The central nervous system is made up of the brain and spine. The surfaces of these structures are bathed in cerebrospinal fluid (CSF) which helps protect them. CSF is produced deep within the brain and flows through a system of interconnecting cavities called ventricles to coat the surface of the brain, spine and major nerve roots where it is resorbed. The total volume of CSF is turned over at least three times on a daily basis.
The paired lateral ventricles are imaged routinely as part of the ultrasound assessment of the fetal brain at 18 to 20 weeks gestation. The size of these ventricles is recorded at this examination, with a measure less than 10mm considered normal. When one or both of the lateral ventricles are dilated and measure larger than normal, it is termed ventriculomegaly.
Ventriculomegaly results from many different causes, which lead either to overproduction or under resorption of CSF. Common causes of ventriculomegaly include structural brain abnormalities, infection, chromosomal abnormalities (e.g. Down syndrome) and tissue injury.
Ventriculomegaly affects approximately one in every 500 babies born in Australia with boys being almost twice as likely to be affected as girls.
Types of ventriculomegaly
Ventriculomegaly is usually described as being mild, moderate or severe based on the measured size of the lateral ventricles. If there are no other abnormalities or specific causes identified at the time of the examination, it is described as isolated. However, ventriculomegaly is often associated with other abnormalities.
Mild ventriculomegaly: the ventricles measure between 10–11 mm.
Moderate ventriculomegaly: the ventricles measure between 12–14 mm.
Severe ventriculomegaly: the ventricles measure 15 mm or more.
To best counsel parents about the likely outcome of a baby with ventriculomegaly, it is important to investigate so that the cause and any associations are identified wherever possible. Approximately 40 per cent of babies with mildly dilated ventricles have some associated abnormalities, which may be structural, infectious or chromosomal in origin. Tests include:
- a detailed ultrasound assessment of your baby's anatomy to identify if there are any other associated abnormalities in the brain or body.
- maternal blood tests to identify if infection may have caused the ventriculomegaly. Infections commonly tested for include toxoplasmosis, cytomegalovirus and herpes simplex.
Your doctor may suggest the further testing, depending on the findings of the ultrasound scan.
- To identify if there are any chromosomal abnormalities. This is more likely if other abnormalities have been identified in the baby or if a Down syndrome screening test reported an increased risk.
- To identify if fetal infection has occurred. Fetal infection may follow if the mother's blood test shows she has had a recent infection.
Fetal MRI (Magnetic Resonance Imaging)
To further investigate the anatomy of your baby's brain. MRI is safe and does not rely on radiation exposure. It is performed to provide additional information about your baby. Fetal MRI is typically performed if a structural abnormality is suspected.
All this information is important to determine a cause and to establish whether the ventriculomegaly is isolated or associated with other problems with your baby. This information is crucial if appropriate counselling regarding outcome and pregnancy management is to be provided.
Causes of ventriculomegaly
This is the most common cause of ventriculomegaly accounting for 30 to 40 per cent of cases. The normal flow of CSF through the aqueduct may be blocked due to a variety of possible reasons, such as structural abnormality, infection or bleeding. With obstruction to flow, there is a build of CSF because resorption cannot occur and the back-pressure forces dilation above aqueduct in the lateral (and third) ventricles. A diagnosis of aqueductal stenosis is usually made when no other abnormalities in the brain have been found. This cause of ventriculomegaly generally has the best prognosis especially if it is isolated and non-progressive. Neurosurgery with the placement of a shunt to drain the excess CSF is required after birth. Normal outcome is reported in at least 63 per cent of cases.
Chiari II Malformation
This accounts for 25 to 30 per cent of cases of ventriculomegaly. This abnormality is a direct consequence of the primary neural tube defect, spina bifida. The spinal abnormality leads to altered CSF flow and abnormal arrangement of structures comprising the hindbrain. Spina bifida is typically easily recognised at ultrasound assessment. Neurosurgery with shunt placement is required in the majority of cases after birth. The abnormality of the spinal cord determines the outcome, which includes disability related to paraplegia and incontinence.
This is a spectrum of abnormalities affecting the structures of the hindbrain, including the cerebellum and cistern magna. It accounts for seven to ten per cent of cases of ventriculomegaly. The Dandy-Walker complex has a high association with additional structural and chromosomal abnormalities, which typically determines the outcome.
Agenesis of the corpus callosum
This is when the structure that connects the two hemispheres of the brain (the corpus callosum) is partially or completely absent. It is often difficult to diagnose before 22 weeks gestation. Callosal abnormalities account for 20 to 30 per cent of cases of ventriculomegaly. Callosal abnormalities may be associated with migrational disorders which reflect problems during brain growth and the establishment of neuronal connections. The outcome varies considerably and is affected mainly by the presence of other abnormalities.
This describes cases with no recognised cause. In particular, mild ventriculomegaly may be considered a variant of normal if isolated and not associated with progressive change over the course of the pregnancy. Outcomes are described as normal in over 93 per cent of cases.
Follow up during pregnancy
Ongoing management varies according to the severity and cause of the ventriculomegaly. Follow-up involves serial ultrasound assessment of fetal growth and of the ventricles to document any progressive change in the degree of ventriculomegaly. Progressive increase in ventricle size over the course of the pregnancy is associated with an increased possibility that the developmental outcome for your baby may not be normal.
Multidisciplinary counselling with a neonatologist (specialist for newborn babies), neurologist and/ or neurosurgeon will be arranged as appropriate to the diagnosis. Delivery planning is determined individually depending on the cause and progress of the ventriculomegaly.
Generally the recurrence risk for future pregnancies is estimated to be four per cent. However individual counselling about this is recommended as there is an increased risk for a small number of women.
If you would like more information we recommend you talk to your doctor.
Centre for Maternal Fetal Medicine
Mater Mothers' Hospital
South Brisbane Qld 4101
Phone 07 3163 1896
Fax 07 3163 1890
Mater acknowledges consumer consultation in the development of this patient information.
Last modified 16/11/2015.