Patient Guide to Spinal Muscular Atrophy

Spinal Muscular Atrophy: Types and Symptoms

It’s so important to recognize the signs of spinal muscular atrophy, a serious condition that affects babies and young children.

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As a parent, you’d do anything to protect your children. That’s what makes spinal muscular atrophy (SMA) is so terrifying: because it largely affects babies and small children.

SMA is a genetic disease may affect the ability to walk, eat, and breathe, with symptoms varying based on SMA type. The main symptoms of SMA are muscle weakness and wasting or breakdown (atrophy) that get worse over time.

But there’s hope. This section describes different SMA types and typical symptoms in patients without treatment. Much progress has been made in treating SMA, and new therapies may help stop the disease from getting worse.

“Fortunately, the natural history of this disease has been changed with treatment,” says Claudia A. Chiriboga, MD, MPH, Professor of Neurology and Pediatrics at Columbia University Medical Center in New York, NY. “Early diagnosis is extremely important, because it can lead to early use of treatments that can prevent disease progression.”

Spinal muscular atrophy symptoms most commonly start in babies and children younger than age 2, but can also begin in adolescence or adulthood. In general, the earlier SMA symptoms begin, the more severe the form of SMA disease, Dr. Chiriboga says. Spinal muscular atrophy is grouped into five different types based on when symptoms start and the severity of symptoms.

Spinal muscular atrophy symptomsKnow the signs of SMA so you can start treatment ASAP.

SMA Type 0

Spinal muscular atrophy type 0 is found in babies before birth or immediately after birth. SMA type 0 is rare, and it’s also the most severe form of SMA disease. These babies move less in the womb and are born with extremely weak muscle tone, weak breathing muscles, and often joint deformities. Without treatment immediately after birth, these babies often do not live past the first few months of life.

SMA Type 1

SMA type 1 is the most common form, and affects about 50% of people with SMA disease. This form emerges in babies before the age of 6 months, with symptoms that can include:

  • Decreased movement of the legs and arms
  • Reduced muscle tone (lack of muscle tension even at rest)
  • Poor reflexes in tendons (the tissue that attaches muscles to bone)
  • Muscle twitches
  • Swallowing and feeding difficulties
  • Breathing problems

Babies with SMA type 1 are very floppy and cannot lift their heads or sit without help. They may also develop a bell-shaped chest because of weakness in the rib muscles required for breathing.

SMA Type 2

Spinal muscular atrophy type 2 is found in children between the ages of 6 and 18 months. Babies with SMA type 2 have poor muscle tone, but can initially sit if they are helped into a seated position.

 Over time, they slowly lose muscle strength and function, and may lose the ability to sit on their own by adolescence and have severe breathing problems. They have limited range of motion at joints caused by contractures, which are a shortening of the muscles and/or tendons around a joint. They often have hand tremors and most develop scoliosis (curving of the spine).

“Babies with SMA type 2 are floppy early on, but unlike in SMA type 1, they don’t have severe respiratory and swallowing problems that can lead to early death,” Dr. Chiriboga said. “The majority of children with SMA type 2 reach young adulthood, and many even reach adulthood. Eventually, without treatment, they do develop major musculoskeletal limitations—namely scoliosis and limb contractures (limited range of motion) as well as progression of weakness over time—and are not able sit on their own at some point.”

SMA Type 3

SMA type 3 begins after 18 months of age and primarily affects the legs, while the arms stay strong until much later in life, Dr. Chiriboga says. People with SMA type 3 typically can walk on their own, but muscle weakness may lead to inability to keep up with peers, falls and trouble walking up and down stairs.

This form of SMA disease progresses slowly until puberty, when a more rapid decrease is found. Without early treatment, many people with SMA type 3 will eventually lose the ability to walk and need a wheelchair later in adulthood.

The age that symptoms of SMA type 3 start plays a big role in how long a person can walk on their own, Dr. Chiriboga said. “Only 70% of children who develop SMA type 3 before the age of 3 are walking 10 years after symptom onset, compared to 97% of those who develop symptoms after age 3.

SMA Type 4

SMA type 4 begins in early adulthood (20s and 30s) and causes mild to moderate muscle weakness, primarily affects the hips and upper legs (psoas and quadriceps), shoulders (deltoids), and back of the upper arm (triceps). Symptoms typically are either not found in the arms or are much less severe than those in the legs. Patients may have tremors and mild breathing problems.

“A majority of people with SMA type 4 can walk unaided until approximately age 60,” according to Dr. Chiriboga. Later on, some people may need a wheelchair.


SMA also can lead to the following complications:

  • Poor growth in children because of difficulty feeding
  • Gastrointestinal problems, including constipation, poor absorption of nutrients, delayed gastric emptying (the stomach doesn’t empty food as fast as it should), and gastroesophageal reflux (stomach contents back up) that can lead to aspiration (stomach fluids entering the lungs)
  • Bone/joint problems, including scoliosis, hip dislocation, joint contracture (limited range of motion caused by shortening of muscles or tendons around a joint), and bone thinning (osteopenia/osteoporosis)
  • Sleep issues, including hypoventilation (breathing too shallow or slow leading to low oxygen levels) during sleep
  • Metabolism issues: People, especially infants, with SMA should not skip meals as this can lead to metabolic problems, such as high/low blood sugar (hyperglycemia/hypoglycemia) or acidosis from abnormal fat metabolism (the body produces too much acid)

Next Steps

If you or your child is showing symptoms of SMA, talk to your doctor right away. You may need a genetic testing to diagnose SMA, or at least rule it out. A neurologist typically diagnoses SMA, but any physician can order the genetic test.

Table. Types of Spinal Muscular Atrophy

SMA Type

Age When Symptoms Start

Symptoms (If Left Untreated)


Before birth

  • Reduced movements in the womb
  • Babies are born with joint deformity, lack of muscle tone, and breathing problems


<6 months

  • Muscle weakness
  • Muscle twitches
  • Swallowing and breathing problems
  • Unable to hold head up or sit on their own
  • Bell-shaped chest


6 to 18 months

  • Babies need help getting into a seated position, but can sit without support
  • Muscle weakness that worsens during childhood
  • Hand tremor (shaking)
  • Scoliosis
  • Eventually cannot stand or walk on their own
  • May develop breathing problems


>18 months

  • Children can stand and walk, but over time have difficulty walking and running, climbing steps, and getting up from a chair
  • Hand tremor (shaking)
  • Fatigue (tiredness)
  • May develop scoliosis and joint contractures (shortened muscles or tendons around a joint)
  • Many need a wheelchair later in life


>21 years

  • Mild to moderate muscle weakness, most commonly muscles in hips and upper legs (psoas and quadriceps), shoulders (deltoids), and upper arms (triceps)
  • Hand tremor (shaking)
  • Mild breathing problems
  • May lose the ability to walk later in life


Updated on: 10/19/20
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