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Normal head growth is affected by the growth and alteration of structures within the cranium (brain, blood, cerebrospinal fluid, and bone) and the timing of these changes in relation to closure of the fontanels. Standards have been determined for head growth in healthy children up to 18 years of age, and the Centers for Disease Control and Prevention has constructed revised standard head circumference for age, based on national survey data from the general US population.1 In addition, special adjusted growth curves have been developed for children with specific underlying medical conditions, such as neurofibromatosis type 1.2

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Deviations from normal head growth may be the first indication of an underlying congenital, genetic, or acquired problem. Because of the rapid pace of neuronal and glial cell proliferation and the relative restriction in time during development, it is understandable that an appropriately timed insult could result in profound effects on the number of neurons that result.3 For example, insults that are sustained before embryonic day 40, when the number of neurons is increasing exponentially, would be expected to have a larger impact than those that occur after day 40, when postmitotic neurons have already been formed. A number of disorders exist that presumably represent excessive or deficient production of neurons and glia.

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Macrocephaly typically refers to a head circumference that is greater than two standard deviations above the mean and can be caused by an increase in size of any of the intracranial contents (brain, cerebrospinal fluid, blood or bone). Megalencephaly refers to an abnormally large and heavy brain. Macrocephaly can be classified as anatomic or metabolic, distinguishing increases in head size resulting from an increase in the size or number of brain cells from increases in head size that result from the deposition and subsequent accumulation of metabolic products, without a concomitant increase in the number of cells. A large brain that results from cerebral edema or neoplasia is not classified as megalencephaly.

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Familial macrocephaly is the most common type of anatomic macrocephaly, and refers to an autosomal dominantly inherited increased head size associated with normal somatic size, development, and neurologic examination.4,5 The next most common etiology for anatomic macrocephaly is increased intracranial pressure with hydrocephalus (see Chapter 553). Macrocephaly may also be associated with a variety of cutaneous or systemic anomalies, such as neurofibromatosis, tuberous sclerosis, linear sebaceous nevus syndrome, and hypomelanosis of Ito.6,7 Lastly, macrocephaly may also be associated with abnormalities of somatic size, such as cerebral gigantism (Sotos syndrome)8,9 and Beckwith-Wiedemann syndrome.10

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Although most metabolic disorders result in decreased head size, several metabolic disorders are characterized by progressive macrocephaly.11 Examples include aminoaciduria and leukodystrophies, such as Alexander disease,12 Canavan disease,13 and lysosomal storage disorders (Tay-Sachs, mucopolysaccharidosis and gangliosidosis).14

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Microcephaly, by contrast, refers to a head circumference that is greater than two standard ...

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