The course of the disorder varies but is generally slowly progressive. Kennedy syndrome is an X-linked recessive disorder, which means that it occurs only in men. Women carry the gene. A diagnostic DNA test is available. Adult SMA may begin between 40 and 60 years of age and progresses rapidly, with an average life expectancy of about 5 years.
Most cases prove to be variants of amyotrophic lateral sclerosis.
Symptoms include progressive limb weakness and atrophy of the muscles, difficulty speaking and swallowing, and respiratory problems. Treatment of all forms of SMA is symptomatic and supportive and includes treating pneumonia, scoliosis, and respiratory infections, if present. Also, physical therapy, orthotic supports, and rehabilitation are useful. Genetic counseling is imperative. The prognosis for individuals with SMA varies depending on the type of SMA and the degree of respiratory function.