How Spinal Muscular Atrophy (SMA) Is Diagnosed

You may need screening if you have a family history of SMA

Spinal muscular atrophy (SMA) can be diagnosed with genetic testing. If you or your child has the condition, it can take weeks, months, or even longer to confirm that SMA is the cause of symptoms such as muscle weakness and breathing difficulties. If your medical team is concerned about SMA, a genetic test may be ordered. Several states routinely screen newborns for SMA, and advocates for newborn SMA screening believe that the list is growing.

There are other conditions that can cause effects similar to those of SMA, and you or your child may need to have a medical evaluation that includes diagnostic tests for other conditions as well.

Self Checks/At-Home Testing

The warning signs of SMA that you should look for at home differ based on the age at which the condition begins to have clinical effects. If you are concerned that you or your child may have weak muscles, there are a number of things you can do to check if you need to see a doctor.

Babies and Young Children 

New parents who haven’t had other children before might not know what to expect in terms of a baby’s movement. It is perfectly fine if you aren’t sure if there is a problem—if you are concerned, it is better to ask for help. Your child’s pediatrician will be able to recognize early signs of SMA.

A child who has SMA would have the following features: 

  • Trouble eating: Your baby may have difficulty swallowing, sucking, or moving their head towards a bottle or nipple.
  • Muscle movements: Your baby might not spontaneously move their arms and legs, stretch out their body, reach for objects, or turn their head.
  • Low muscle tone: Your baby’s muscles may seem weak and floppy, and their arms or legs may drop down when you aren’t lifting them. For example, if you lift your baby’s arms during a sponge bath, their arms may drop down when you let go. Or if you lift your baby’s legs for a diaper change, their legs may drop when you let go.
  • Sitting up unassisted: Babies who have very early onset SMA (type zero or type one) will not learn to sit up. Babies who have type two SMA may learn to sit up and then lose that ability.
  • Struggling to breathe: You may notice that your baby is taking shallow breaths or working very hard to breathe.

Adolescents and Adults

Later onset SMA types three and four begin in late childhood, adolescence, or adulthood. You may notice problems walking up stairs or lifting heavy or large objects. Sometimes, muscle twitches can occur.

Exhaustion can be the main issue with these late-onset types of SMA.

In general, at-home signs of SMA are non-specific—you know something is wrong, but not necessarily what it is. If you or your child experience muscle weakness, twitches, breathing problems, or exhaustion, be sure to see the doctor as soon as possible.

At-Home Monitoring

Babies, children, and adults who have SMA may need to have oxygen levels monitored at home with a non-invasive device. This device, which is placed on the finger, can approximate oxygen levels in the blood.

Your doctor may recommend using it all the time, or when sleeping, to detect a sudden drop in blood oxygen level.

Labs and Tests

A genetic test, performed on a blood specimen, is the most definitive test for SMA. If you or your doctors are concerned about the possibility of SMA, a genetic test will probably be needed.

If you have a family history of SMA, the genetic test may be done even before any symptoms begin. In fact, SMA screening of all newborns is standard in several states, even for babies who do not have a family history of the condition.

If your medical team is ruling out other medical conditions besides SMA, you may also need to have other diagnostic tests.

Timely diagnosis is considered advantageous because the treatments used for disease management of SMA are believed to be more effective when they are started early. Additionally, complications such as respiratory emergencies and infections can be prevented when the illness is recognized at an early stage.

Blood Tests

Blood tests are used to identify the cause of muscle weakness in children and adults or to monitor respiratory function in SMA.

Creatine kinase: If you are showing signs of muscle weakness or breathing problems, you may have blood tests such as a creatine kinase level. This protein can be elevated when a person has muscle damage—which can occur in some neuromuscular conditions. Creatine kinase is expected to be normal or near-normal in SMA.

Arterial blood gas: If breathing has become a problem, oxygen levels can be accurately measured using blood that is collected from an artery. This test is usually done in a hospital or rehabilitation setting when a person is receiving oxygen or getting assistance with breathing or is at imminent risk of breathing problems.

