Symptoms and disease severity vary depending on the type of SMA a person has.

Spinal muscular atrophy (SMA) is a group of inherited diseases that damages nerve cells in the spinal cord called motor neurons. This damage gets worse over time (progressive), affecting actions like swallowing, breathing, sitting and walking.

Spinal muscular atrophy is passed on to children by parents through abnormal genes. The most common form of SMA is caused by an abnormal or missing survival motor neuron 1 (SMN1) gene. This gene is responsible for making a protein needed by motor neurons.

This form of SMA is broken into several types, based on how severe the symptoms are and at what age they started. With the more-severe types, a child will not reach adulthood. Treatments can help manage symptoms, prevent complications and extend life.

With this most severe type, a mother can sense a child is moving less or not moving while she's still pregnant. When born, the child will be very weak and have little muscle tone, along with a variety of other possible abnormalities. Children born with type 0 typically do not survive more than six months due to respiratory failure from weak breathing muscles.

Type I, also called Werdnig-Hoffmann disease, or infantile SMA, usually becomes obvious sometime between birth and 6 months of age. Type 1 is characterized by decreased muscle tone and problems with sucking, swallowing and breathing. Most children with SMA have trouble sitting without help. Sometimes children with type 1 have a frog-like leg posture because they lack muscle strength to hold their legs in other positions. Symptoms progress quickly. Most children with type 1 live less than two years because of respiratory failure from weak breathing muscles.

This intermediate type is usually apparent within the first year and a half of a child's life. Most children can sit without help, but they can't stand or walk and need a wheelchair. Because of progressing muscle weakness, the spine usually curves (scoliosis) over time, and a child may lose the ability to sit as he or she gets older. Breathing support is also needed. Some children with type II survive into adulthood.

Symptoms of this juvenile type, also called Kugelberg-Welander syndrome, start after a child's first year and a half of life. Most children can stand or walk without help. But because of progressing muscle weakness, walking and climbing stairs may become difficult over time. A wheelchair is needed later in life. Breathing support is also needed. People with type III have normal life spans.

Symptoms of this late-onset type usually start sometime after the age of 30. It's characterized by a mild-to-moderate trembling (tremor) and muscle weakness, as well as muscle twitching. People with type IV have normal life spans and remain mobile throughout life.

Sept. 23, 2020