What is SMA
SMA is a progressive genetic disease that leads to the destruction of motor neurons, which affects skeletal muscle activity. In SMA, muscle loss and weakness become more obvious over time. There is no cure for SMA, but different treatments are available to manage symptoms, prevent complications and improve quality of life.
Types of SMA
There are four types of SMA. Signs and symptoms range from mild to severe and vary depending on the type of SMA and the age when symptoms appear.
Type 1
This is the most severe type of SMA, and if left untreated a child may not live past 2 years of age. Symptoms appear before 6 months of age and include:
- inability to sit unsupported
- decreased muscle tone
- decreased limb movements
- decreased tendon reflexes
- muscle twitching
- difficulty swallowing and feeding
- impaired breathing
- scoliosis (curvature of the spine) and other skeletal abnormalities
Type 2
Symptoms usually appear between 6 and 18 months of age and include:
- breathing difficulties (especially during sleep)
- delayed motor milestones and inability to walk
- difficulty coughing and swallowing
- joint contractures (shortening of muscles or tendons around joints)
- tremors
- scoliosis
Type 3
Symptoms usually appear after 18 months of age and include:
- challenges with getting up from sitting, walking, climbing steps and running
- muscle weakness of the shoulders, upper arms, hips and/or thighs
- scoliosis
- joint contractures
- aching muscles
Type 4
This is the rarest and least severe type of SMA. There is a slow progression of symptoms, and they usually appear after 21 years of age. Symptoms include:
- mild to moderate muscle weakness
- muscle tremors
- muscle twitching
Cause of SMA
The central nervous system (CNS) consists of the brain and spinal cord, which communicate with all the muscles in the body. Motor neurons are nerve cells that connect the brain and spinal cord to muscles and are responsible for controlling muscle movement. Motor neurons need SMN (survival motor neuron) protein to work properly. In SMA, there is a mutation in the gene that is responsible for making the SMN protein. Without the SMN protein motor neurons die, and the CNS cannot send signals to muscles. Without these signals, muscles lose their strength and function over time.
SMA affects about 1 in 10,000 children.
How SMA is inherited
SMA involves having two copies of a mutated SMN1 gene or having no copies of the SMN 1 gene. SMA is usually passed down in an autosomal recessive pattern. In autosomal recessive inheritance both copies of the gene passed down to a child are abnormal and result in disease. Parents usually show no signs or symptoms of the disease as they typically have one normal copy of the gene and one abnormal copy of the gene (they are known as “carriers”). There is a 25% chance a child will develop SMA if both parents are carriers of SMA.
SMA can also be caused by alterations in genes other than SMN1.
Disease severity
The severity of SMA can be determined by the presence of the SMN2 gene. The SMN2 gene acts as a back up to the SMN1 gene and is involved in making SMN protein. In SMA, the number of SMN2 genes is linked to the severity of the disease. A milder course of SMA occurs when symptoms begin appearing later in life and when the SMN2 genes generate more SMN protein. In people with SMA, SMN2 genes are unable to make enough SMN protein to supply all motor neurons.
Diagnosis of SMA
A diagnosis of SMA is suspected in children with muscle weakness and decreased muscle tone. Their health-care provider may order tests to confirm a SMA diagnosis or refer them to a pediatric neurologist.
- Genetic testing: This is the most common test to diagnose SMA. The SMN gene deletion test looks for alterations or deletion of gene and the SMN2 gene copy number. This test takes 2 to 4 weeks to get results.
- Lab tests: This looks for high levels of the muscle enzyme creatine kinase
- Muscle biopsy: A sample of muscle is collected and analyzed
- Electrophysiological testing: This includes electromyography [EMG] and nerve conduction studies. They assess electrical activity of the nerves and muscles
Most recently, SMA can be diagnosed before symptoms begin to appear using newborn screening (NBS). NBS is a blood test done in infants at about 24 hours of age, which can detect more than 25 treatable conditions, including SMA. The NBS can determine any deletions in SMN1 gene and the number of SMN2 gene copies. A positive NBS result means the infant most likely has SMA, but tests need to be done to confirm a diagnosis. SMA detected by NBS allows for earlier detection and therapy initiation, leading to improved health and quality of life.
