Spinal Muscular Atrophy: Types 0 & 1

Spinal muscular atrophy (SMA) of all types belongs to a group of hereditary diseases that cause weakness and wasting of the voluntary muscles in the arms and legs of infants and children.

Alternate Names:

Prenatal onset arthrogryposis multiplex congenital (SMA0),

Werdnig-Hoffman disease-Infantile Muscular Atrophy (SMA1)

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 is determined by the age of onset and the severity of symptoms.

• Type 0 is prenatal.
• Type 1 (also known as Werdnig-Hoffman disease or infantile-onset SMA) is evident at birth or within the first few months.

Symptoms include floppy limbs and trunk, feeble movements of the arms and legs, swallowing difficulties, a weak sucking reflex, and impaired breathing. Legs tend to be more impaired than arms.

The clinical evaluation includes a history and physical examination.

The history may reveal abnormalities during the pregnancy especially with onset of fetal movements or may reveal another affected family member. The history should define the onset of the disease and its progression.

The physical examination reveals weakness, hypotonia, absent reflexes, and muscle fasciculation in an alert infant. It may reveal contractures, muscle atrophy, labored breathing with use of accessory muscles and malnutrition. Molecular testing of the SMN1 gene is needed for confirmation of diagnosis. Carrier status must be defined before prenatal diagnosis is attempted.

There is no cure for SMA.

There is no treatment for the progressive weakness caused by the disease.

Treatment consists of managing the symptoms and preventing complications.

Individuals with SMA Type 0 or 1 require little, if any, involvement of an orthopedist due to their short life span.

Supportive care is important. When nutrition/feeding become concerns, tube feeding via nasogastric tube or gastrostomy may be offered.

Attention must be paid to the respiratory system, because affected people have difficulty clearing secretions. Respiratory complications are common.

The prognosis is poor for infants with SMA Types 0 and 1.

SMA Type 0 infants never achieve any motor milestones and usually die between 2-6 months of age.

SMA type 1 children fare only slightly better in that they may achieve sitting with support only and survive to 2 years or less without respiratory assistance.

SMA 1 children may survive longer if offered non-invasive respiratory support (NIPPV or tracheotomy).