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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 ...