What to Know About Spinal Muscular Atrophy (SMA)

Spinal muscular atrophy (SMA) is a genetic disease that causes muscle weakness, making it difficult to stand, walk, move the arms and head, and even breathe or swallow. The symptoms are caused by a loss of nerve cells in the spinal cord and brain stem.

It is a progressive motor neuron disease. SMA has several types, all caused by the same genetic change and protein deficiency. but with differing ages of onset and symptoms. SMA that begins at an early age has a worse prognosis and shorter life expectancy than forms of SMA that begin later in life.

This article will discuss spinal muscular atrophy, its different types, symptoms, causes, diagnosis, treatment, and caring for a child with the condition. New treatments are offering hope of altering the progress of SMA.

Spinal Muscular Atrophy (SMA) Types

SMA affects approximately 1 in 10,000 newborns. The different types of SMA are defined by their age of onset and presenting symptoms.

Several types have other names that can be used to define them, including:

• Type 0: The most severe type of SMA can begin to cause symptoms even before birth. The pregnant woman might notice that the fetus is not moving, and a prenatal ultrasound can detect significantly decreased movement. Usually, babies with SMA type 0 do not survive for more than a few days after birth.
• Type 1 (Werdnig-Hoffmann disease): This type of SMA develops within the first six months of life. Children with this type of SMA will not learn how to sit up on their own without support. Abilities are substantially impaired, and survival beyond early childhood is limited.
• Type 2: Symptoms begin to develop between ages of 6 months and 2 years. The symptoms involve backsliding of previously attained physical abilities, such as sitting up or standing. Children with this type of SMA will need substantial day-to-day support and will usually be wheelchair dependent. They can have a normal life span. However, medical complications due to the disease (such as pneumonia) can increase the risk of early death.
• Type 3 (Kugelberg-Welander disease): This type develops symptoms after age 18 months. The effects of the disease can have a significant impact on a person’s physical self-care abilities. Life expectancy can be normal or near normal with lifestyle adjustments and treatment.
• Type 4 (adult SMA): This type of SMA can begin during early adulthood. People who develop this type of SMA have a very slow progression of symptoms and may eventually need to use a walker or a wheelchair. This type of SMA can prevent a person from being able to use their upper extremities the way they would like to.

Symptoms of SMA

The symptoms and effects of SMA are all related to muscle weakness. However, the severity of the effects varies based on the SMA type that a person has.

Children who have type 0 SMA or type 1 SMA do not develop physical strength, while people who have type 2 SMA, type 3 SMA, and type 4 SMA will experience a decline in their abilities after beginning to develop motor strength and control.

In addition to muscle weakness, people with all types of SMA will experience muscle atrophy, which is degeneration and thinning of the muscles.

Symptoms of SMA can include: 

• Babies who develop SMA during infancy are not able to hold their heads up, cannot sit up without support, may struggle to breathe, and have trouble swallowing.
• Toddlers with SMA do not learn to walk, have trouble sitting up without falling over, and may need assistance eating and breathing.
• Young adults or older adults with SMA may need to use a wheelchair or a walker and may have arm weakness.

People with mild forms of SMA who can breathe on their own may have difficulty breathing when they become sick or develop an infection. During these times, a person with SMA may need respiratory support, even if they do not regularly need respiratory support.

What Causes SMA?

All types of SMA have the same cause. They are caused by a genetic defect in the SMN1 gene (encoding survival motor neuron protein). This gene encodes for SMN protein, which is crucial for survival of the α-motor neurons in the spinal cord.

Without an adequate amount of SMN protein, the neurons degenerate, and SMA progresses over time. It can progress very rapidly for people who have a very low amount of the SMN protein.

There is another gene, SMN2, that also codes for the SMN protein. People have several copies of the SMN2 gene, but the number varies. With more copies of SMN2, a person who has SMA will have a milder form of the disease, with a later age of onset, less nerve damage, less disability, and a longer life expectancy. 

Hereditary Pattern of SMA

The genetic mutation that causes SMA is almost always expressed in a recessive pattern. This means that a person would develop clinical effects of SMA only if both of their SMN1 genes are altered and cannot produce SMN protein. As the gene is not on a sex chromosome, the risks are the same for people of any sex.

A child with parents who do not have any signs of SMA can develop the condition if their parents each carry one copy of SMN1 gene with the mutation and the child inherits a copy of the altered gene from each parent.

How Is SMA Diagnosed?

The diagnosis of SMA is confirmed with genetic testing. For people who have a family history of this condition, genetic testing may confirm or rule out the diagnosis even before any symptoms occur. 

