If your baby has spinal muscular atrophy (SMA), you likely have questions about what to expect when it comes to their growth and development. The muscle weakness caused by SMA can interfere with developmental milestones — tasks that most children can perform at certain ages — such as sitting up without support, crawling, and walking.

In the past, babies with SMA weren’t likely to meet many of their developmental milestones. Better treatments have helped improve development in babies with SMA, and many now achieve milestones considered impossible before.

In this article, we’ll discuss how treatment fosters development and how you can best support your child with SMA.

How SMA Affects Development

SMA affects the body differently depending on what type a person has. There are five types of spinal muscular atrophy, which are classified partly based on which milestones have historically been achieved in untreated people.

Let’s look at how this condition has affected children’s development based on which type of SMA they have.

SMA Type 0

In SMA type 0 — also called prenatal SMA — symptoms begin while the fetus is still growing in the mother’s uterus. Mothers might even notice that the fetus doesn’t move much in the last few months before birth. These babies are born with extreme muscle weakness and often other defects and are thus considered unable to meet any motor developmental milestones.

SMA Type 1

Symptoms of SMA type 1 (sometimes called Werdnig-Hoffman disease) start in the first six months of life. They can cause difficulties and delays in language and motor development.

By the age of 6 months, a baby is expected to be able to:

• Coo
• Make “raspberry” sounds with their mouth
• Grasp things
• Roll over
• Raise their head
• Support themselves with their arms while sitting

Without treatment, a baby with SMA type 1 may not be able to reach these developmental milestones when they are expected to, if at all, because of muscle weakness.

Babies this age are also expected to be able to nurse from breast or bottle and to eventually start using their fingers to eat solid foods. Infants with SMA type 1 may have problems with reaching these milestones because they sometimes have difficulty latching onto the nipple, due to weak mouth muscles, and difficulties with swallowing.

SMA Type 2

SMA type 2, sometimes called intermediate SMA or Dubowitz disease, typically develops in babies between the ages of 6 months and 18 months. When left untreated, a child with SMA type 2 may be delayed in meeting motor milestones — or may never meet them.

Babies with SMA type 2 develop as expected until symptoms begin. Perhaps a baby can roll over or sit up unassisted before they’re 6 months old, but as their symptoms progress, they may have difficulty with those abilities or lose them entirely.

At the age of 9 months, infants usually refine their motor skills to the point where they can bang two objects together. At age 1, they are expected to be able to drink from a lidless cup. However, babies with SMA type 2 often develop a fine tremor in their fingers and hands. This tremor, together with muscle weakness, can make holding onto objects difficult.

By 18 months, most children can feed themselves solid food using their fingers. Children with SMA type 2 often have difficulty grabbing onto, chewing, and swallowing solid food. They might also need help with gross motor skills. These skills involve large muscles in the the arms, legs, back, and abdomen, such as pulling themselves up on furniture and walking.

SMA Type 3

Also known as Kugelberg-Welander disease, SMA type 3 can develop anywhere from 18 months of age to a child’s teenage years. If the condition is left untreated after symptoms appear, the child’s muscle weakness will increase over time and they may partially or fully lose skills they’d previously mastered.

Children with SMA type 3 typically are able to achieve all major motor developmental milestones. Although children with SMA type 3 are able to walk at some point in their lives, they’re at risk of falling and might need extra support — such as a wheelchair — over time. They can have difficulty running as well.

The leg muscles are often affected most in SMA type 3, so children with this condition may be able to meet motor milestones that don't involve the legs, such as drinking from a cup or scribbling. They usually don’t have difficulty with eating or speaking, either.

SMA Type 4

SMA type 4 is the mildest form of SMA and develops in adulthood after age 21, although most people are diagnosed after age 35. The most common symptom is mild to moderate muscle weakness in the legs. People with SMA type 4 are likely to have reached all of their motor developmental milestones during their childhood.

Testing Muscle Function

Knowing specifics about your child’s muscle function can also help you anticipate what kinds of developmental milestones they’ll be able to achieve. Children with SMA should have their muscle function tested regularly, usually every six months. Your baby’s health care professional — typically a pediatrician, family doctor, or pediatric neurologist — can use different tests to determine how well your baby’s muscles are functioning. They’ll choose which test to use based on how old your child is and what type of SMA they have.

One test is called the Hammersmith Infant Neurological Examination — Part 2 (HINE-2), which tests and monitors muscle function in babies with SMA. HINE-2 can help with determining if a child will be able to meet their developmental milestones and with fine-tuning their treatment plan, based on their individual strengths and weaknesses.

Another common test is called the The Children’s Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP-INTEND). It is mainly used to test and monitor muscle function in babies and young children with SMA type 1 as well as in children with other neuromuscular conditions, such as muscular dystrophy. Research has shown that CHOP-INTEND is also a useful tool for monitoring muscle function in older children and adults with SMA type 2.

New Treatments Mean More Milestones

In the not-too-distant past, the only treatments available for SMA were focused on symptom control and supportive care — they didn’t actually affect the underlying disease processes. As gene-modifying treatments have become available in the past 10 years, children with SMA now have a much better chance of maintaining and improving their muscle function and meeting their milestones — especially if treatments are started before they turn 2.

In 2016, the U.S. Food and Drug Administration (FDA) approved Spinraza (a formulation of nusinersen), the first treatment to address the root genetic cause of SMA. Since then, the FDA has approved two more gene-modifying medications for treating SMA: Zolgensma, a formulation of onasemnogene abeparvovec-xioi, and Evyrsdi, a formulation of risdiplam. These medications have quickly become the standard of care for SMA.

These medications are most effective when given to children under 2 years old. In one study, researchers found that children with SMA type 1 who received onasemnogene abeparvovec showed dramatic improvements in motor function and the speed at which they reached milestones

Another study showed that children with SMA type 1 who take risdiplam for two years maintained their motor function better than those who didn’t. They are better able to hold up their heads, sit up without help, and stand and walk with support.

Babies with SMA type 1 who are treated early with gene therapy achieve the most improvements in muscle function. In one research study, children who underwent treatment sooner were more quickly able to achieve the milestone of sitting without help compared to those who were treated later. Moreover, their CHOP-INTEND scores improved to levels previously unseen in children with severe SMA.

Once these new treatments became available, health experts quickly realized that the sooner children started gene therapy, the better the results were for the child. Routine newborn screening for SMA is the standard of care in the United States, with approximately 99 percent of all newborn babies now being screened.

Genetic testing for SMA allows treatment to start immediately, even before symptoms start. This rapid treatment response can:

• Preserve muscle function that would have otherwise been lost
• Extend life spans
• Reduce the need for breathing support

SMA isn’t curable, but people with the condition are living longer than ever and preserving more function. Your baby has a better chance of reaching their developmental milestones now than ever before.

Talk With Others Who Understand

On mySMAteam, the social network for people with SMA and their loved ones, more than 2,200 members come together to ask questions, give advice, and share their stories with others who understand life with SMA.

Do you have more questions about your child’s developmental milestones? Share your experience in the comments below, or start a conversation by posting on your Activities page.