The skull provides a fixed space for the brain, blood vessels, and cerebrospinal fluid (CSF) in adults and older children, thus causing symptoms of increased intracranial pressure (ICP) in the case of a space-occupying lesion. However, in infants it has the capability of growth that can accommodate expansion of any intracranial compartment. Macrocephaly is defined by head circumference greater than two standard deviations from the mean for gender (i.e., 98th percentile). Large head size arises from the enlargement of any of the four major constituents of the head: ventricles (hydrocephalus), brain parenchyma (megalencephaly), skull (osteodysplasia), or extracerebral fluid collections (subdural, subgaleal, or subperiosteal).

Hydrocephalus is typically described as communicating (with free flow of CSF between the ventricles and the subarachnoid space) or noncommunicating (obstructive). Either type can be congenital or acquired. Communicating hydrocephalus is usually caused by overproduction of CSF or blockage of CSF reabsorption at the arachnoid villi. Common causes include intraventricular hemorrhage, infection, or inflammation (e.g., bacterial, tuberculous or cryptococcal meningitis, viral encephalitis), meningeal infiltration by tumor, and metabolic conditions (e.g., Hurler syndrome with associated mucopolysaccharidosis deposition at the leptomeninges).

Congenital hydrocephalus presents with enlarged head at birth, but it is increasingly being diagnosed in utero by routine ultrasonography (US). It usually presents during the newborn period or in early infancy with lethargy or irritability, vomiting, poor feeding, or poor weight gain. Acquired hydrocephalus shows a rapid rate of head growth crossing percentiles. Common findings include developmental delay, hypertonia, and hyperreflexia. Hydrocephalus of any etiology can show signs of increased ICP including apnea, bradycardia, a tense and bulging fontanelle, split sutures, the “setting sun sign” (i.e., downward deviation of the eyes showing sclera superiorly), vomiting, sixth nerve palsies, and increased blood pressure with bradycardia. Scalp veins may be prominent and tortuous.