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Isomerism
This leaflet explains more about isomerism. If you have any further questions, please
speak to a doctor or nurse caring for you.
What is isomerism and why has my baby got it?
Isomerism is the name for a condition associated with complex congenital (present at birth)
heart disease.
Normally the major organs of the body develop and remain either on the left or the right side of
the body. For example, the heart and stomach normally lie on the left, and the liver on the right
side of the body. In isomerism there is confusion between left and right.
The upper collecting chambers of the heart, known as the atria, are normally formed differently
in order to fulfil their different functions. The right atrium collects deoxygenated blood from the
body via two large veins called the inferior and superior vena cava. The left atrium collects
oxygenated blood from the lungs via four pulmonary veins. The lower chambers, known as the
ventricles, are the pumping chambers. The right chamber pumps blood to the lungs and the left
chamber pumps blood around the body.
In a baby with isomerism, there are either two right atria (right atrial isomerism), or two left atria
(left atrial isomerism), instead of one of each. In addition, there are usually other associated
structural defects of the heart. The most common defect in both types of isomerism is an
atrioventricular septal defect (AVSD). This is where all the four chambers of the heart are
connected by one large hole, and there is a single common inlet valve, instead of two separate
inlet valves.
There may be a number of other problems with the baby’s heart, which will be explained by the
doctor. These include stenosis (narrowing) or a complete blockage of the pulmonary valve (to
the lungs). The pulmonary veins (from the lungs back to the heart) may also be abnormal.
Sometimes there are two pumping chambers (ventricles) of equal size but often one of the
ventricles is much larger than the other.
The heart beat begins with electrical impulses in the right atrium. Babies with left atrial
isomerism may have a slow heart rate, or heart block, when the impulses originating in the right
atrium are not transmitted to the ventricles, resulting in the ventricles beating independently at a
much slower rate. In severe cases of heart block, babies in the womb may develop fluid around
the heart and other organs, known as heart failure. If this happens, the risk of losing the baby
before birth is increased. If the heart rate remains slow after birth, a pacemaker may be
required.
In isomerism, the confusion between the right and left sides is not exclusive to the heart, and
other organs may be affected. This may include:
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The bowel: the baby’s bowel may take up an abnormal position known as malrotation of the
bowel. Surgery may be required to prevent the bowel from twisting.
The spleen: the spleen is important for a baby’s ability to fight some types of infection. Babies
with isomerism can have multiple spleens or no spleen. If the spleen is absent then regular
penicillin may be advised to prevent some types of infection.
The liver: in rare instances, the system that produces bile may not work properly (biliary
atresia). Bile is needed for digestion and this will need surgery soon after birth and ongoing
monitoring.
Will the baby need any tests after birth to confirm the diagnosis?
Isomerism may vary in its severity, but can be associated with complex heart problems. Nonheart related problems involving the bowel, liver and spleen may also exist. Investigations after
birth will be required to identify some of these associated conditions.
What treatments are available?
The exact treatment strategy, including surgery, will be determined by each individual condition.
Is there any risk in future pregnancies?
For future pregnancies, the overall risk of the same problem happening again is five to 10 in one
hundred. Therefore it may be important for you to consider genetic counselling before future
pregnancies.
Useful sources of information
Antenatal Results and Choices (ARC). ARC is a registered charity that offers continued
support and advice to parents who face difficult decisions regarding fetal abnormalities. They
have extensive experience in helping parents at this time and aim to offer both support and
advice in meeting your unique needs.
t: 020 7631 0285 w: www.arc-org.uk
Evelina Children’s Heart Organisation (ECHO). ECHO is a charity that supports the families
of children with congenital heart disease who receive treatment at the Evelina Children’s
Hospital. They have a rota of trained prenatal support volunteers who offer comfort and
reassurance at this stressful time. ECHO also sponsors specialist antenatal classes for parents
of heart babies.
t: 07730 146154 e: [email protected] w: www.echo-evelina.org.uk
Little Hearts Matter (LHM). LHM is a national charity that offers support and information to
families where a single ventricle heart condition has been diagnosed.
t: 0121 455 8982 e: [email protected] w: www.lhm.org.uk
Heartline. Heartline is a national charity that provides support and help for children with
congenital heart conditions and their families.
t: 01276 707636 w: www.heartline.org.uk
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Children's Heart Federation (CHF). CHF is a national charity and the umbrella body for
voluntary organisations working to meet the needs of children and young people with congenital
and acquired heart conditions and their families.
Freephone infoline: 0808 808 5000 (Monday to Friday, 9.30am-4.30pm)
w: www.childrens-heart-fed.org.uk
British Heart Foundation (BHF). BHF is a national charity providing information and support
on anything heart-related.
Helpline: (local rate) 0300 330 3311 w: www.bhf.org.uk
Contact us
If you have any questions or concerns about isomerism please contact Fetal Cardiology on
020 7188 2308 (Monday to Friday, 9am to 5pm).
Patient Advice and Liaison Service (PALS) – To make comments or raise concerns about the
Trust’s services, please contact PALS. Ask a member of staff to direct you to the PALS office or:
t: 020 7188 8801 at St Thomas’ t: 020 7188 8803 at Guy’s e: [email protected]
Knowledge & Information Centre (KIC) – For more information about health conditions,
support groups and local services, or to search the internet and send emails, please visit the
KIC on the Ground Floor, North Wing, St Thomas’ Hospital.
t: 020 7188 3416
Language support services – If you need an interpreter or information about your care in a
different language or format, please get in touch using the following contact details.
t: 020 7188 8815 fax: 020 7188 5953
NHS Direct - Offers health information and advice from specially trained nurses over the
phone 24 hours a day.
t: 0845 4647 w: www.nhsdirect.nhs.uk
NHS Choices – Provides online information and guidance on all aspects of health and
healthcare, to help you make choices about your health.w: www.nhs.uk
Leaflet number: 3651/VER1
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Date published: April 2013
Review date: April 2016
© 2013 Guy’s and St Thomas’ NHS Foundation Trust