What is Muscular Dystrophy?
The term covers over 40 separate neuromuscular diseases which have in common the progressive and irreversible wasting of muscle tissue. Some of these diseases are known as dystrophies, the wasting of the muscles from within themselves. Others are atrophies, wasting arising from a disorder originating in the nerve system which causes loss of the ability to use muscles.
What is Spinal Muscular Atrophy?
The name Spinal Muscular Atrophy (SMA) refers to several related neuromuscular diseases that all have the same basic cause, although they differ considerably in age of onset and severity. SMA occurs in approximately 1/10,000 births. SMA Type I, the most severe form of this disease, is the most common single genetic cause of death in infancy. There is currently no cure for SMA or treatment to stop its progression. Medical care and physical therapy may help prevent complications and ensure the best possible quality of life for those affected.
The shared feature of all the forms of SMA is progressive muscle weakness and wasting caused by degeneration of the anterior horn cells of the spinal cord (also called the lower motor neurons). Normally, these nerve cells relay messages from the brain to the muscles and stimulate them to contract. Without this stimulation, the muscles atrophy. Eventually, in addition to generalized muscle weakness and wasting, particularly in the trunk, upper arms and thighs, respiratory problems can develop. The brain and the sensory nerves that allow us to feel sensations such as temperature, touch and pain are not affected. Intelligence is normal.
Types of Spinal Muscular Atrophy
There are three types of childhood Spinal Muscular Atrophy and one adult form, classified in terms of the age at which weakness becomes obvious and by the severity of its progression.
- SMA Type I: The most severe form, is also known as Infantile Spinal Muscular Atrophy or Werdnig-Hoffmann disease. The onset of SMA Type I is either in utero or within the first few months of life.
- Course: The baby often has severe weakness at birth requiring respiratory support and intubation. There is a lack of normal movement and motor development; bulbar and respiratory involvement; tongue fasciculations. Head control is poor. The child does not sit unassisted. The major management issue is the prevention and treatment of respiratory infections.
- Prognosis: This disease is the most common cause of genetically determined neonatal death. A lifespan to three years is possible.
- SMA Type II: Intermediate Spinal Muscular Atrophy or benign Werdnig-Hoffman disease is usually apparent within the first six months to three years of life.
- Course: There is weakness in the skeletal muscles. The child will never stand or walk unassisted. There may be fasciculations in the tongue or hands. Scoliosis or joint contractures may develop. Ventilator support may be required at some time.
- Prognosis: A person with Type II frequently lives a very satisfying and productive life, well into adulthood.
- SMA Type III: Juvenile Spinal Muscular Atrophy or Kugelberg-Welander disease is usually apparent from one to 15 years of age.
- Course: There is general weakness of the skeletal muscles. The child walks unassisted for a period of time. A wheelchair is often required by age 30. Scoliosis and contractures may develop. The afflicted may show hand fasciculations.
- Prognosis: Lifespan is generally unaffected.
- SMA Type IV: Adult Spinal Muscular Atrophy begins later in life, usually between the ages of 15 and 50. The degree of disability is often mild and life expectancy is not usually affected. Type IV is less common and less clearly understood at the present time than the three childhood forms.
- Prognosis: Normal lifespan Fasciculation: involuntary twitching of muscles
- Contracture: permanent tightness or shortening of a muscle due to a disease Scoliosis: curvature of the spine
How is SMA Transmitted?
The childhood SMAs are caused by a genetic defect on chromosome 5, which is transmitted through a pattern of recessive inheritance. Affected children are born to parents who are both carriers, with both a normal and mutated form of the SMA gene. The mutation is harmless in carriers, but a child who inherits a “double dose” of the gene (that is, one copy of the faulty gene from each parent) is born with SMA. When both parents are carriers, there is a 25% risk in each pregnancy that their child will be born with the disease. SMA is a relatively common disease. An estimated one in 80 people in Canada is a carrier of SMA, so the chance of a carrier mating with another carrier is appreciable even if the disorder is completely unknown in previous generations of their families.
Source: Muscular Dystrophy Canada.