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Ventriculomegaly (Unilateral or Bilateral) in Excess of 14 mm and/or Progressive Ventriculomegaly - Hydrocephalus


What is this and what is its significance?

On a cross-sectional scan using the ultrasound transducer it can be seen that the brain is divided into two hemispheres. Within each hemisphere there is a ventricle (cavity or chamber) that contains fluid that lines and protects the surrounding soft brain tissue. The fluid in the ventricles is produced by spongy tissue, known as the choroid plexus, that appears to float in the ventricular cavity. Mild ventriculomegaly is defined as dilation of the lateral ventricles of the brain that does not raise the intracranial pressure and does not increase from one scan to the next. In the second and third trimester, the normal width of the ventricle (including mild ventriculomegaly without significant mental handicap) is up to 10 mm. See information sheet: Ventriculomegaly (mild dilation of the ventricles of the brain). Ventriculomegaly in the range of 10 - 14 mm is considered mild (see information sheets: Bilateral ventriculomegaly of 10 - 14 mm and Unilateral ventriculomegaly of 10 - 14 mm).

In this case, the dilation is in excess of 14 mm, which is significant, especially if there is an increase in the width of the ventricles from week to week. This is an extreme state of ventriculomegaly, usually with increased pressure on the brain tissue itself, and progressive dilation of the ventricles f

rom week to week. It is caused by an obstruction of the fluid passages out of the ventricles and the resultant increased accumulation of fluid in the ventricles. Usually this is first detected in the third trimester of pregnancy.

Causes of hydrocephalus or marked ventriculomegaly:

The common causes are:

  • Obstruction of the fluid passages out of the ventricles because of bleeding and the formation of a clot in the drainage outlets. The fluid formed in the ventricles then accumulates and results in ventriculomegaly with an increased head circumference.

  • Sometimes, especially if there is progressive ventriculomegaly, the cause is genetic. This genetic disorder, which is responsible for hydrocephalus, is more frequent in males and is usually transmitted by X-linked inheritance. There are a few cases of autosomal recessive inheritance, particularly in the Israeli Arab community.

  • If there are additional defects, there may be a more complex genetic cause such as a chromosomal syndrome.

  • Complex defects in the structure of the brain arising from genetic causes that may or may not be hereditary that disrupt the normal, complex development of the brain. In cases in which further defects are detected in the brain in particular and in other organs in general, there is an increased risk that the fetus may have a syndrome, with a significantly increased risk of mental retardation.

  • Environmental effects such as disrupted blood flow to the brain or infections (viral or other pathogen) during pregnancy. The mother-s blood can be examined to test for infection by some of the common pathogens. If the cause is found to be a viral infection that is known to cause ventriculomegaly, there is, of course, an increased risk of fetal developmental retardation.

  • Chromosome disorders. If the ventriculomegaly is an isolated finding, the risk of an associated chromosome disorder is between 2% and 5%. However, the presence of other abnormal findings on ultrasound examination significantly raises the risk - according to some reports to as high as 25%. In these cases it is worth recommending that the mother undergo amniocentesis for chromosome analysis.



It is worth noting that apart from obstruction of the fluid passages, which usually leads to a larger than average head circumference, the other causes tend to be associated with a decreased brain volume, and therefore a relatively small head circumference.

What are the risks associated with bilateral ventriculomegaly?

In some cases, the cause of the ventriculomegaly is bleeding and the formation of a clot that obstructs the drainage outlets.

In these cases, inserting a drain into the ventricles after birth can relieve the increased pressure on the brain. In other cases, the implications for mental functioning in the fetus are more severe.

The extent of the effect on functioning depends on the severity of the dilation, the presence of additional findings, the cause of the hydrocephalus, the gestational age, and the degree of progression of the ventriculomegaly.

If the ventriculomegaly is part of a syndrome, the problem is more severe. However, even if there are no other specific signs, marked ventriculomegaly constitutes a high risk for neurological problems and postnatal developmental disorders.

Inheritance pattern

This depends on what syndrome the patient has. Investigations should be undertaken to establish whether the patient has the hereditary form of hydrocephalus transmitted by X-linked inheritance that only affects males. These males have a highly specific thumb structure where the thumbs are permanently folded in towards the hand. The existence of other cases in the family should arouse suspicion.

There is also a type that is transmitted by autosomal recessive inheritance. This type is common among Israeli Arabs, particularly in consanguineous marriages.

