Spinal muscular atrophy is a group of genetic diseases that damage and kill
motor neurons — nerve cells in the spinal cord and lower brain that
control movement in the arms, legs, face, chest, throat and tongue. As they
die, muscles weaken and atrophy, and over time the damage affects
speaking, walking, swallowing and breathing.
One gene
Most types are caused by a change in the SMN1 gene, which makes a protein
motor neurons need to stay healthy. Where part of SMN1 is missing or
abnormal, there is not enough of that protein, and the motor neurons die.
Most people have two copies, one from each parent. **SMA normally only
happens when both copies carry the change.** With one copy changed there are
usually no symptoms — but the gene can be passed to a child. Some less common
types come from changes in other genes.
Four types, defined by when they start
| Type | Onset | What it looks like | Outlook |
|---|---|---|---|
| I (Werdnig-Hoffman, infantile-onset) | Usually before 6 months; in the most severe cases before or just after birth (types 0 or 1A) | Trouble swallowing and breathing, little movement, contractures (chronic shortening of muscles or tendons); usually cannot sit up unaided | The most severe and the most common. Without treatment, many children die before age 2 |
| II | 6 to 18 months | Most can sit without support but cannot stand or walk unaided; may have trouble breathing | Usually live into adolescence or young adulthood |
| III (Kugelberg-Welander) | After 18 months | Can walk alone, but may struggle to run, get up from a chair or climb stairs; may have scoliosis, contractures and respiratory infections | The mildest type affecting children. With treatment, most have a normal lifespan |
| IV | After 21 years | Mild to moderate leg weakness, tremors, mild breathing problems, slowly worsening | Rare and often mild. Normal lifespan |
Treatment
There is no cure. Treatments manage symptoms and prevent complications:
- Medicines that help the body make more of the proteins motor neurons
need - Gene therapy for children under 2 years of age — which, set against the
outlook for type I above, is the most consequential line on this page - Physical, occupational and rehabilitation therapy to improve posture and
joint mobility, improve blood flow, and slow weakening and atrophy - Therapy for speaking, chewing and swallowing, where those are affected
- Assistive devices — supports or braces, orthotics, speech synthesisers,
wheelchairs — to keep people independent - Good nutrition and a balanced diet to maintain weight and strength; some
people need a feeding tube - Breathing support where there is muscle weakness in the neck, throat and
chest — devices for daytime breathing and to prevent sleep apnoea at night,
and for some people a ventilator
*Source: National Institute of Neurological Disorders and Stroke, via
MedlinePlus.*
Licence: CC0 1.0 (public domain) · Adapted from medlineplus.gov
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