Spinal muscular atrophy (SMA) is an inherited neurodegenerative disorder that affects lower motor neurons. Lower motor neurons are nerve cells that control voluntary muscle movements and are involved in walking, speaking, swallowing, and breathing.

There are five types of SMA, 0, 1, 2, 3, and 4. The forms differ in the severity, progression, and age of onset of symptoms.

The most common symptoms in most types of SMA are:

  • Muscle weakness and decreased gross motor skills
  • Impaired mobility
  • Breathing problems
  • Problems swallowing
  • Scoliosis (curvature of the spine)

However, different types of SMA vary in age of onset and severity of the symptoms. They are summarized as:

SMA type 0

SMA type 0, also known as prenatal SMA, is the most severe form of the disease.

It is characterized by reduced fetal movements in utero between 30 and 36 weeks of gestation. At birth, babies are deprived of oxygen caused by respiratory failure. They are also born with joint abnormalities and have problems swallowing.

Infants with SMA type 0 usually die of the disease at 2-6 months old.

SMA type 1

SMA type 1 is also known as infantile-onset SMA or Werdnig-Hoffmann disease. It is the most common form of SMA. The age of onset is around 6 months.

Symptoms include reduced muscle tone and impaired limb movements, fasciculations (spontaneous muscle contractions), loss of tendon reflexes, tremors (involuntary, rhythmic muscle contractions), and problems with swallowing and breathing.

These children usually die of respiratory failure before age 2. Respiratory support can extend survival.

SMA type 2

SMA type 2 is also known as intermediate SMA or chronic infantile SMA. Symptoms usually appear between ages 7-18 months.

Muscle weakness and reduced muscle tone are common symptoms of SMA type 2. Further symptoms include difficulties swallowing, scoliosis, and deformities of the hands, feet, and chest. Many milestones, such as sitting, are delayed. Some children can crawl and stand without bracing, but most are never able to walk.

The severity and prognosis depend mainly on the age of onset. The later the onset, the better the prognosis. Children may die prematurely of the disease, or may live a full and productive life with the right care.

SMA type 3

SMA type 3 is also known as Kugelberg-Welander disease and is a relatively mild form of SMA. The age of onset is 18 months to 17 years, with most cases diagnosed before age 2.

Children with SMA type 3 learn to walk, but frequently fall and have difficulties with balance and walking up and down stairs. Later in life, however, they are able to walk well. Legs are more severely affected than arms, something also seen in other types of SMA.

Respiratory problems occur later in life and swallowing difficulties are less common than in types of SMA with an earlier onset.

Patients with SMA type 3 usually have a normal lifespan.

SMA type 4

SMA type 4 is also called adult-onset SMA because symptoms usually appear in the second or third decade of life.

Patients with SMA type 4 experience mild muscle weakness, scoliosis, tremors, and twitching. Swallowing and breathing are usually not affected, and life expectancy is normal.


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