Spinal muscular atrophy type 0

Last updated Feb. 2, 2023, by Marisa Wexler, MS
Fact-checked by Ana de Barros, PhD

Type 0 is the rarest and most severe form of spinal muscular atrophy (SMA), a rare genetic disease that causes muscle weakness and wasting. Like all other main types of SMA — types 1 to 4 — type 0 is caused by mutations in the SMN1 gene, and results in the progressive loss of motor neurons, the nerve cells that control voluntary movement.

What are the symptoms of SMA type 0?

Spinal muscular atrophy type 0 is defined by symptoms that appear before birth, while a developing fetus is still in the womb. Usually, the first detectable symptom is a reduction in the fetus’ movement, or a complete absence of fetal movement, in the later stages of pregnancy. Because it manifests prior to birth, type 0 is sometimes referred to as prenatal SMA.

At birth, babies with type 0 SMA have severe symptoms of muscle weakness and low muscle tone. Facial paralysis is common, and many infants with this disease type are born with contractures, where muscles are shortened and hardened.

Infants born with SMA type 0 often aren’t able to respond to the world around them. Many babies with this disease type are born with heart disease, and most have difficulty breathing and feeding on their own when they are born.

How is SMA type 0 diagnosed?

As with all forms of SMA, the gold standard to diagnose the disorder is genetic testing, using a blood sample to identify disease-causing mutations in the SMN1 gene.

Because SMA-causing mutations are present from the moment of conception, it is possible to perform genetic testing of a fetus during pregnancy — a placenta sample, involving the liquid surrounding the fetus, can be used for such testing.

Genetic testing also can be performed prior to implantation of an embryo when using assistive reproductive technologies such as in vitro fertilization.

Newborn screening, which involves collecting a blood spot from a heel prick shortly after birth and analyzing it for disease-causing mutations, also can detect the disease. This approach is already available in several states in the U.S. and in other countries.

Because SMA is an autosomal recessive disease, a person will only develop symptoms if both copies of SMN1 — one inherited from each biological parent — contain a disease-causing mutation. Someone with only one mutated copy of the gene is called an SMA carrier because such individuals will not develop the disease, but may pass the mutated gene to their biological children. If two carriers have a child, there’s a 25% chance of any offspring developing SMA.

What are the treatment options for SMA type 0?

There is no cure for people living with SMA, and no specific SMA type 0 treatment. In recent years, several disease-modifying therapies have been developed that may slow the progression of SMA.

Three treatments for SMA are now approved in the U.S.; they also are available in other countries, though specific indications may differ. These treatments are:

• Spinraza (nusinersen), the first approved therapy for SMA. This therapy is administered via injection into the spine, and is approved in the U.S. to treat all ages and all main types of SMA. After a loading dose, the injection is given every four months.
• Evrysdi (risdiplam), a daily oral therapy that can be administered at home. It’s approved in the U.S. for all main types of SMA in patients ages 2 months and older.
• Zolgensma (onasemnogene abeparvovec-xioi), a one-time gene therapy. Zolgensma is approved in the U.S. as a gene therapy for children with all main types of SMA, ages 2 years and younger.

All three of these treatments have proven to be effective at slowing the progression of SMA, but their ability to repair damage that has already accumulated is limited. Consequently, it’s not clear to what extent these treatments may benefit babies with SMA type 0, who already have advanced disease when they are born.

Because SMA type 0 is so rare, and these disease-modifying therapies only became available within the last decade or so, there is little published data on outcomes of treatment for babies with type 0 SMA.

A handful of case reports have described outcomes for babies with SMA type 0 who were treated with Spinraza and/or Zolgensma. While some mild benefits have been reported, all of these children continued to experience substantial motor weakness and breathing difficulties.

Supportive care for type 0 SMA

Because of problems with respiratory function, babies with type 0 SMA generally require medical intervention as soon as they are born to help keep them alive. Most infants are placed on invasive ventilation, in which a machine is used to help them keep breathing.

Because these babies usually have trouble feeding by mouth, a gastrostomy tube, also called a feeding tube or G-tube, often is placed to allow nutrients to be delivered directly into the child’s stomach.

What is the life expectancy for people with SMA type 0?

Most babies born with SMA type 0 do not survive past 6 months of age, and many die within the first month of life. Supportive treatments like ventilation and feeding tubes can help, to some extent, prolong life, but they do not alter the fatal course of the disease. Because there is little data on the use of disease-modifying therapies in this rare type of SMA, it’s not yet known whether these treatments might alter SMA type 0 life expectancy.

SMA News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.