What is Spinal Muscular Atrophy?

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 dystrophy's, 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, and SMA Type I, which is 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.

The 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 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 TypeI is either in utero or within the first few months of life. Course: Often have severe weakness at birth requiring respiratory support and intubation; lack of normal movement and motor development; bulbar and respiratory involvement; tongue fasciculations; head control is poor; 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; lifespan to 3 years possible.

SMA Type II - Intermediate Spinal Muscular Atrophy or benign Werdnig-Hoffmann disease. Usually apparent within the first 6 months to 3 years of life Course: Weakness in skeletal muscles; never stand or walk unassisted; may have fasciculations in tongue or hands; may develop scoliosis or joint contractures; may require ventilator support at some time. Prognosis: Frequently live very satisfying and productive lives well into adulthood.

SMA Type III - Juvenile Spinal Muscular Atrophy or Kugelberg-Welander disease. Usually apparent from 1-15 years old. Course: General weakness of skeletal muscles; walks unassisted for a period of time; wheelchair often required by age 30; scoliosis and contractures may develop; may show hand fasciculations. Prognosis: Lifespan 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 40 people 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.

Most cases of SMA are thought to be inherited in what is called an autosomal recessive manner, which means that both parents must be carriers for a child to be born with it. The easiest way for me to try to explain how the process works, would be to use myself as an example. Both of my parent's had the abnormal gene that causes SMA. When they had me, there was a 25% chance that I would be a non carrier, a 50% chance that I would be a carrier, and a 25% chance that I would actually develop the disease. Since I did develop the disease, I am also obviously a carrier. As I understand it, if I were to have children, all my children would inherit the abnormal gene from me, so they would all be carriers, but if my husband is *not* a carrier, there would be almost no chance that our children would be born with SMA. But, if my husband *is* a carrier, the chances of us having a child who develops SMA would be approximately 50%.

As to the likelihood of other members of my family being carriers of the defective gene, the likely option would be that each of my parents inherited the abnormal gene from one of their parents, so each of their siblings (my Aunts and Uncle) have a 50% chance of being carriers, and a 50% chance of being noncarriers. The chance of my other relatives inheriting the gene, my cousins for example, would be about 25%

OK, now that we're done with the clinical aspect of what SMA is, let's try for a more personal point of view. SMA certainly does not affect us all in the same way. Two of us of the same age, same general body build, with the same diagnosis can be affected very differently. The researcher's current thinking on this is that we with SMA are born with a certain amount of strength, and it will not grow in our lifetimes, and what we lose as we get older is not necessarily strength, but function. The consensus seems to be that the reason we lose so much function in childhood is that as we grow and gain weight, our limited functioning muscles have more of a load to bear than they can manage, and we become weaker--but that it's possible that once we become full grown adults, we may only lose function because of things like illnesses, surgeries, and extra weight gain. I also personally believe that the normal aging process hits those of us with SMA much harder than other people. Everyone loses some muscle function/strength as they age, but we have so much less to lose, that even the smallest change is huge to us.

The following is a note written by a fellow list member on the Strength-L discussion list, an incredible woman who is a senior at the State University of West Georgia. She will soon have a Bachelor's of Science in Biology. Her plans are to immediately go on to get her Master's (staying at West Georgia), which will be in Molecular Biology. She hopes to work at the Centers for Disease Control in Atlanta, but would also be happy working in the pharmaceutical industry or as part of a team investigating genetic disease of any kind. She not only has clinical knowledge of SMA, but has intimate knowledge of SMA as well. Not too long ago, she lost her son, Brandon Sherrer, to SMA Type I.

This is the most comprehensive note I have ever read about SMA, it helped me to understand my own disease so much better, and I thank her profusely for writing it.

Please understand though, the area of research is ever changing, and theories could change in the future. I am definitely not a Doctor, and am not advocating any kind of treatments or anything, I am just posting this note here in hopes that it will help other's gain a better understanding of the complexities of SMA, as it did me :-)

Lynne's Post:

First of all, the term" progressive" is confusing and misleading when you are discussing a disease like SMA. Many people with SMA believe that their condition gets worse as time goes by. For many, many years doctors and scientists believed that this meant SMA was a "progressive" disease. More recently, research indicates that this is not the case.

