Down Syndrome

Dr Scott Salisbury Obstetrician & Gynaecologist Watkins Medical Centre Level 3 225 Wickham Terrace Brisbane Q 4000

Appointments 07 3010 2121 After Hours 07 3899 4455

©Dr Scott Salisbury 2010 Down Syndrome WHAT EXACTLY IS DOWN SYNDROME?

Down Syndrome is a genetic condition involving an extra chromosome, which means that the person with Down Syndrome has 47 chromosomes instead of the usual 46. The extra chromosome is chromosome 21 and therefore it is occasionally called Trisomy 21. It is a result of a genetic accident before, during or soon after conception. The reason for this accident in the vast majority of people is unknown.

HOW DOES DOWN SYNDROME AFFECT PEOPLE?

Everyone is affected differently. However people with Down Syndrome have distinctive physical characteristics shared by others with the condition. They also have developmental difficulties in that they are slow to develop and learn more slowly when compared with other children. There is also a range of physical disabilities which are not found in all children with Down Syndrome. For example, 40% of babies born with Down Syndrome are born with heart defects which can be serious and may require surgery. Hearing, vision and intestinal problems are also more common than ordinarily seen in the population.

WHAT IS THE INCIDENCE OF DOWN SYNDROME?

The overall incidence of Down Syndrome is about 1:800 live born children. This ratio is consistent across all races as well as socio-economic groups and is not dependant on geographical or environmental factors. Down Syndrome can at times be passed on from generation to generation, this constitutes the smallest group of affected children.The majority of Down Syndrome affected children are a chance phenomenon which increases dramatically with maternal age.

Age Related Risk of Down Syndrome AGE RISK 21 1520 23 1450 25 1350 27 1200 29 1010 30 890 31 775 32 660 33 545 34 445 35 355 36 300 37 220 38 165 39 125 40 90 41 70 42 50

©Dr Scott Salisbury 2010 Down Syndrome WHAT TESTS ARE AVAILABLE FOR SCREENING DOWN SYNDROME?

These fall into two main groups. Firstly that of screening tests and then confirmatory or diagnostic tests. The three most commonly used screening tests that are currently available are discussed although knowledge continues to be acquired about the screening for Down Syndrome. The first screening test is the 18-20 week ultrasound which you will be offered routinely. The second is a blood test commonly referred to as the triple test and the third is a early pregnancy scan at 11-13 weeks which measures a fluid filled space at the back of the neck (Nuchal Translucency Test)

Ultrasound An ultrasound at 18-20 weeks, although is a excellent screening test for a host of foetal abnormalities, is limited in its ability to screen for Down Syndrome as not all Down Syndrome children have physical defects that can be determined on ultrasound. Therefore at least 50% of Down Syndrome children that are scanned by the most experienced operators will still be missed by ultrasound alone.

The Triple Test The Triple Test is not available until after 15 weeks of gestation. It measures three proteins produced by pregnancy, alpha fetoprotein, unconjugated oestriol and human chorionic gonadotrophin in a single blood sample. This will detect approximately 60% of Down Syndrome children. There is no direct risk to the child with these blood tests although it is important to appreciate these tests are not precise. If these tests showed a significant increased chance of having a child affected by Down Syndrome, then you would be offered an amniocentesis test. The suggestion is that if your risk is greater than 1:250 on this blood test, then an amniocentesis would be offered.

Nuchal Translucency Ultrasound This early pregnancy scan at 11-13 weeks aims to measure the thickness of an area of fluid at the back of the neck which has correlation such that as the translucency area increases in thickness so too does the risk of having a child affected by Down Syndrome. As with the triple test only a probability of likelihood is given. Conversely only 70-80% of pregnancies with Down will be detected with about 5% of these people being falsely alarmed into thinking their pregnancy is affected and will have an invasive test such as amniocentesis or chorionic villus sampling unnecessarily.It is important to appreciate that if your risk is 1:1000 then this does not exclude the possibility of Down Syndrome as you could be the one. Equally if you have a risk of 1:5 which sounds on face value very high, there is still an 80% chance that your baby is in fact normal.

It can now be combined with a blood test done 4 days prior to your scan (but > 10.5 weeks) which is similar to the triple test but on this occassion it’s statistical power is combined with that of the neck lucency thickness to hopefully pick up 90% of Down Syndrome.

©Dr Scott Salisbury 2010 Down Syndrome My suggestion is if you are considering having these tests as an easy and simple way of increasing your detection rate of Down Syndrome, you should also think about its consequences. By that I mean you should consider whether you would have an amniocentesis/CVS if you were at risk by these tests and also whether you would consider termination of pregnancy. It is important to appreciate that amniocentesis and chorionic villus sampling are done to confirm the presence or absence of Down Syndrome and they have a small rate of miscarriage at 1:200 and 1:50 respectively. Any termination that ensured would often be done at least at 18 weeks of gestation and would need a labour type process. If you would not consider either of these management options, I would question the value of having the blood test in the first place given the subsequent emotional decisions if you were to be screened and then considered at risk.

THE DIAGNOSTIC TESTS

The only way of being certain if an unborn child has Down Syndrome is by detecting the presence of an extra chromosome in the baby’s cells. This therefore involves taking a sample of cells either from the amniotic fluid (the fluid around the baby) or indeed from the placenta. These are called amniocentesis and chorionic villus sampling (CVS) respectively.

AMNIOCENTESIS

This is now a well established and widely used technique at 16 weeks. An ultrasound scan is used to check the position of the baby in the womb. A needle is then inserted through the mother’s abdominal wall into the womb and a sample of amniotic fluid is taken. The fluid contains cells from the baby which can be grown in the laboratory. It is a relatively painless procedure. It is usually the local anaesthetic that is used to numb the area that is the most disagreeable. Results from this can take upwards of three weeks for results but usually a result is to hand within two weeks. This test is almost 100% accurate. There is a small chance that the cells may not grow in the laboratory and that would constitute less than 1% of these cases. There is a risk of miscarriage which is about 1:200. Termination of pregnancy usually is via a labour process.

CHORIONIC VILLUS SAMPLING (CVS)

A similar procedure which requires an ultrasound to guide a needle into the placenta via the abdominal route and a small piece of the developing placenta is removed. This contains actively growing cells and therefore a result is usually available within 10 days or so. It is usually performed at 11-13 weeks. CVS has a slightly higher rate of miscarriage of 1 to 2% and again has the same risks of failing to grow the cells.Termination can often be performed by a surgical procedure. There is also the special risk associated with the CVS of the fact that occasionally the cells obtained do not accurately reflect the pathology in the baby and a further follow up amniocentesis may be required to clarify the result. It is important you give careful consideration of all the facts and full discussion as a couple is important before coming to the final decision.

Chorionic Villous Sampling

Amniocentesis