Respiratory muscles are involved to varying degrees in all forms of the disease. In SMA type 1, the most severe form, the onset of the disease is noted within the first 6 months of the child's life. Children with SMA type 1 are never able to sit without support, and death usually occurs before age 2. It is characterized by degeneration of the anterior horn cells of the spinal cord and motor nuclei of the brainstem, causing weakness. There are many forms of the disorder, which may affect specific muscle groups or be generalized and may manifest anywhere from in utero to adulthood.
Most forms are genetic.
Prenatal diagnosis in known affected families is also under investigation. There are three main types of SMA occurring in childhood, conventionally divided by age of onset. These will be described in their most typical presentation, although it is important to realize that clinical features and progression overlap. Acute Infantile SMA, or Werdnig-Hoffman disease, is the most severe form. Symptoms may be present in utero with decreased fetal movement.