Spinal Muscular Atrophy (SMA) is a genetic condition that impacts muscle movement, arising from issues within motor neurons located in the brain and spinal cord. Despite the challenges SMA presents, it is possible to detect the condition during pregnancy through various genetic testing methods. This early detection provides families with important information for informed decision-making and to prepare for a child with SMA.
Understanding Spinal Muscular Atrophy and Its Inheritance
Spinal Muscular Atrophy is a genetic disorder affecting the central and peripheral nervous systems, leading to the loss of motor neurons in the spinal cord. These nerve cells control voluntary muscle movement. When motor neurons are lost, muscles do not receive necessary signals, causing weakness and wasting.
SMA is primarily caused by a mutation in the SMN1 gene, which produces a protein essential for motor neuron function. Insufficient protein leads to motor neuron degeneration, impacting voluntary movements in areas like the head, neck, chest, and legs.
SMA is an autosomal recessive disorder, meaning a child must inherit two mutated SMN1 gene copies—one from each parent—to develop the condition. Individuals with one mutated copy are carriers and usually asymptomatic. SMA has various types, from Type 0 (most severe, prenatal onset) to Type 4 (mildest, adult onset), with differing severities and onset ages. The SMN2 gene can partially compensate for a mutated SMN1 gene; more SMN2 copies often result in less severe symptoms.
Screening for SMA Before or During Pregnancy
Carrier screening for SMA is commonly performed to identify individuals or couples at increased risk of having a child with SMA. This screening can occur before conception or in early pregnancy. It typically involves a blood or saliva sample from the parents to determine if they carry a mutated SMN1 gene.
If one parent is identified as a carrier, the other partner is usually tested. If both parents are carriers, there is a 25% chance with each pregnancy that their child will inherit two mutated SMN1 gene copies and be affected by SMA. Carrier screening accurately detects the most common SMN1 gene mutation, though it cannot detect all rare mutations. This screening is a first step, helping families understand their risk before considering more invasive diagnostic tests.
Diagnostic Testing for SMA During Pregnancy
Diagnostic tests may be recommended during pregnancy for high-risk couples, such as when both parents are SMA carriers or there is a family history. These tests confirm if the fetus has inherited the SMN1 gene mutations. Two methods are used for prenatal diagnosis: Chorionic Villus Sampling (CVS) and Amniocentesis.
Chorionic Villus Sampling (CVS) is typically performed between 10 and 13 weeks, taking a small tissue sample from the placenta for fetal DNA analysis. Amniocentesis is usually conducted later, between 15 and 20 weeks, by collecting amniotic fluid containing fetal cells. Both CVS and amniocentesis are highly accurate for detecting SMA, but they carry a small risk of complications like miscarriage, infection, or fluid leakage.
Interpreting Test Results and Next Steps
Prenatal diagnostic test results provide important information for expectant parents. A positive result confirms SMA in the fetus, while a negative result indicates the fetus does not have the condition. In some cases, the test might show the fetus is a carrier, inheriting one mutated gene but not developing SMA.
Genetic counselors play a role in helping families understand these results. They explain the specific type of SMA, its potential severity, and implications for the child’s health. Counselors also discuss various available options, which may include continuing the pregnancy with preparations for a child with SMA, palliative care, or termination of pregnancy. They provide support and connect families with resources to navigate decisions and prepare for the future.