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Human head size is largely determined by the size of the brain, cerebrospinal fluid (CSF) volume, and blood vessels and their content. Destructive processes or errors in proliferation of cells can lead to small heads that are less than the second percentile, a condition known as microcephaly. An increase in the volume of any 1 of these contents can lead to a head circumference greater than the 98th percentile, a condition known as macrocephaly. The development of a child can be affected depending on the processes that are involved in these disorders.

Head shape also determines head circumference. For a given volume, a perfect sphere is the structure with the least surface area. Any deviation from such a spherical shape can lead to larger circumferences.


In the evaluation of a child with a head circumference less than the second percentile, one must first consider body size and gestational age if the child is a newborn. Head circumference percentiles can be corrected for gestational age if a child is born prematurely; symmetric less than second percentile body size and head size may not be as concerning as isolated microcephaly. In some families, a head circumference less than the second percentile may be normal and can be associated with normal developmental outcomes, although this is unusual. When presented with a child with a head circumference that measures less than the second percentile, one must consider whether this is an acquired lesion or a genetic one. Acquired microcephaly is often accompanied by hypertonia, whereas genetic disorders often present with hypotonia.

Acquired Microcephaly

In Utero Infection

Acquired microcephaly may be present in infants with TORCH (toxoplasmosis, other [syphilis, varicella-zoster, parvovirus B19], rubella, cytomegalovirus [CMV], and herpes) or other in utero infections. Typically, neuroimaging demonstrates calcifications in this form of acquired microcephaly, and the pattern of these calcifications may help to determine the etiology. Scattered parenchymal calcifications would suggest in utero CMV infection, whereas periventricular calcifications may suggest toxoplasmosis infection. Although CMV is ubiquitous and may cause a mononucleosis-like illness in the mother, very little is done to educate expectant mothers regarding the risks for acquiring this virus in pregnancy, and typically, preconception titers are not assessed. It is estimated that 50% of pregnant women have not had exposure to CMV, and unfortunately, in the 1% to 4% of pregnant women in which the virus is acquired, the outcome can be devastating (Fig. 543-1). Classically, having young children in daycare or working in a school or daycare center increases risk of acquiring CMV while pregnant; according to the Centers for Disease Control and Prevention, hand washing after contact with bodily fluids from young children may prevent infection with the virus.

Figure 543-1

Microcephaly with calcifications on computed tomography axial imaging of the brain due to ...

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