Carrier screening: Spinal Muscular Atrophy (SMA)

RAISING A healthy child to adulthood is an utmost priority to parents looking to start a family and preconception genetic screening is a vital step towards achieving that goal. By getting tested, parents who are carriers of genetic diseases can prevent passing on these life-threatening conditions to their children.
Spinal Muscular Atrophy (SMA) is a genetic disease affecting 1 in 10,000 live births with a carrier frequency of 1 in 50. Most carriers display no symptoms. Therefore, it is vital that couples learn more about the disease and take the necessary steps toward screening and prevention.
SMA is a severe neuromuscular disease which degenerates the neurons in the spinal cord. This leads to extreme muscle weakness and paralysis. After cystic fibrosis, SMA is the second most common recessive disorder found in infants. The condition is passed down to infants through genetics.
Most infants affected with SMA do not display any unusual symptoms after birth. Symptoms usually surface around three months of age when the baby begins to display symptoms of the “floppy child” syndrome, meaning that the infant will not be able to move, cough or breathe normally. This condition then leads to chest infections and respiratory failure. In most cases, the child dies during the first year of life. Other symptoms of SMA include:
l Inability to chew or swallow food
l Inability to sit
l Inability to crawl or walk
l Inability to cry loudly
l Inability to breastfeed
There are the three types of SMA:
l Type 1 SMA – muscle weakness and hypotonia (“floppy baby” syndrome) which occurs at birth or during the first three months of life. Respiratory failure usually leads to death before the child turns one year old. To prolong the child’s life, a hole in the airway can be opened (a tracheostomy) to help connect the child to a respirator. 
l Type 2 SMA – symptoms begin after six months. The child cannot walk or stand, but is able to sit and usually lives beyond four years of age.
l Type 3 SMA – a milder form of SMA occurring during or after the first year of life. Patients can walk unaided and live until adulthood.
SMA affects all populations regardless of ethnicity or race. Most carriers of SMA are silent carriers; they do not display any unusual symptoms and have no family history of SMA. However, there is a 25-per-cent chance that a child born from carrier parents will be affected with the disease. Therefore, SMA carrier testing is highly recommended. Getting tested before pregnancy or during early pregnancy can help couples make informed choices going forward.
The carrier test requires only a tiny sample of blood from the patient. The test result is returned within the week. You can ask for the carrier test from your OB/GYN. If you have further questions, you might want to meet and consult with genetic counsellors or geneticists at the hospital.
If the test reveals that you or your partner is carriers for SMA, discuss with your doctor how to proceed. Being carriers for SMA does not necessarily mean an end to your dream of having children. Here are the options your doctor might present you with:
l Start normal conception and take the risk. At four months of pregnancy, a prenatal diagnosis will be carried out. A needle will be used to pick up fluids around the infant. If the baby is affected by SMA, parents can choose to terminate the pregnancy or they can choose to proceed with it.
l Preimplantation genetic diagnosis (PGD) – for couples who do not want to risk normal conception. The PGD process can be applied during in vitro fertilisation (IVF). It identifies gene mistakes in embryos, before transferring only healthy ones back into the mother’s womb.
l Adoption
For each option, the doctor will provide you with information on what you should expect.

DR OBJOON TRACHOO is a Geneticist attached to the Women’s Health Centre of Samitivej Sukhumvit Hospital. |Call (02) 022 2555-6.