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Unilateral ventriculomegaly between 10 and 14 mm

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: Borderline ventriculomegaly (up to 10 mm). Ventriculomegaly in the range of 10 - 14 mm is considered mild (see information sheets: Bilateral ventriculomegaly of up to 14 mm and Unilateral ventriculomegaly of up to 14 mm). Dilation in excess of 14 mm is significant dilation, especially if there is an increase in the ventricular width from week to week. See information sheet: Ventriculomegaly in excess of 14 mm and/or progressive ventriculomegaly - hydrocephalus. There are no major differences between unilateral and bilateral ventriculomegaly, but the unilateral type is slightly more benign. Ventriculomegaly in the range of 10 - 14 mm is considered mild and can be unilateral or bilateral. The bilateral type is covered in the information sheet: Bilateral ventriculomegaly of up to 14 mm. The discussion here is of unilateral ventriculomegaly, where there is asymmetry between the sizes of the two ventricles, with one being at least 10 - 14 mm wide.

The common causes are:

  • In some cases, it may be a variation of the norm.

  • In a relatively small number of cases there may be unilateral obstruction of the fluid passages out of the ventricle on one side because of bleeding and the formation of a clot in the drainage outlet. The fluid formed in the ventricle then accumulates, and results in ventriculomegaly with an increased head circumference.

  • 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. This is possible but has not been proved. 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 that is usually bilateral, there is an increased risk of fetal developmental retardation.

  • Chromosome disorders. In cases where the ventriculomegaly is an isolated finding, the risk of an associated chromosome disorder is 4%; however a more careful analysis of cases indicates that the true risk is lower.

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 mild unilateral ventriculomegaly?

In about 30% of cases, the ventriculomegaly is associated with other defects in the brain or in other tissues, or with chromosome disorders or other anomalies.

In such cases, there is a high risk that the child will have a developmental neurological problem and/or other problems.

It is therefore important to refer the family for genetic and other professional counseling in order to discuss the ramifications of the condition and what the future will hold.
The risk for chromosomal problems is about 4%. These can be detected by amniocentesis. In most cases in which a chromosomal problem is detected, there are additional reasons for performing amniocentesis, such as advanced maternal age or increased risk for Down syndrome based on screening tests ( biochemical screening or increased nuchal translucency). It is open to question whether a 35-year-old woman with normal screening test results that indicate a low risk for Down syndrome should undergo amniocentesis based solely on the presence of mild ventriculomegaly. It must be assumed that the risk for a chromosomal problem in this case is not very high, but because there is no definitive answer to this question, there is apparently an indication for amniocentesis that is not based solely on the presence of unilateral ventriculomegaly.

What are the risks associated with mild unilateral ventriculomegaly when no other defects are found on ultrasound examination and no further expansion of the ventriculomegaly is detected on monitoring?

In about a third of the cases, it resolves by birth. Most researchers agree that the risk for developmental problems or convulsions after birth is not significantly higher than that in a regular pregnancy. The problem is that up to now too few cases have been described in the medical literature to be able to estimate from these whether or not there is a slight increase in the risk for convulsions (some of the patients had epilepsy) or developmental delay. It should be noted that there is a risk of 3% - 5% for defects and mental retardation in each pregnancy. Because follow-up studies of development in children born with mild unilateral ventriculomegaly have not been undertaken for long enough to know what the long-term consequences are, it is not known whether these children have learning difficulties or milder developmental disorders. The estimate is that the frequency of such disorders is low, if it exists at all.

What should be done if mild unilateral ventriculomegaly is diagnosed?

  • 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 monitor the width of the ventricles and the diameter of the head once every two to three weeks 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 mild 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 in 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.

  • 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 bilateral ventriculomegaly. For unilateral ventriculomegaly, in the absence of other defects detected on ultrasound examination, it is not currently recommended that MRI be performed. It can be done privately, however, but it should be kept in mind that it is on trial status in these cases.
 
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