Penetrance

This depends on what syndrome the patient has. In X-linked or autosomal recessive hydrocephalus, penetrance is full, but sometimes ventriculomegaly cannot be seen on ultrasound examination until the advanced weeks of pregnancy (after 30).

Tests that can be performed in pregnancy to demonstrate associated defects

  • A thorough ultrasound system scan should be carried out to look for additional defects in the structure of the brain or other organs. It is also important to look for defects in the genitalia, eyes, heart and growth rate. Normal fetal movement should be ascertained. It is important to examine the position of the thumb (see above under X-linked hydrocephalus thumb position). It is also important to monitor the width of the ventricles and also the diameter of the head once every two to three weeks in order to ascertain that it is not expanding more than the normal amount.

  • Investigation for intrauterine infection should be carried out by testing the mother-s blood in order to look specifically for the known teratogenic pathogens (agents that are known to infect and harm the fetus) such as TORCH, varicella-zoster virus, Epstein-Barr virus, etc.

  • Fetal echocardiography.

  • Genetic counseling should be offered in order to give the parents a thorough explanation about the implications of the finding and to plan further follow-up such as whether it is advisable to undergo amniocentesis to rule out a chromosome disorder in the fetus. As noted above, when there is ventriculomegaly, there is a definite indication to perform amniocentesis. Each case, however, must be considered individually, based on how important the pregnancy is, the week of gestation, the couple-s wishes, the presence of other defects, the results of the screening tests for Down syndrome (biochemical screening and nuchal translucency), etc. These are discussed within genetic counseling. Based on the severity of the ventriculomegaly and the presence or absence of additional findings or a family history, further follow-up and molecular or other tests should be considered.

  • In cases of mild or more severe ventriculomegaly, it is advisable to refer the parents to a multidisciplinary clinic in a hospital for professional counseling by a pediatric neurologist, a geneticist, an expert in ultrasound examinations and a neonatologist, in order to collate all the data and discuss the various possibilities and courses of treatment that are available, etc. The decisions taken will depend on the severity of the problem, the presence of additional findings, etc.

  • In the last few years, high-speed MRI machines have been developed that enable examination of the fetal brain (this could not be done before because the fetus often moves and the older MRI machines were not quick enough). Cumulative experience shows that this test has a great advantage in identifying defects that cannot be detected by ultrasound, and it is recommended in cases of mild or more severe ventriculomegaly, especially when other defects are also present. The question of whether the result of an MRI scan can be normal and there still be a high risk for a developmental problem has been asked. It can be assumed that the risk in this case is lower than that described above, but there are no definitive answers to this question.

What is the risk of recurrence in a subsequent pregnancy?

In genetic counseling, the risk for recurrence can be evaluated after a precise diagnosis has been made. It depends on the type of hydrocephalus and its cause. In X-linked hydrocephalus, the risk of recurrence is 50% for each pregnancy with a male fetus. Female fetuses are not affected. In the autosomal recessive type, which is usually rare, the risk of recurrence is 25% in every pregnancy.

The risk that more distant relatives will be affected depends on the number of affected individuals in the family, their relationship to the affected individuals, etc. This is ascertained within genetic counseling.

Molecular genetic information

The gene for the disease

This depends on the specific syndrome and/or its cause. In X-linked hydrocephalus, the gene, L1CAM, is known and mutation analysis can be performed. This gene is associated with a specific clinical picture featuring hydrocephalus, fingers contracted into the hand, absent corpus callosum (the structure that connects the two hemispheres of the brain), mental retardation and spastic muscle weakness.

The gene for the autosomal recessive type has not yet been identified.

Location

L1CAM is on the X chromosome.

Genetic testing

Diagnostic testing

This depends on the specific syndrome and/or its cause. In the X-linked type, the mutation can be identified immediately by establishing the gene sequence. See information sheet: Finding the defect (mutation) by establishing the gene sequence - X-linked disease.

Carrier testing

This depends on the specific syndrome and/or its cause. In the X-linked form, as soon as the mutation is found in a fetus or a newborn with hydrocephalus, all the females in the family can be tested for carrier status. See information sheet: Testing the disease-causing gene for mutations that are common in a specific ethnic group - X-linked diseases.

Fetal testing

This depends on the specific syndrome and/or its cause. In the X-linked form, as soon as the mutation is found in a fetus or a newborn with hydrocephalus, all the carriers in the family can undergo prenatal diagnosis for each pregnancy. The condition can be diagnosed in weeks 11 - 13 of gestation by DNA testing. See information sheet: Testing the disease-causing gene for mutations that are common in a specific ethnic group - X-linked diseases.
 
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