Spinal Muscular Atrophy (SMA) Types I, II, and III belong to a group of hereditary diseases that cause weakness and wasting of the voluntary muscles in the arms and legs of infants and children.
The disorders are caused by an abnormal or missing gene known as the survival motor neuron gene (SMN1), which is responsible for the production of a protein essential to motor neurons. Without this protein, lower motor neurons in the spinal cord degenerate and die.
The type of SMA (I, II, or III) is determined by the age of onset and the severity of symptoms.
All forms of SMN-associated SMA have a combined incidence of about 1 in 6,000. SMA is the most common cause of genetically determined neonatal death. The gene frequency is around 1:80, and thus approximately one in 40 persons are carriers. There are no known health consequences of being a carrier, and presently the only way one may know to consider the possibility is if a relative is affected.
There is no cure for SMA. Treatment consists of managing the symptoms and preventing complications. Individuals with SMA are living longer and fuller lives with the help of assistive technology such as ventilators, power wheelchairs, and modified access to computers.
Type I (also known as Werdnig-Hoffman disease, or infantile-onset SMA) is evident at birth or within the first few months.
Werdnig-Hoffman disease is inherited in an autosomal recessive pattern, which means two copies of the gene - one from each parent - are required to inherit the disorder. The parents of an individual with an autosomal recessive disorder both carry one copy of the defective gene, but do not have the disorder.
The prognosis is poor for babies with SMA Type I. Most die within the first two years.
Type II (also known as juvenile SMA, intermediate SMA, or chronic SMA) has an onset between 6 and 18 months.
Legs tend to be more impaired than arms. Children with Type II are usually able to sit without support if placed in position. Some may be able to stand or walk with help.
For children with SMA Type II, the prognosis for life expectancy or for independent standing or walking roughly correlates with how old they are when they first begin to experience symptoms - older children tend to have less severe symptoms. Children with onset after 18 months are often able to walk and are fully functional for years before they need assistance. They may have a normal life expectancy.
Type III (also called Wohlfart-Kugelberg-Welander disease, or mild SMA) can begin as early as the toddler years or as late as adolescence.
Children can stand alone and walk, but may have difficulty getting up from a sitting position.