Children with this type may have swallowing problems that can lead to difficulty feeding and poor growth. Most affected children cannot control their head movements or sit unassisted.

It is a severe form of the disorder with muscle weakness evident at birth or within the first few months of life. Spinal muscular atrophy type I (also called Werdnig-Hoffmann disease) is the most common form of the condition. Some infants with spinal muscular atrophy type 0 also have heart defects that are present from birth (congenital). Their respiratory muscles are very weak and they often do not survive past infancy due to respiratory failure. They have extremely weak muscle tone (hypotonia) at birth. Affected infants move less in the womb, and as a result they are often born with joint deformities (contractures). Spinal muscular atrophy type 0 is evident before birth and is the rarest and most severe form of the condition. Other forms of spinal muscular atrophy and related motor neuron diseases, such as spinal muscular atrophy with progressive myoclonic epilepsy, spinal muscular atrophy with lower extremity predominance, X-linked infantile spinal muscular atrophy, and spinal muscular atrophy with respiratory distress type 1 are caused by mutations in other genes. The types differ in age of onset and severity of muscle weakness however, there is overlap between the types. There are many types of spinal muscular atrophy that are caused by changes in the same genes. The muscle weakness usually worsens with age. The weakness tends to be more severe in the muscles that are close to the center of the body (proximal) compared to muscles away from the body's center (distal).

It is caused by a loss of specialized nerve cells, called motor neurons that control muscle movement. Spinal muscular atrophy is a genetic disorder characterized by weakness and wasting ( atrophy ) in muscles used for movement (skeletal muscles).

Spinal muscular atrophy type 3