A common pediatric neurosurgery referral is a young child with a rapidly increasing head circumference crossing percentiles. The differential diagnosis is broad and includes benign expansion of the subarachnoid spaces (BESS), subdural collections, hydrocephalus, and neoplasm. When the fontanelle is open, the etiology can usually be diagnosed on head ultrasound, and this should therefore be the initial screening test of choice. In the absence of an open fontanelle, or if there are other signs and symptoms of acute raised intracranial pressure (vomiting, headache, irritability, drowsiness, full fontanelle, sun setting eyes), the etiology should be diagnosed with MRI, if available, in order to limit radiation exposure. There is growing evidence that exposure to radiation through CT imaging increases a child’s life long risk of cancer, and so all care should be taken to minimize this exposure as much as possible. Ultrasound (when fontanelle open), and/or MRI (when fontanelle closed), are therefore the screening tests of choice to investigate macrocephaly.
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