Treatment of SMA
In the past children with severe forms of SMA did not live past 2 years of age. There is no cure for SMA, but early treatment can help prevent motor neuron loss. Treatment involves using SMN-enhancing therapy and other approaches (e.g., rehabilitation and physical and occupational therapy) to manage symptoms and prevent complications.
SMN-enhancing therapies
Nusinersen (Spinraza)
Nusinersen helps increase the production of SMN protein for maintenance of motor neurons. It is given via lumbar puncture every 4 months following the initial 4 doses that are given over a few weeks.
Onasemnogene abeparvovec (Zolgensma)
Onasemnogene abeparvovec is a gene transfer therapy that involves a functional human SMN gene being placed inside a safe virus (called adeno-associated virus 9 [AAV9]). This is then delivered to the body’s motor neuron cells. The SMN gene then tells surviving motor neuron cells to make SMN protein, ultimately increasing muscle movement, function and survival in children with SMA
Risdiplam (Evrysdi)
Risdiplam works by increasing and maintaining levels of SMN protein and is given orally once daily.
Care management
Manage breathing problems
- To support weakened respiratory muscles, respiratory devices such as positive airway pressure can help with movement of air into and out of the lungs
- Cough assist devices can help remove secretions
Manage feeding and nutrition problems
- Gastrostomy (G tube) or nasogastric feeding tube (NG tube) provides nutrition directly into the stomach, bypassing the weakened muscles normally used for chewing and swallowing
Manage movement and daily activities
- Physical therapy can help maintain and restore muscle function and strength via exercise
- Range of motion can be maintained with stretching
- Occupational therapy can enhance activities of daily living and work skills
- Speech therapists can manage challenges with speech and swallowing
- Assistive technologies can help people with SMA optimize care and ensure independence (e.g., pulse oximeter, standers, car bed, walkers, powered and manual wheelchairs and braces [orthoses])
- Back braces can help mitigate scoliosis
Orthopedic care
- An orthopedic surgeon may initiate bracing to provide postural support or surgery for scoliosis
Prevention
SMA is an inherited genetic condition, but preventative steps can be taken prior to pregnancy to decrease the risk of SMA. A genetic counsellor can help explain to couples their chances of having a child with SMA or being a carrier, especially if you and/or your partner are carriers.
If a parent is using in vitro fertilization (IVF) then a preimplantation genetic diagnosis can be done to screen which embryos do not have the mutated SMN1 gene to make sure the child has two healthy copies of the SMN1 gene. As a result, only healthy embryos would be implanted.
Prenatal testing can be used to determine if a child is at risk for SMA. Two tests can be done:
- Amniocentesis: A thin needle is inserted into the abdomen to remove and test amniotic fluid, which contain fetal DNA. This test is done after 14 weeks of pregnancy.
- Chorionic villus sampling (CVS): Chorionic villi form the placenta and contain fetal DNA that can be removed and tested for SMA. This test is done as early as 10 weeks of pregnancy.
Complications of SMA
- Pneumonia and respiratory infections
- Breathing problems and weak lungs requiring assistance with ventilation (breathing support)
- Scoliosis, hip dislocation and bone fracture
- Feeding and swallowing difficulties that may need to be managed with a feeding tube
Parents collaborate with different health-care providers to manage and coordinate the care of children with SMA. Children with SMA may require assistance with the activities associated with daily living and their physical needs. Families of children with SMA may need to make home modifications, customize everyday items and learn how to use different equipment.
Children with SMA require ongoing assessment as they grow and develop and will be cared for by a multidisciplinary health-care team.
At SickKids
SickKids has a specialized multidisciplinary SMA clinic within the Neuromuscular program. The team includes a pediatric neurologist, nurse practitioner, long term ventilation program/respirology, physiotherapist, registered nurse and social worker. Our clinic visits range from simple follow-ups to more specialized procedures, such as nerve conduction studies and electromyography and administration of SMA therapies.
https://www.sickkids.ca/en/care-services/clinical-departments/neurology/neuromuscular-program/
Resources
Canadian Organization for Rare Disorders (CORD)
raredisorders.ca
Cure SMA
curesma.org
Spinal Muscular Atrophy (SMA) - Muscular Dystrophy Association (MDA)
mda.org/disease/spinal-muscular-atrophy
SMA360 Support Program
sma360.ca/ca
Treat-NMD
treat-nmd.org
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