When a person doesn’t have a family history of SMA, the diagnostic process involves an evaluation of symptoms and a physical exam before tests are ordered. Depending on the symptoms and initial medical evaluation, tests that might be ordered could include blood tests, electromyography (EMG), nerve conduction studies (NCV), muscle biopsy, and spine or brain imaging tests. 

Medical Observation and Surveillance After Diagnosis

After a definitive diagnosis, ongoing evaluation of neuromuscular function is necessary. It is important for children and adults who have SMA to get regular physical examinations so that changes in muscle function and posture can be detected, even before symptoms become obvious.

This medical surveillance can help with treatment planning and with knowing when to begin to use supportive devices and mobility devices. Families can also benefit from anticipating the type of assistance their loved one is expected to need and whether it’s best to look for professional caregivers.

Spinal Muscular Atrophy Treatment

Treatment for SMA has been substantially changed within the past decade. New disease-modifying treatments can impact the disease process. Additionally, therapies and interventions that help alleviate complications are also a necessary part of treatment.

Disease-modifying therapies for SMA include:

• Spinraza (nusinersen): This medication is injected intrathecally (into the fluid surrounding the spinal cord) with four loading doses of injections timed every few weeks and then a maintenance schedule of every four months. This treatment works by binding to the SMN2 gene to increase SMN protein production.
• Zolgensma (onasemnogene abeparvovec): This genetic therapy is injected into a vein, with a onetime dose. The treatment provides a copy of the SMN gene to direct SMN protein production.
• Evrysdi (risdiplam): This treatment is taken by mouth on a daily schedule. It increases the production of SMN protein by acting on the SMN2 gene.

These therapies are more effective when they’re started at early stages of the disease before substantial disability has developed. However, the effects can be variable, and the protocols for starting treatment or for combining therapies are modified as changes and familiarity with the medical treatments increase.

Physical therapy can help a person with SMA slow muscle weakness and atrophy through stretching and strengthening exercises. Occupational therapy can help the person perform everyday tasks of self-care and recommend assistive devices and home modifications.

Caring for a Child with Spinal Muscular Atrophy

Care and daily life with SMA require substantial lifestyle considerations. The day-to-day care is guided by each person’s level of abilities and their need for support. It can be helpful to learn what to expect and what the typical progression may be for your type of SMA or your child’s type of SMA.

If you have SMA, you might need assistance with grooming, bathing, or using the toilet. You might need to consider accessibility when using public spaces, such as calling ahead to ask if the place where you want to go has ramps or wide hallways.

Devices used for care may include:

• External braces for support of arms or legs as needed
• A cane or walker for assistance with mobility
• A wheelchair
• A feeding tube
• A mechanical device to assist with breathing

Some interventions are external, such as braces, but some are more invasive and require surgery or a minimally invasive procedure. A person with SMA should try to be as active as possible to help maintain muscle function.

SMA Outlook

The outlook and life expectancy for SMA varies, depending on the type of SMA that a person has.

When medical therapy and intervention begin at an early stage of the disease, the outlook can substantially improve for those who have a milder type of SMA. People with types 3 or 4 SMA may have a normal life expectancy. Those with type 2 have a variable outlook, but most live into their 20s or 30s.

Life expectancy with SMA can be as short as a few days with type 0 SMA and just a few months or up to a year with type 1 SMA.

SMA Support

Awareness of SMA has been increasing. A number of different groups are available to help you navigate life as a person with SMA or a caregiver of a loved one with SMA.

If you have a child who is at risk or who has been diagnosed with SMA, you might benefit from joining a group so that you can receive and provide emotional support and practical advice about what to expect and how to manage.

If you are living with SMA, you might benefit from being part of a community of people who have experienced and continue to experience the same challenges and triumphs that you are going through.

Groups you might consider reaching out to include Cure SMA, Muscular Dystrophy Association, and SMA Foundation.

Summary 

Spinal muscular atrophy (SMA) is a disease of the motor neurons in the spinal cord and brain stem. It is hereditary, and there are different types that are all caused by the same genetic mutation and protein deficiency, but with varying levels of severity and impact on quality of life. If you or your child has a family history of SMA, it is important to have a genetic test to determine whether you or your child has the condition.

Many people who develop SMA do not have a family member with the condition, and the diagnosis in those cases may occur after symptoms begin. There are medical disease modifying treatments that can help alleviate the severity of SMA, and supportive care is an important part of managing life with SMA.

Support groups and services can help you with advice, resources, access to services, and with staying up-to-date on the most recent developments in SMA therapy.