Family doctor

What is Down's Syndrome?

Human cells consist of 46 chromosomes which contain the genetic material which makes us all unique, e.g. have blue eyes, brown hair etc. These 46 chromosomes are made up of 23 pairs. The most common abnormality of chromosomes is a condition called Trisomy 21 or Down's Syndrome. Discovered by John Down in the late 19th century.

Trisomy 21 is when the cells have 22 normal pairs of chromosomes but 3 chromosomes of the number 21. This form of DS is the most common, but a small percentage of DS individuals have some normal cells between and are called mosaic.

Physical characteristics of DS:

*Note: All these characteristics do not appear in all DS individuals.

• Facial: flat face, upward and slanted eyes, epicanthic folds (skin folds at the eyes), speckled irises (part of the eye) called Brushfield spots. High arched palate.
• Hands: Simian crease (continuous line in the palm of the hand), short broad hands, underdevelopment of middle portion of the 5th finger resulting in the finger bending towards the outside of the hand.
• Cardiac malformations - discussed later in this article.
• Pelvis: Abnormal shape of the pelvis - flattened.
• Intestine: underdevelopment.
• Short stature with height and skull circumference less than the normal populations.

Mentally: The intellectual handicap of DS individuals varies, with the mosaic types having the highest IQs.

Incidence of DS:

The incidence of DS in the population is very linked to maternal age as is shown in the table below:

Screening for DS:

A variety of tests can be carried out on pregnant women to determine the presence of DS:

1. Amniocentesis consists of sampling of some of the amniotic fluid which surrounds the fetus.
2. Chorionic villus sampling (CVS) consists of sampling the placenta which is sampled through the cervix.
3. Maternal blood tests for maternal serum alpha-fetoprotein and unconjugated oestriol are low when the fetus has DS. Human chorionic gonadotrophin (HCG) which is an indicator for pregnancy, is excessively raised in the presence of DS.
4. Ultrasound study of the fetus's nuchal (neck skin) fold thickness and long-bone length.

Numerous criteria are considered in deciding which test(s) are appropriate to screen for DS.

Problems developing in DS individuals:

1. Thyroid disease.
2. Cardiac (heart) defects.
3. Respiratory problems.
4. Macroglossia (large tongue).
5. Leukemia.
6. Alzheimer's disease.
7. Atlanto-axial instability.

1. Thyroid disease:

The thyroid gland is in the front of the neck and controls the general metabolism of the body. In DS thyroid dysfunction can be clinically more difficult to diagnose.

Hypothyroidism (underactive thyroid) can present with slow speech, thick tongue, slow thinking, fatigue, constipation, dry skin, poor concentration. All these features can be found in DS individuals with normal thyroid functions.

2. Cardiac defects:

The normal heart is divided into 4 chambers. 2 atria on top and 2 ventricles below. The left and right sides of the heart are separated by a septum (wall) and valves separate the upper and lower chambers.

The common cardiac defects in DS are: openings between the 2 atria, openings between the 2 ventricles, openings where all 4 chambers meet, persistent patent ductus arteriosus which is part of the circulation in a fetus which should disappear at birth.

3. Respiratory problems:

DS individuals, particularly under the age of 5 years, are prone to respiratory disease. Their lack of muscle tone, poor cough reflex and reduced immunity make them more prone to these infections and prolong recovery time once infected. Common causes for hospitalisation are due to pneumonia, bronchiolitis, croup and asthma.

4. Macroglossia:

Macroglossia is a large tongue. Macroglossia can interfere with breathing and cause difficulty swallowing. Also, it can be cosmetically unacceptable and the young DS individual can often be seen with their tongue protruding. Surgery is available for this condition.

5. Leukemia (cancer of the blood cells):

Two common types of leukemia that occur in DS are : Acute lymphocytic leukemia and Acute megakaryoblastic leukemia.

6. Alzheimer's disease:

Alzheimer's disease is a condition resulting in general disintegration of intellectual abilities. It has been found that DS individuals are more prone to Alzheimer's disease and at a younger age than the rest of the population.

7. Atlantoaxial instability:

The neck bones are called the cervical spine and the first 2 bones, or vertebrae, are called the Atlas and the Axis. DS individuals are susceptible to instability of these 2 vertebrae, called atlantoaxial instability. Spinal cord compression may be a consequence of atlantoaxial instability.

Screening tests for individuals with DS:

1. Thyroid testing. Due to the problems with abnormal thyroid detection as mentioned above, regular testing should be carried out. At 6 months of age, again at 12 months and then annually. A simple blood test is required.
2. X-ray of neck: Physical activities, e.g. contact sport, could affect these vertebrae. X-rays should be obtained at 5-8 yrs of age, 10-12 yrs and at 18 yrs.

Future of DS individuals:

Today people with DS live at home with their families and seldom in institutions. They are active participants in the educational, vocational, social and recreational activities of the community. They are integrated into the regular educational system and take part in sports, camping, music, art programmes and all the other activities of their communities. In addition they are socialising with people with and without disabilities. As adults they are obtaining employment and living in group homes and other independent housing arrangements.

General comments:

This article gives only a basic overview of Down Syndrome. Should you have concerns about your pregnancy contact your general practitioner to discuss the screening tests appropriate for you.

If you have a child with DS take your child to your GP early on when they get an infection as the DS individual can have a respiratory infection for some time before they present with a serious cough. Also, ongoing ear infections can reduce their hearing and prevent intellectual development.