Ultrasound for Evaluating Large Head Circumference in a 2-Month-Old Infant
Yes, cranial ultrasound is an appropriate and recommended first-line imaging modality for evaluating a 2-month-old infant with large head circumference to assess for hydrocephalus and other intracranial abnormalities. 1, 2
Why Ultrasound is Appropriate
Cranial ultrasonography can be safely performed at the bedside and avoids the risks associated with transporting infants for MRI. 1 At 2 months of age, the anterior fontanelle remains open, providing an acoustic window for comprehensive evaluation. 1
What Ultrasound Can Detect
Ultrasound reliably identifies several critical conditions in infants with macrocephaly:
- Hydrocephalus and ventricular dilation can be confidently diagnosed, with measurements including the Levene ventricular index and anterior horn width (AHW normal <3 mm, abnormal >6 mm). 1, 2
- Intraventricular hemorrhage (Grades I-IV) is reliably detected within the lateral ventricles. 2
- White matter changes, including cystic periventricular leukomalacia, are visible on ultrasound. 2
- Extracerebral fluid collections (subdural effusions, subarachnoid spaces) can be identified, which are common causes of macrocephaly beyond hydrocephalus. 3
- Intracranial cysts (subependymal and subarachnoidal) may be detected. 3
Important Limitations to Consider
While ultrasound is an excellent initial study, clinicians must understand its diagnostic gaps:
- Hypoxic-ischemic injury has low sensitivity for detection via ultrasound. 2, 4
- Cerebellar hemorrhage is significantly underdetected, with only 23% of cases identified by ultrasound. 2
- Diffuse white matter injury, the most common current form of white matter damage, is difficult to visualize. 2
- Small infarctions and cortical malformations require MRI for detection. 2, 4
Clinical Algorithm
Begin with cranial ultrasound as the initial imaging study for any 2-month-old with large head circumference. 1, 2, 3
When Ultrasound Alone is Sufficient
If ultrasound demonstrates clear hydrocephalus with ventricular dilation and the clinical picture is consistent (progressive head growth, bulging fontanelle, splaying of sutures), proceed with appropriate management including serial ultrasounds to monitor progression. 1
When to Advance to MRI
MRI with diffusion-weighted imaging should be obtained if:
- Ultrasound is normal but clinical concern persists (abnormal neurological examination, seizures, developmental delays). 2, 4
- The infant has risk factors for conditions poorly detected by ultrasound (history of birth trauma, perinatal asphyxia, suspected cortical malformation). 2, 4
- Additional structural detail is needed for surgical planning or prognostication. 1
MRI provides additional diagnostic information beyond ultrasound in approximately 40% of patients and is the gold standard for identifying hypoxic-ischemic encephalopathy and subtle structural abnormalities. 4
Critical Pitfalls to Avoid
Do not assume all macrocephaly is hydrocephalus. Macrocephaly has heterogeneous causes including familial megalencephaly, extracerebral fluid collections without increased intracranial pressure, and various genetic syndromes. 3 The distinction between pathological hydrocephalus requiring intervention versus benign enlargement of subarachnoid spaces (external hydrocephalus) is crucial and can be made with ultrasound. 3, 5
Progressive splaying of the sagittal suture width is the most reliable clinical indication of increased intracranial pressure when combined with imaging findings. 1 Serial head circumference measurements crossing percentiles upward warrant urgent evaluation. 3, 5
CT scanning should be avoided in routine evaluation due to radiation exposure, reserved only for acute hemorrhagic emergencies when MRI is unavailable. 1, 2