The signs and symptoms of spinal muscular atrophy vary but involve muscle weakness, decreased muscle tone, limited mobility, and in some cases, issues with breathing and swallowing. The most common form of SMA can be classified into four types. We distinguish these types based on the highest motor milestone achieved, the age of disease onset, and the severity of the individual’s symptoms.
Type I, also known as Werdnig-Hoffman disease or infantile-onset SMA, is typically apparent by six months of age. It can have symptoms like reduced movement or muscle tone, shortening of muscles and tendons, lack of tendon reflex, skeletal abnormalities, and issues with breathing, feeding, and swallowing. Without treatment, many children with this type of SMA have a life expectancy of two years of age. With proactive treatment and ongoing care, life expectancy can be longer and children may achieve some motor milestones like sitting and walking. Learn about a recent treatment for Type 1 SMA in our monthly Brain Health Roundup.
Type II, or Dubowitz disease, usually becomes evident between six and 18 months of age. Children with SMA Type II can sit but cannot stand or walk unassisted, and some also have respiratory problems. Older children typically have less severe symptoms and a better prognosis. While people have a shorter life expectancy, many individuals live to adolescence or young adulthood, and with continued care, some individuals live a normal lifespan.
Children with Type III, or Kugelberg-Welander disease, develop symptoms after 18 months of age. While they can walk independently, they often have difficulty with other motor movements like rising to stand from a chair, running, and climbing. They can also experience problems with spine curvature, shortening of the muscles and tendons, and respiratory issues. With proper treatment, life expectancy is typically in the normal range.
Type IV has an onset after 21 years of age. People with SMA Type IV primarily have mild to moderate leg muscle weakness and other symptoms.