Spinal muscular atrophy, lower extremity, dominant (SMA-LED) is a genetic disorder that causes muscle weakness and loss of muscle tissue (atrophy) in the legs (lower extremities). SMA-LED mainly affects the muscles in the thighs (quadriceps). Symptoms usually present in childhood and slowly get worse with time (progress). Children with this condition have trouble walking well (unsteady gait) and may also be delayed in walking. They may also have difficulty with standing from a seated position.
SMA-LED is caused by a change (mutation) to the DYNC1H1 gene. This gene provides the instructions for the body to produce a protein called dynein, which helps nerve cells (neurons) that control muscle movement send chemical signals to each other. When the DYNC1H1 gene has a mutation, the dynein protein is not made and neurons cannot send signals to each other. This abnormal chemical messaging is what causes the muscle weakness in SMA-LED. As its name suggests, this disease is inherited in an autosomal dominant way. This means a mutation in only one of the two copies of the DYNC1H1 gene a person has is enough to cause the condition.
SMA-LED is usually considered in a child who has proximal muscle weakness. Genetic testing of the DYNC1H1 gene is used to confirm the diagnosis. Muscle weakness seen in SMA-LED is similar to the muscle weakness seen in other genetic conditions. A doctor may collect and examine a small muscle sample (biopsy) to look for clues for the diagnosis. There is no cure for SMA-LED, but physical therapy can be helpful in strengthening the affected muscles. If your child has been diagnosed with SMA-LED, talk with their doctor about all current treatment options. Support groups can provide additional information and connect you with other affected families.