Ventriculomegaly

Definition

Fetal VM is an ultrasound (US) finding that refers to dilatation of the lateral ventricles of the brain, typically defined as greater than 10 mm at the level of the atria, with or without dilatation of the third or fourth ventricles. In the pediatric literature, the term hydrocephalus usually refers to expansion of the ventricular system associated with increased cerebrospinal fluid (CSF) pressure, secondary to a disturbance in the dynamics of CSF circulation. Because it is impossible to measure CSF pressure in utero prenatally, this condition is preferably referred to as VM.

The ventricular atrial width is roughly constant after 15 weeks' gestation. VM is defined as a width at the level of the atria of the lateral ventricle greater than 10 mm; this corresponds to about 4 standard deviations above the mean of 7.6 mm. Most authors consider an atrial width of less than 10 mm normal, a width between 10.0 and 15.0 mild VM, and a width greater than 15 mm severe VM. However, definitions vary in the literature. The terms mild, moderate, and severe VM for atrial widths 10 to 12 mm, 13 to 15 mm, and greater than 15 mm are also often used ( Figs. 43.1 and 43.2 ).

Prevalence and Epidemiology

Because the incidence of VM depends on the definition used, it has been reported to range from 1 : 50 to 1 : 1600. The incidence of severe VM is about 1 : 1000 newborns. Discrepancies in reported incidences are also due to differences in measuring techniques or gestational age at examination, assessment of only one hemisphere, a lack of precise correlation in the diagnosis between prenatal and neonatal life, and the possible prenatal “resolution” of the condition. Male fetuses are diagnosed with VM slightly more often than female fetuses.

Etiology and Pathophysiology

Many cases of VM are due to an imbalance between production and absorption of CSF caused by an obstructive event. CSF is formed mainly at the level of the choroid plexus and flows from the lateral ventricles through the foramina of Monro into the third ventricle, and then via the aqueduct of Sylvius into the fourth ventricle. At this level, it passes through the foramen of Magendie (in the roof of the fourth ventricle), or through the foramina of Luschka (at the sides of the fourth ventricle) into the subarachnoid space around the cerebral hemispheres. From there, CSF is reabsorbed by the arachnoid granulations that are mainly distributed along the superior sagittal sinus. If there is obstruction in the ventricular system, VM is defined as noncommunicating or obstructive . The obstruction can occur at different levels of the ventricular system, and different etiologies are known—either congenital, or secondary to intrinsic or extrinsic lesions. Generally, the more distal the obstruction is from the foramina of Monro, the greater the degree of the involvement of the ventricular system (see Fig. 43.2 ).

Noncommunicating VM includes the unilateral and bilateral obstruction of foramina of Monro, congenital narrowing of the aqueduct of Sylvius, aqueductal forking, or the presence of a septum within the aqueduct. Some cases of aqueductal stenosis may be inherited as an X-linked recessive trait or, more rarely, as an autosomal recessive disease.

Ventricular obstruction can also be caused by intrinsic processes such as inflammation, secondary to infections such as cytomegalovirus and toxoplasmosis ( Fig. 43.3 ); by hemorrhage; or by other extrinsic events such as tumors, arachnoid cysts, or malformations of the CNS that impair the normal circulation of CSF between the ventricular and the extraventricular system. Open neural tube defects and posterior fossa abnormalities are also often associated with VM ( Fig. 43.4 ).

The term communicating VM describes conditions such as sagittal sinus thrombosis, in which the cause is outside the ventricular system and a communication with at least part of the subarachnoid space is possible, or conditions with increased CSF production, such as choroid plexus papilloma without an obvious obstruction. VM can also develop without obstruction, in prosencephalic developmental anomalies (e.g., holoprosencephaly, agenesis of the corpus callosum [see Fig. 43.5 ]) and disorders of neural proliferation (e.g., microcephaly, megalencephaly) or neuronal migration (e.g., lissencephaly, schizencephaly) ( Chapter 34, Chapter 36, Chapter 39 ). Finally, VM can be the consequence of acquired destructive processes of the brain, resulting from vascular accidents or infections ( Chapter 40 ).