Imaging the Infant or Child with an Abnormal Head Circumference

The assessment of an infant or child with an abnormal head circumference commonly includes imaging of the head with neurosonography, computed tomography (CT), or magnetic resonance imaging (MRI). The choice of imaging modality depends on the patient's age, presentation, clinical condition, and suspected underlying abnormality.

Macrocephaly, a head circumference more than 2 SD above the mean, or more than the 98th percentile for patient age, is a nonspecific finding. It may reflect benign enlargement of the subarachnoid spaces or may herald a significant intracranial abnormality: an extra-axial collection, a hydrocephalus, or a developmental, metabolic, neoplastic, posttraumatic, or infectious etiology.

Microcephaly, a head circumference more than 2 SD below the mean, or less than the third percentile for patient age, may be primary or secondary, acquired or genetic. Common causes include chromosomal abnormalities, intrauterine infection, intrauterine ischemia, maternal exposure to drugs and toxins, perinatal or postnatal brain injury, and craniosynostosis. Reduced brain volume and intellectual disability frequently accompany microcephaly.

Neurosonography is the initial imaging modality of choice when assessing a neonate or young infant with abnormal head circumference. Neurosonography is portable, readily available, cost-effective, and easily performed in preterm and term neonates as well as older infants with an open fontanelle. It allows rapid evaluation of suspected intracranial abnormalities without sedation or the use of ionizing radiation.

Neurosonographic images are obtained using the anterior fontanelle as an acoustic window (typically in infants younger than 6 months) in the coronal and axial planes. Imaging through the mastoids or posterior fontanelle provides improved visualization of the posterior cerebrum and posterior fossa. Abnormalities seen on initial imaging can be assessed with serial studies, particularly …