhages compared to CT/MRI 6. However, these limitations are outweighed by the safety profile and diagnostic yield for the most clinically important pathology in a 1-month-old with bulging fontanelle.

Technique matters: Use dedicated neonatal convex and high-frequency linear transducers through both anterior and mastoid fontanelles for optimal visualization, including posterior fossa structures 6, 3. Proper operator training is essential as this is a heavily operator-dependent technique 6.

Correlation with clinical context: In well-appearing infants with bulging fontanelle and fever, viral meningitis (particularly enterovirus) is far more common than bacterial meningitis (8.4% vs 0.4%), and imaging findings should be interpreted in this clinical context 5.