Spinal muscular atrophy is more accurately described as a "nerve" disease than a "muscle" disease. The spinal cord has two major functions. One is to carry sensations (pain, touch, heat) from the body to the brain. Specific cells in the spinal cord carry out this function. They are completely unaffected by SMA. This is why persons with SMA have normal sensation.

The second major function of the spinal cord is to carry motion impulses from the brain to the body. This function is performed by anterior gray horn (AGH) cells. In SMA, some of the AGH cells are killed. As yet, scientists are unsure of why this happens.

Nerve cells that are mature cannot divide. Nerve cells in the spinal cord are matured at an early age in life. If a cell cannot divide, the body has no way to replace cells that do not function properly or are dead. This is why nerve damage is permanent and cumulative.

Once an AGH cell has died or stopped working, the impulses from the brain do not pass through it. So the impulses don't reach the muscle. Muscle cells must have stimulation to stay healthy. When a muscle does not get the stimulation it needs, it begins to break down (atrophy). This is where the name of the disease comes from. The muscular atrophy is produced by spinal disease.

When an embryo is growing in the womb, it produces many more brain and spinal cells than what it needs. Some of these cells are killed. This is a normal process called apoptosis that normally leaves the right amount of cells alive to function. After this event and nerve maturation, the cells of the spinal cord should (for the most part) live for the rest of the person's life.

In SMA, massive killing of the AGH cells occurs when it isn't supposed to. This process happens in a short period of time. When in the life cycle this takes place and how extensive the damage is can be correlated to the most common classifications of SMA (Types I, II, and III). In general, the earlier in life the abnormal death of the AGH cells takes place, the more extensive the damage will be. For example, child A and child B both will have SMA. Child A experiences the loss of the AGH cells in the womb. He will be born with some manifestations of SMA, and his loss of AGH cells will continue for 6 months after birth. He will be severely affected by Type I SMA. After the first 6 months of life, no additional death of AGH cells occurs, but the compromise is already so great that the child cannot sustain breathing and eating. The child is very likely to die before the 6 months is over.

Child B is born "normal" and develops normally. When he is 15 months old he begins to experience loss of AGH cells. The loss occurs over the next 6 months (as with Child A) but not nearly as many cells are killed. Child B's parents notice awkward walking and symptoms such as difficulty standing up from the floor. Child B has Type III SMA. After the six months of AGH cell death, no more cells die, but the child's disability keeps getting worse because of factors such as growing. At age 12 he requires a wheelchair. Child B will live to grow up because his breathing and ability to eat are much, much less affected.

IT IS VERY IMPORTANT TO UNDERSTAND THAT THIS IS AN EXAMPLE.

Science does not know how long the cells continue to die, so my example of 6 months is only used for clarity. What is generally agreed upon, however, is that after the specific period of cell death, no more abnormal killing occurs. It is also important to note that this is theory, and someone may come along and prove that the cells die forever. As far as what we know today, though, this theory is the most well accepted one.

Because the death of the AGH cells starts, and stops, and then never starts again, SMA is a "non-progressive" disease.

In contrast:
1. Cancer is a disease caused by abnormal growth of cells. The cells will grow abnormally forever unless medical care stops them or the person dies. They will also migrate to other parts of the body and start growing abnormally there. Cancer is progressive. It starts, keeps going forever, gets worse, and never stops.

2. Some diseases are "relapsing-remitting" diseases. They start, get worse, and then go away for a while. Then they start over again. How bad the "relapsing" phase is and how often relapses occur may or may not change over time.

3. Diseases such as a cold start in the body, get worse, then go away permanently unless the person is infected again. They are "acute" diseases. Cancer and neuromuscular diseases are "chronic"- they last a long time.

Some of the references I have seen on SMA suggest that if a person has not had any detectable loss of nervation (NOT the same thing as loss of function or loss of strength, but if there is no loss of strength or function there has been no loss of nervation. On the other hand, having a loss of strength or function does not imply that loss of nervation has occurred) for a period of 2 years, then that person will not experience any further loss of nervation due to SMA. Basically this can be construed as this: if you have SMA, and you have the same strength now that you did 2 years ago, you don't have to worry that your SMA is going to worsen. However, other things than the SMA may still cause your disability to get worse. If you have SMA and you are significantly weaker now than you were 2 years ago (assuming you are physically mature now and 2 years ago) then it is worth trying to pinpoint a cause because it may be something as simple as lack of exercise.

Again, my deepest thanks to Lynne for taking the time to share her knowledge.

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