Agenesis of the corpus callosum may be partial or complete and may occur in an isolated fashion or in association with other anomalies of cellular migration. Dandy-Walker malformation is diagnosed on the basis of the classic triad: complete or partial agenesis of the cerebellar vermis, cystic dilation of the fourth ventricle, and enlarged posterior fossa. There may be associated hydrocephalus, absence of the corpus callosum, and neuronal migration abnormalities. Intelligence may be normal or impaired, depending on the degree of associated cerebral dysgenesis.
Holoprosencephaly represents varying degrees of failure of the forebrain (prosencephalon) to divide into two distinct cerebral hemispheres. Holoprosencephaly is often associated with midline facial defects (hypotelorism, cleft lip, cleft palate). This anomaly may be isolated or associated with a chromosomal or genetic disorder. The prognosis for infants with severe (alobar)holoprosencephaly is uniformly poor, but those with milder forms (semilobar, or lobar) may have less severe neurological outcomes. Children with trisomy 13 and trisomy 18 characteristically have varying degrees of holoprosencephaly.
Hydranencephaly is a condition in which the brain presumably develops normally, but then is destroyed by an intrauterine, probably vascular, insult. The result is virtual absence of the cerebrum with an intact skull. The thalamus, brainstem, and some occipital cortex are typically present. Children may have a normal external appearance at the time of birth, but do not achieve developmental milestones.
Macrocephaly represents a head circumference above the 97th percentile and may be the result of macrocrania (increased skull thickness), hydrocephalus (enlargement of the ventricles; see Chapter 184), or megalencephaly (enlargement of the brain). Diseases of bone metabolism or hypertrophy of the bone marrow cause macrocrania. Megalencephaly may be the result of a significant disorder of brain development or an accumulation of abnormal metabolic substances (Table 1871). Most often, however, macrocephaly is a familial trait of no clinical significance. If the child is developmentally normal but has macrocephaly, plotting the parents’ head circumference on a growth chart can provide reassurance and help to avoid unnecessary neurodiagnostic testing.
Microcephaly represents a head circumference below the 3rd percentile. Myriad syndromes and metabolic disorders are associated with microcephaly, some of which are hereditary (Table 187-2). In most instances, a small head circumference is a reflection of a small brain. Brain growth is rapid during the perinatal period, and any insult (infectious, metabolic, toxic, vascular) sustained during this period or during early infancy is likely to impair brain growth and result in microcephaly. Rarely, a small head is the result of premature closure of one or more skull sutures, called craniosynostosis. This diagnosis is readily made by the abnormal shape of the skull. As a rule, macrocephaly and microcephaly raise a concern about cognitive ability, but head circumference alone should never be used to establish a prognosis for intellectual development.
Macrocrania (increased skull thickness) Achondroplasia Hypochondroplasia Fragile X syndrome Osteopetrosis Chronic, severe anemia
Hydrocephalus (enlargement of the ventricles; see Chapter 184) Masses
Megalencephaly (enlargement of the brain) Embryologic disorder causing abnormal proliferation of brain
Accumulation of abnormal metabolic substances Alexander disease Canavan disease Gangliosidoses Mucopolysaccharidoses
Benign extracerebral collections of infancy Familial macrocephaly
Disorders of Neuronal Migration