Spinal muscular atrophy (SMA) is a severe genetic disorder that results in degeneration of the anterior horn cells and progressive muscle weakness.

Published: August 31, 2020

Spinal muscular atrophy (SMA) is a severe genetic disorder that results in degeneration of the anterior horn cells and progressive muscle weakness. SMA affects the muscles needed for essential functions such as breathing, feeding and movement.

The main types of SMA are delineated by severity and age of onset of symptoms. Infants with SMA (Type 0 and Type 1) progressively become weaker. The progression of disease is slower in other forms of SMA (Type 2, Type 3, Type 4).

Steps to take after diagnosing spinal muscular atrophy

Diagnostic evaluation

• Inform the family of the newborn screen results.
• Schedule an appointment for the infant for genetic testing and counseling.
• Refer to pediatric neurology for consultation and treatment with a confirmed diagnosis.
• Confirmatory testing will assess the number of copies of SMN2, which may help determine the severity of disease.

Clinical considerations

• SMA can present at any age, from neonatal period to adult onset, depending on severity of disease and symptoms. Progressive loss of motor neurons causing muscle weakness is the primary issue in SMA.
• In younger patients, it can present as hypotonia (floppy baby); weakness; motor regression/loss of motor milestones; delay in motor milestones; aspiration; failure to thrive; weak cry and cough; tremor. Due to weakness, infants with severe types cannot hold their head up, roll over, sit or stand.
• In older patients, it presents with progressive loss of motor milestones (ability to stand or walk), weakness, loss of muscle bulk, scoliosis and contractures.

Management considerations

• Novel treatments can stop the progression of disease and potentially reverse some symptoms. There are currently three Food and Drug Administration-approved disease-modifying therapies for SMA.
  • Nusinersen (or Spinraza)

• Onasemnogene abeparvovec-xioi (or Zolgensma)

• • Risdiplam (or Evrysdi)
• These disease-modifying therapies have been shown to work better if they are started early in the course of disease. They work best if given in pre-symptomatic patients. Delaying treatment may result in irreversible loss to anterior horn cells.
• Respiratory muscles are weak, limiting the ability to take effective breaths and cough effectively to clear mucus from lungs. This can lead to recurrent pneumonias, even with common infections.
• Swallowing difficulty and aspiration may prompt the need for gastrostomy tube. Failure to thrive may be seen in severe forms of SMA.
• Ongoing rehabilitation with therapies as well as use of orthotic devices is important to prevent joint contractures. Due to progressive neuromuscular scoliosis, spinal surgery may be required.