Genetic Testing

This test is done with a simple non-invasive blood test. A number of hereditary myopathies (muscle diseases) and metabolic conditions can cause symptoms similar to those of SMA, and your doctor may also send genetic tests for any other potential conditions that you could have as well.

The genetic test can identify a mutation (alteration) in the SMN1 gene, which is found on chromosome 5. If a person has the mutation on both copies of chromosome 5 (one from the father and one from the mother), they are expected to develop the physical effects of SMA.

The gene test also identifies the number of copies of SMN2 gene, which is located on chromosome 5 as well. If a person has few copies, the effects of SMA are expected to begin early in life and to be severe. If a person has many copies (up to eight or 10), then the condition is expected to begin later in life and to have mild effects.

A person is considered a carrier for SMA if one of their copies of chromosome 5 has an SMN1 gene with the genetic alteration. A carrier can transmit the gene to their children if the child also receives another altered SMN1 gene from the other parent.

There are also a few other genes that can cause SMA—the cytoplasmic dynein 1 heavy chain 1 (DYNC1H1) gene on chromosome 14 or the ubiquitin-activating enzyme 1 (UBE1) gene on the X chromosome. A person who inherits one defective copy of either of these genes would develop SMA.

Imaging, Electrical Studies, and Biopsy

Imaging tests are not especially helpful in the diagnosis of SMA. As with some of the other diagnostic tests, they are generally only needed if there is a concern about other potential diagnoses.

As the condition advances, imaging tests are often needed to evaluate complications, such as bony spine changes and infections.

Imaging tests that may be used in the evaluation and management of SMA include: 

  • Brain MRI: A brain MRI can show anatomical changes. This test is expected to be normal in SMA, but several of the other illnesses that cause weakness (such as cerebral adrenoleukodystrophy) are associated with changes on a brain MRI.
  • Spine X-ray: Often, a spine X-ray is used to diagnosis scoliosis. This may be followed by a spine MRI if further evaluation is needed.
  • Spine MRI: A spine MRI would not be expected to show changes that help in the diagnosis of SMA, but it can show changes associated with complications of SMA, such as scoliosis.
  • Chest X-ray: A chest X-ray is typically helpful in identifying pneumonia, which can occur due to the respiratory muscle weakness of SMA.

Electrical Studies

Electromyography (EMG) and nerve conduction velocity studies (NCV) are diagnostic electrical studies that are often used in the evaluation of muscle weakness.

NCV is a non-invasive test that uses electrical shocks placed in the skin to assess motor and sensory nerve function based on the recorded speed of the nerve. An EMG involves the placement of a thin needle into the muscle to measure muscle function.

Both of these tests can be a bit uncomfortable, especially for a young child. You should be reassured, however, that these electrical tests are safe, useful, and don’t cause any side effects.

EMG and NCV can show different patterns depending on whether a person has a muscle disease, peripheral nerve disease, or motor neuron disease. An EMG or NCV may show evidence of a motor neuron deficit in people who have SMA, although these tests are not always abnormal in SMA.

The EMG can show evidence of denervation (loss of nerve stimulation to a muscle) and fasciculations (tiny muscle contractions), while the NCV can show evidence of slowed motor nerve function. Measures of sensory nerve function are expected to be normal in SMA.

Biopsy

Biopsies of the nerve, muscle, or spinal cord can show abnormalities in SMA, but these tests are not usually needed. The genetic test for SMA is non-invasive and reliable, while a biopsy is an invasive procedure with results that don’t always help in verifying or ruling out SMA.

A muscle biopsy would be expected to show signs of atrophy (shrinking of the muscle). A nerve biopsy may be normal or can show signs of nerve degeneration. And a biopsy of the anterior horn of the spine would show severe atrophy of the motor neuron cells.

Differential Diagnosis

There are a number of neuromuscular and metabolic conditions that can cause muscle weakness and decreased muscle tone. The other illnesses considered in the differential diagnosis of SMA are different for children than for adults because some of these illnesses typically begin during childhood, while some begin during adulthood.

