Some doctors use the term SMA type 4 for adult-onset SMA, regardless of the inheritance pattern. They differ mostly in their time of onset and rate of progression. All forms affect the skeletal muscles of the trunk and limbs. In general, those muscles closer to the center of the body are more affected than those farther away. SMA type 1 and SBMA affect the neurons controlling the mouth and throat muscles more and therefore involve more problems with chewing and swallowing.
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.