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Spinal Muscular Dystrophy

Spinal muscular atrophy (SMA) causes the muscles to weaken and waste away (atrophy) due to degeneration of nerve cells in the spinal cord (motor neurons). Under normal conditions, the brain sends signals to motor neurons, which than are received and sent to muscle cells. When these motor neurons malfunction, the muscles deteriorate. Intelligence and sensory nerves are not affected.

SMA is inherited as an autosomal recessive trait. This means a person needs to get the defective gene from both parents in order to be affected. Approximately 4 out of 100,000 people are affected.

Three different types of SMA in childhood can be distinguished by the age of onset and the severity of the condition.

SMA type I, severe infantile SMA or Werdnig-Hoffmann disease, is the most severe form. Usually diagnosed in the first 6 months of life, the child is unable to roll or sit unsupported. Infants are born floppy with weak, thin muscles eventually causing feeding and breathing problems. Their lifespan usually does not surpass 18 months of age.

SMA type II patients have symptoms less severe during early infancy, however, they become progressively weaker with time. Their life span rarely exceeds 2 to 3 years.

SMA type III is the least severe form of the disease. Symptoms such as weakness in the shoulder muscles and proximal leg muscles may not appear until between 2 and 17 years of age. Atrophy and weakness of proximal limb muscles, primarily in the legs, is followed by distal involvement. Usually the cases are diagnosed as limb-girdle muscular dystrophy until they are studied fully. Twitchings (fasciculations) are an important differentiating sign. Muscular biopsy and electromyography show the true nature of the process as a lower motor neuron disease. Pulmonary dysfunction is often a cause of morbidity in these patients Weakness in the muscles is progressive and eventually become more severe; however, children with type III disease can survive well into early adulthood.

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