Medical conditions that can have characteristics similar to those of SMA including: 

Myopathy (muscle disease): There are many types of myopathy. The severity of muscle weakness varies with different types. Diagnostic testing with blood tests, electrical studies, and possibly a biopsy may be needed if myopathy is considered a possible cause of your symptoms.

Muscular dystrophy: Muscular dystrophy is a subset of myopathy; there are nine types of muscular dystrophy, including myotonic muscular dystrophy. They can begin at different ages (usually during childhood) and they cause weakness and decreased muscle tone. Often, diagnostic tests (such as a biopsy and genetic tests) are needed to differentiate between SMA and muscular dystrophy.

Botulism: This is an infection characterized by severe muscle weakness, decreased muscle tone, and difficulty breathing. Botulism is caused by exposure to the bacteria Clostridium botulinum. It can be transmitted through contaminated food or contaminated open wounds. Botulism can affect people of all ages and tends to be more severe in children than in adults (although adults can have severe effects too). A physical examination can differentiate between botulism and SMA.

Adrenoleukodystrophy: A rare hereditary disease, adrenoleukodystrophy begins during childhood, causing muscle weakness and vision changes, as well as many neurological problems. This illness is usually characterized by increased muscle tone rather than the diminished muscle tone typical of SMA. Adrenoleukodystrophy usually causes recognizable changes that can be seen on a brain MRI. 

Prader-Willi syndrome: This hereditary condition begins in early childhood and can cause muscle weakness and decreased muscle tone, as well as cognitive and behavioral effects. Because it is caused by a genetic defect, it can be identified with a genetic test.

Angelman syndrome: A hereditary condition that causes severe developmental issues, Angelman syndrome can cause muscle weakness in young children. This condition causes a wider range of neurological problems than SMA.

Myasthenia gravis: This is an autoimmune condition (the body’s immune system harms a person’s own body) that affects the neuromuscular junction, which is the area between a nerve and a muscle. It usually causes eyelid drooping, but it can cause proximal muscle weakness and respiratory muscle weakness like SMA. Myasthenia gravis affects adults more often than children. 

Neuropathy: There are a number of neuropathies (nerve diseases), and they affect adults more often than children. Neuropathies cause muscle weakness and decreased muscle tone, and may cause decreased sensation as well.

Guillain-Barre syndrome (GBS): Guillain-Barre syndrome is a progressive neuropathy that usually affects adults. It generally causes leg weakness that can rapidly spread up the body, causing weakness of the respiratory muscles.

Multiple sclerosis (MS): MS usually affects adults and not children. It can cause a variety of neurological symptoms, the most prominent of which is weakness. MS also often has effects that are not characteristic of SMA, such as sensory loss, vision loss, and cognitive changes. 

Amyotrophic lateral sclerosis (ALS): This rare condition is, like SMA, a motor neuron disease. It causes muscle weakness in affected adults. ALS does not affect vision, sensation, or cognition (thinking).

It can be difficult to distinguish between adult-onset SMA and ALS. Genetic testing for the SMA gene can differentiate between the two conditions. ALS has a worse prognosis than adult-onset SMA.

Kennedy disease: A genetic disease that is often referred to as spinobulbar muscular atrophy (SBMA), Kennedy disease is a motor neuron disease that can cause symptoms similar to those of ALS and adult-onset SMA, including arm and leg weakness. This condition can be diagnosed with a genetic test.

Was this page helpful?

Article Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial policy to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. National Human Genome Research Institute. About Spinal Muscular Atrophy. Updated February 19, 2012.

  2. National Institute of Neurological Disorders and Stroke. Spinal Muscular Atrophy Fact Sheet. Updated August 13, 2019.

  3. Luxner L. SMA Newborn Screening Expands as More States Enact Mandatory Testing. SMA News Today. Published March 26, 2019.

  4. Prior TW, Finanger E. Spinal muscular atrophy. 2000 Feb 24 [Updated 2016 Dec 22]. In: Adam MP, Ardinger HH, Pagon RA, et al., editors. GeneReviews. Seattle (WA): University of Washington, Seattle; 1993-2019.

Additional Reading