SMA with lower extremity predominance (SMA-LED)
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SMA with lower extremity predominance (SMA-LED) is a rare type of spinal muscular atrophy (SMA) that affects the lower limbs.
SMA is a rare and heritable disease characterized by the loss of motor neurons, or nerve cells that control voluntary muscle movement. Without these nerve cells, muscles weaken and atrophy.
The symptoms of SMA-LED usually start in infancy or early childhood, and progress slowly, weakening the lower limbs, especially the muscles of the thigh. About one-quarter of those with SMA-LED may not develop muscle weakness until they are adults.
SMA-LED is also known as Kugelberg-Welander syndrome, and autosomal dominant childhood-onset proximal spinal muscular atrophy with contractures.
Other forms of SMA, which are caused by mutations in the SMN1 gene, are divided into five main types — 0, 1, 2, 3, and 4. Other rare types include Finkel type SMA, SMARD1, Kennedy’s disease, and X-linked infantile SMA.
What causes SMA-LED?
SMA-LED is caused by mutations in the DYNC1H1 (dynein cytoplasmic 1 heavy chain 1) on chromosome 14 or the BICD2 (BICD cargo adaptor 2) gene on chromosome 9. Both genes provide instructions for making proteins that are part of a protein complex called the dynein-dynactin complex, which is involved in moving materials within cells.
The dynein-dynactin complex binds to various materials within cells, moving them along a system of small tubes called microtubules. The dynein-dynactin complex has very important functions in protein transport, positioning of cell compartments, and the mobility of structures within cells, among several other cell processes. In nerve cells, dynein helps neighboring cells to communicate by moving synaptic vesicles that contain chemical messengers.
In SMA-LED, mutations in either DYNC1H1 or BICD2 cause changes in the amino acid sequence (the building blocks of proteins). As a result, the movement of proteins and cellular materials is impaired, resulting in reduced chemical signaling among neurons that control muscle movement. Poor synaptic vesicle transport also affects nerve cell growth, and is thought to contribute to muscle weakness in patients.
Research suggests DYNC1H1 variants may cause not only lower, but also upper motor neuron disease.
SMA-LED is inherited in an autosomal dominant manner, meaning that a single copy of the disease-causing mutation is sufficient to result in disease development. The children of people who are affected by SMA-LED have a 1 in 2 risk of inheriting the defective gene and developing the disease themselves.
What are the symptoms of SMA-LED?
SMA-LED is characterized by muscle weakness in the lower limbs, with symptoms most pronounced in the large muscles of the thighs (quadriceps). Symptoms may start in infancy or childhood, and progress slowly. Some people with SMA-LED may also have muscle weakness in their upper limbs. About one-quarter of those with SMA-LED do not develop muscle weakness until adulthood, and generally, the symptoms do not get worse over time.
Symptoms can include:
- delayed motor milestones
- waddling or unsteady gait
- difficulty rising from a seated position
- difficulty climbing stairs.
How is SMA-LED diagnosed?
Diagnosis of SMA-LED involves a physical exam, reviewing the health history in the family, and undergoing genetic testing.
Diagnosis may involve:
- blood samples for genetic testing looking for mutations in either the DYNC1H1 or BICD2 genes
- muscle biopsy
- magnetic resonance imaging (MRI)
- electromyography (EMG).
Some people may initially be diagnosed with other forms of SMA, or other neurological diseases.
What treatment options are available for SMA-LED?
SMA-LED treatments involve managing disease symptoms and focusing on improving quality of life. Physical therapy can be used to preserve range of motion, increase muscle strength, and prevent joint contractures (shortened muscles and tendons at joints).
People with SMA-LED may benefit from using specialized equipment, such as walkers and wheelchairs for mobility, and other adaptive aids for living with spinal muscular atrophy.
At this point, there is no gene therapy available for SMA-LED.
What is the life expectancy for people with SMA-LED?
Most patients with SMA-LED have a normal lifespan. Some may need support, such as braces or a wheelchair, for mobility later in life.
SMA News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.
FAQs about SMA-LED
Spinal muscular atrophy with lower extremity predominance (SMA-LED) is a rare type of spinal muscular atrophy, with symptoms usually starting in infancy, and marked by weakness in the lower limbs.
Yes. Due to the difficulty in diagnosing spinal muscular atrophy with lower extremity predominance (SMA-LED), some people may receive a diagnosis as late as adulthood. In fact, about one-quarter of those with SMA-LED may not develop symptoms, such as muscle weakness, until they are adults.
Children with spinal muscular atrophy with lower extremity predominance (SMA-LED) may have delayed motor development, and some may never walk. Others may have a waddling or unsteady gait, and some may require specialized equipment.
There is no cure for SMA-LED at this point in time. Available treatments can help ease the disease symptoms and contribute to a better quality of life.
At this point, there is no gene therapy for SMA-LED, which is caused by mutations in the DYNC1H1 or BICD2 genes.
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