Approximately one in 20,000 babies is affected with severe SMA.
SMA type II is the most common form.
General clinical signs are that of lower motor neurone weakness:
- Flaccid weakness (muscles soft and floppy)
- Reduced or absent tendon reflexes
- Normal or absent plantar reflexes
- Muscle fasciculation
- Muscle atrophy
The severity of the symptoms depends on the type of SMA but within a type group there is much variation between those affected.
Features: the most severe form. Severe muscle weakness, hypotonia (no support of head when pulled up from lying to sitting; floppy when held in ventral suspension), poor suck and swallow reflexes, respiratory failure. Ocular and facial muscles and cerebral function are preserved. May be deformities of limbs/joints at birth from in-utero hypotonia. May be a history of reduced fetal movements in-utero.
Mortality/morbidity: median survival 7 months - 95% die before 18 months.
Features: developmental motor delay (delay in sitting, standing). Can usually eventually sit unsupported. Some can crawl or stand but these abilities may reduce as body weight increases. May be finger tremor. Musculoskeletal deformities, respiratory failure. Pseudohypertrophy of gastrocnemius muscle.
Mortality/morbidity: can survive into adulthood. Respiratory infections are a common problem and respiratory support may be required at some point.
Features: a milder disorder. Slowly progressive proximal weakness. Difficulty with more complex motor skills, e.g. climbing stairs. May have gastrocnemius pseudohypertrophy. Chewing and swallowing may be affected later.
Mortality/morbidity: can have a normal lifespan.
Features: similar to type III but tends to be less severe.
Mortality/morbidity: can have normal lifespan.
Features: similar to SMA types I-IV but the predominant symptom is severe respiratory distress due to involvement of the diaphragm muscles. Respiratory problems are generally the first symptoms. Distal muscle weakness. Sensory and autonomic nervous systems may also be involved.