Prenatal ultrasound scans showing isomerism

Isomerism is a congenital (present at birth) disorder where organs develop on the ‘wrong’ or opposite side of the body to usual. This page from Great Ormond Street Hospital (GOSH) explains about isomerism detected during a prenatal ultrasound scan and what this might mean for your child.You have been given this information sheet as the scan has shown that your baby has some features suggestive of an underlying diagnosis of right or left isomerism. In addition to the detailed report from your doctor, this information sheet gives general information to help you understand the other features that can be seen when this diagnosis is made.

The human body is not symmetrical. In early development, there are certain structures and organs that develop on one side rather than the other. For example, normally the heart and stomach are on the left and the liver is more to the right.

In isomerism, an abnormality occurs very early in the development of the embryo where there is ‘confusion’ over whether structures develop on the left or the right. This may result in some of the organs being on the opposite side to usual.

Normally in the heart, there are two filling chambers (atria) at the top of the heart. The left and the right atria are not identical – they each have a characteristic appearance and features.

In isomerism there are either two copies of a left atrium (left isomerism), or two copies of a right atrium (right isomerism).

In some cases, all the rest of the heart structures are normal, but often with isomerism there are also abnormalities in the way the chambers and vessels join together and abnormalities in the size of chambers.

The details and severity of your baby’s heart abnormality will have been discussed with you by your doctor and be written in your report from clinic.

In some cases of left isomerism, the heart beat is abnormally slow (heart block). If the slow heart beat is tolerated during pregnancy then this may require a procedure to place a pacemaker during childhood. If it is very slow, the baby may become unwell in the womb and develop a condition called hydrops where fluid collects within the baby due to poor heart function.

As well as the abnormal atria in the heart, isomerism can also affect other areas of the body.

The abnormal position of the bowel increases the risk of twisting, which can result in the baby having features of bowel obstruction before or after birth. If this occurs surgery is needed to untwist the bowel and fix it in position. It may be recommended, after birth, to fix the bowel in position to reduce the risk.

The spleen can be important in protecting the baby against some types of infection. If the spleen is absent or not working normally, a daily dose of antibiotics (penicillin) would be needed to protect against bacterial infection. The information sheet about removal of the spleen (splenectomy) also explains the implication of having a missing or dysfunctional spleen

This is relatively rare but an important condition sometimes seen with left isomerism. This would require an operation over the first few months of life and long term follow up by the liver team. Information about biliary atresia is available from the Children’s Liver Disease Foundation.

After birth, your baby will be referred to relevant specialists for related conditions which don’t involve the heart.

The risk of having another pregnancy with isomerism is approximately 5 to 10 per cent. Your local hospital can refer you to a genetic counsellor before planning another pregnancy. This would be particularly important if another family member has any abnormality of the position of the heart as this may increase the risk.