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Spinal muscular atrophy is a group of genetic diseases that affect the anterior horn cells, causing the wasting and weakness of the affected muscle without any sensory loss (Anterior horn cells are the nerve cells of the spinal cord and brain stem that have motor functions).
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There are three types of SMA, classified by the onset age and the severity of the disease.
- Infantile SMA (Werdnig-Hoffman
disease)
- The most severe form
- It manifests within the first 3 months of life and shows a rapid progression leading to death in about 3 years
- Infantile SMA is characterized by extremely week muscle tones and involuntary movements (floppy infants)
- Other symptoms include difficulties in swallowing, feeding (poor sucking) and breathing due to respiratory muscle weakness.
- Intermediate SMA (Chronic Werdnig-Hoffman disease)
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The age of onset is usually after six months of age
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Main clinical findings include muscle weakness and a wasting of the limbs. However, this is still a less severe condition than infantile SMA. Swallowing and feeding problems are less common occurrences.
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Intermediate SMA takes a
more benign course than infantile SMA, slowly progressing
in many patients, allowing them to survive into
early adulthood. However, this usually leads
to severe disabilities with contractures.
- Juvenile SMA (Kugelberg-Welander disease)
- The mildest form of SMA is characterized by a proximal limb weakness
- The age of onset is after childhood or early adolescence
- A slow progression will lead to disabilities in early adult life
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- In general, SMA occurs in 1 out of
25,000 people.
- SMA is inherited as an autosomal
recessive trait (both parents have the gene responsible for
SMA).
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- Family history of SMA in some patients
- Muscle enzyme -- elevated CPK (muscle enzyme) levels in the blood
- Muscle biopsy
- Electromyography
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- There is no treatment that reverses or arrests the progression of muscle weakness
- The treatments are supportive to prevent the complications, such as contractures and pneumonia.
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