Head size is an easily measured clinical finding that can be followed over time. The head circumference is measured around the forehead and the occipital protuberance. Microcephaly is defined by a head circumference more than two standard deviations below the mean for a given demographic (age, gender, race). Small head size is an indication of the underlying brain size except in cases of craniosynostosis, or premature closure of the bony sutures.
The posterior fontanelle should close by age 3 months and anterior fontanelle by 20 months. Fibrous union of the suture lines occurs by about age 6 months. The craniobasal bones are ossified by age 8 years. The remaining sutures are visible on x-ray until approximately age 20 years but can resist elevated intracranial pressure by age 12 years.1
Primary microcephaly occurs because of genetic or environmental factors that produce small brain size (micrencephaly). The genetic form of primary microcephaly can be inherited by either autosomal dominant or autosomal recessive mechanisms.2 A variety of neuronal migrational disorders—lissencephaly, schizencephaly, agensesis of the corpus callosum, and polymicrogyria—can be associated with microcephaly. Incomplete neurogenesis can also cause microcephaly secondary to insufficient neuronal production. Disorders such as anencephaly can also result in microcephaly (Figure 8-1). While the inheritance for microcephaly has been linked to chromosomes 1, 9, and 19, linkages are likely to be found with genes mapping to a number of other chromosomes.3–5 Chromosomal disorders including Down syndrome and other trisomy disorders can produce microcephaly.
Exposure to ionizing radiation during the first two trimesters can result in microcephaly.6,7 It does not appear that exposure to low levels of radiation associated with diagnostic imaging (x-ray, CT, MRI) pose any significant increased risk for the development of microcephaly.
Prenatal infections do, however, increase the risk for microcephaly. Cytomegalovirus (CMV), rubella virus, and toxoplasmosis are the most well-known causes of microcephaly, but numerous infections in utero can increase this risk.8
Fetal exposure to certain chemicals during neuronal induction or cellular migration also increases the risk of microcephaly.
As opposed to disorders interrupting cerebral development associated with primary microcephaly, secondary microcephaly arises following neuronal injury. Prenatal cerebral infarction, meningitis, encephalitis, trauma, hypoxia, and metabolic disorders can result in this neuronal injury.
Craniosynostosis is premature closure of at least one of the cranial bony sutures.9 Primary craniosynostosis is caused by abnormalities of the mesenchymal matrix. Secondary craniosynostosis can occur in association with a number of hematologic, metabolic, and in some cases mechanical disorders.
Craniosynostosis is associated with a number of genetic disorders including Aperts syndrome, Crouzon syndrome, Jackson-Weiss syndrome, Muenke syndrome, Pfeiffer syndrome, Saethre-Chotzen syndrome, and Shprintzen-Goldberg syndrome.10 Secondary craniosynostosis can occur in patients with hyperthyroidism, vitamin D deficiency, hypophosphatemia, inherited metabolic disorders such as Hurler disease, and hematologic disorders such as sickle cell anemia and thalassemia.
Microcephaly is defined by a head circumference more than two standard deviations below the mean for a given demographic (age, gender, race). Since small head size is usually an indication of the underlying brain size, it is associated with a variety of neurological disorders.
This physical finding does not guarantee abnormal development since approximately 2% of otherwise healthy school children will have a small head size with a head circumference more than two standard deviations below the mean size. The head circumference should be monitored along with the height and weight. Obviously a small child with a small head circumference would be less worrisome than a small head circumference associated with average height and weight.
Craniosynostosis has numerous subtypes depending on which sutures are involved in the premature closure (Table 8-1). Oxycephaly is associated with the most severe neurological consequences, including increased intracranial pressure, anosmia, optic atrophy, weakness, and spasticity. Patients with Crouzon syndrome or craniofacial dysostosis have premature closure of multiple sutures, hypertelorism, high forehead, exophthalmos, choanal atresia, and relatively intact intelligence.
| Suture Prematurely Closed | Name of Subtype |
|---|---|
| Sagittal | Scaphocephaly or dolichocephaly |
| Sagittal and coronal | |
| Sagittal and lambdoid | |
| Bilateral coronal | Acrocephaly |
| Unilateral coronal | Plagiocephaly |
| Metopic | Trigonocephaly |
| All in vault | Oxycephaly |
After the presence of microcephaly has been confirmed, additional neuroimaging is warranted. Routine radiography gives an excellent view of the bony sutures. CT provides information regarding calcifications associated with infectious and metabolic causes of microcephaly. Finally, MRI provides information regarding neuronal migration disorders, heterotopias, and schizencephaly.
Primary microcephaly and secondary microcephaly associated with hypoxia, infarction, trauma, in utero infections, and metabolic disorders do not have any direct treatment other than supportive measures. Craniosynostosis secondary to hyperthyroidism or vitamin deficiencies is rarely as severe as the primary forms and should be treated via the management of these disorders.
Craniosynostosis involving multiple sutures is treated surgically to prevent increased intracranial pressure.11 Surgery for craniosynostosis involving only one suture is much more controversial since this is typically a cosmetic procedure.
As with microcephaly, macrocephaly is easily identified by monitoring the head circumference. Macrocephaly is defined by a head circumference more than two standard deviations above the mean for a given demographic (age, gender, race). Unfortunately, the skull enlargement occurs after significant enlargement of the ventricles in cases of obstructive hydrocephalus.
