Spinal muscular atrophy (SMA) is a group of inherited diseases that attack and destroy motor neurons, which are the nerve cells in the brainstem and spinal cord that control muscles.
Signs and symptoms of spinal muscular atrophy (SMA) vary in severity depending on the type.
Signs and symptoms of SMA type I (also called Werdnig-Hoffmann disease or infantile-onset SMA) usually present before 6 months of age and may include:
- Severe weakness
- Reduced muscle tone (hypotonia)
- Diminished limb movements
- Newborns don’t reach motor milestones
- Lack of tendon reflexes
- Muscle twitching (fasciculations)
- Swallowing and feeding difficulties
- Difficulty breathing
- Facial paralysis
- Lack of reaction to stimuli
- Congenital heart defects
- Scoliosis or other skeletal abnormalities may develop as children get older
- Death in the first year of life without treatment
Signs and symptoms of SMA type II (aka Dubowitz disease, or intermediate SMA) usually show between 6 and 18 months of age although some can present earlier and may include:
- Muscle weakness close to the center of the body
- Weakness involves the lower limbs more than the upper limbs
- Tremor in the hands
- Limited joint movement (contractures)
- Respiratory complications
Signs and symptoms of SMA type III (also called mild SMA, juvenile onset, or Kugelberg-Welander disease) develop after 18 months of age and may include:
- Foot deformities
- Respiratory muscle weakness
Signs and symptoms of SMA type IV (late-onset SMA) develop after 21 years of age and may include:
- Muscles closer to the center of the body (proximal muscles) usually are more affected
- Legs tend to weaken before the arms
- Hands may eventually weaken, but they often stay strong enough for typing on a computer keyboard and other basic life activities
What Causes Spinal Muscular Atrophy?
The most common form of spinal muscular atrophy (SMA) is caused by genetic defects in both copies of the survival motor neuron 1 gene (SMN1) on chromosome 5q. Except in rare cases, the patient inherits one mutated gene from each parent.
Less common forms of SMA are caused by mutations in other genes.
How Is Spinal Muscular Atrophy Diagnosed?
Spinal muscular atrophy (SMA) is diagnosed with a patient history and physical examination, and a blood test to look for deletions or mutations of the SMN1 gene.
Other tests used to diagnose SMA include:
- Electromyography (records the electrical activity of the muscles during contraction and at rest)
- Nerve conduction velocity studies (measures the nerve’s ability to send an electrical signal)
- Muscle biopsy
- Other blood tests
What Is the Treatment for Spinal Muscular Atrophy?
There is no cure for spinal muscular atrophy (SMA), and treatment is aimed at managing symptoms and preventing complications.
Medications used to treat SMA include:
- Nusinersen (Spinraza) to treat children and adults with SMA
- Onasemnogene abeparovec-xioi (Zolgensma) gene therapy for children under 2 years old
- Rrisdiplam (Evrysdi) to treat patients age two months of age and older
Other treatments for SMA include:
- Physical therapy
- Occupational therapy
- Stretching and strengthening exercises
- Speech and swallowing therapy
- Assistive devices such as supports or braces, orthotics, speech synthesizers, and wheelchairs
- Proper nutrition and calories
- Feeding tube
- Non-invasive ventilation at night
- Assisted ventilation during the day
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