Aqueductal Stenosis
The aqueduct of Sylvius is a narrow structure that allows communication between the third and fourth ventricles. The cerebrospinal fluid produced in the choroid plexus of the lateral ventricles and the roof of the third ventricle travels through the aqueduct of Sylvius to reach the fourth ventricle on its way to the subarachnoid space. Congenital aqueduct of Sylvius stenosis or atresia may be sporadic or of X-linked inheritance. X-linked inheritance (Bickers-Adams syndrome) accounts for 2% of patients with aqueductal stenosis. Boys with Bickers-Adams syndrome often have flexus adductus thumb deformity (Figure 293.1).

A	B

Figure 293.1.— Bickers-Adams syndrome. [A] Macrocephaly due to aqueduct of Sylvius stenosis with bilateral cortical thumbs; [B] flexus adductus thumb deformity.

The head of patients with congenital aqueduct of Sylvius stenosis or atresia is large from birth (Figure 293.2 [A]). Cesarean section is often required because of cephalopelvic disproportion. There is usually frontal bossing, dilated scalp veins, and splitting of the sutures. Transillumination is positive in severe cases (Figure 293.2 [B]). The eyes may deviate downward. The diagnosis is established by MRI of the brain.

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Figure 293.2.— [A] Macrocephaly due to aqueduct of Sylvius stenosis; [B] transillumination of the same patient.

Aqueduct of Sylvius stenosis may also occur with meningitis. The diagnosis is established by MRI of the brain (Figure 293.3).

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Figure 293.3.— Aqueduct of Sylvius stenosis. [A] T1-weighted images demonstrating hydrocephalus due to aqueductal stenosis. [B] Cerebrospinal fluid flow study demonstrating no flow through the aqueduct.

Dandy-Walker Malformation
Patients with Dandy-Walker malformation may present with macrocephaly during the neonatal period. Macrocephaly during the neonatal period usually results from an enlarged posterior fossa (Figure 293.4).

Figure 293.4.— [A] Large posterior fossa with ventral compression of the brainstem; [B] absence of the cerebellar vermis.

The large posterior fossa results in a dolichocephalic appearance which should be differentiated from dolichocephaly and bathrocephaly. Dolichocephaly (keel-shaped head) is due to sagittal suture synostosis. In neonates with premature closure of the sagittal suture most of the dolichocephalic appearance of the head is due to frontal prominence. Bathrocephaly (step in the head) is a normal finding in the neonatal skull that disappears after a few months. Bathrocephaly is characterized by bulging of the interparietal part of the occipital bone (Figure 293.5).

Figure 293.5.— Bathrocephaly. [A] Lateral skull film demonstrating step-like elongation of the posterior aspect of the skull. [B] The occipital bone protrudes more posterior than the parietal bone.