DMD - A Guide for Parents

Section 1_1

Diagnosing DMD

THE CAUSE OF MUSCULAR DYSTROPHY

Boys with Duchenne muscular dystrophy develop weak muscles because the many muscle fibres that were present at birth are destroyed. Muscles in the body are made up of hundreds of thousands of individual muscle fibres. When these fibres contract (or shorten) they make our muscles work. If, for one reason or another, some of these muscle fibres are destroyed or are unable to perform their duties, the muscle will be weaker than normal.

Although Duchenne muscular dystrophy was first recognised over 100 years ago, little was known of its cause, except that it was inherited. Since the late 1980s there have been dramatic advances in our understanding of why the muscles in boys with this disease become weak. Advances came about when scientists discovered exactly which piece of genetic material is missing in Duchenne muscular dystrophy. They were then able to work out which protein in the body this genetic material usually makes. The protein, whose existence was previously unknown, was found and named dystrophin. Boys who have Duchenne muscular dystrophy do not have the genetic material that gives the message to the body to make dystrophin. Without dystrophin Duchenne muscular dystrophy occurs. Dystrophin is a large protein which is found on the inner side of the membrane surrounding each muscle fibre. It seems to be important in maintaining the shape and structure of the muscle fibre (just like joists, rafters and posts are important in keeping a house standing). If dystrophin is missing the muscle fibre breaks down (degenerates) and is unable to do its work properly. Normally, muscle fibres are able to be replaced by a process called regeneration.

During the first few years of life, muscle fibres that break down in boys with Duchenne muscular dystrophy are continually being replaced. Unfortunately, the body has only a limited capacity to continue replacing muscle fibres . Eventually the rate of regeneration cannot keep up with the rate of degeneration. As a result, there is a reduction in the number of good muscle fibres and the whole muscle becomes weaker. In summary, Duchenne muscular dystrophy is caused when a piece of genetic material is missing. As a result the body fails to make the protein dystrophin. Without dystrophin muscle fibres breakdown. Eventually the body's ability to replace these damaged fibres is exceeded. The child becomes weak because he does not have enough strong muscle fibres.

DIAGNOSING DUCHENNE MUSCULAR DYSTROPHY

Duchenne muscular dystrophy is the commonest serious muscle disorder in children and occurs in approximately one in every five thousand boys born. Although Duchenne muscular dystrophy is an inherited disease, and present from the initial stages of fetal development, there is no physical indication at birth that the baby is anything less than perfectly formed.

It is rare for any delay in development to be noticed in the first year of life. Problems are usually not evident until eighteen months to four years of age. On average a diagnosis is not made until the child is five, although with increasing awareness of the problem, some boys are diagnosed earlier. At least half of affected boys do not walk until eighteen months of age or later. In retrospect, almost all parents have noted that their child never walked or ran normally. Over the first few years of life these children have difficulty climbing and getting up from the floor. Parents often comment that their child falls frequently. Walking up on the toes (toe-walking) is common. The boys are not able to keep up with their friends in physical activities at kindergarten or school and they are often regarded as clumsy or lazy. Some parents notice that their son has large calf muscles. This enlargement is called "pseudohypertrophy" and is due mainly to an increase in fatty tissue in the muscle. Pseudohypertrophy is a sign of muscle disease rather than a indication of good strength. Indeed Duchenne muscular dystrophy used to be called "pseudohypertrophic muscular dystrophy". There are some boys with Duchenne muscular dystrophy who have problems with delay in mental or language development. As a result the physical movement problems may be relatively less obvious in the early stages.

Some families seek medical advice because they recognise that their child has a problem. For others the problem has only come to light as a result of a school medical examination or because teachers noticed that the child does not run or perform physical activities as well as most of the other children in his class. Other boys may have been sent for medical assessment because of delay in development, late walking or poor language development. On the surface, many of the early problems may seem to be far removed from a serious muscle disease.

Many parents will have taken their child to a doctor (or even several doctors) with a concern about one of these problems only to be told that the child will "grow out of it". This is not too surprising as the signs of muscular dystrophy can be exceedingly difficult to detect in the early stages and many other conditions can produce similar symptoms. Because of this, doctors are now being taught to do a blood test (CK or creatinekinase test) on young boys with these symptoms, even if they do not find anything when they examine the child. If the CK blood test result is very high, a muscle problem is likely to be present. Referral to a specialist for further examination and other tests is required.

A family doctor or specialist may recognise signs of Duchenne muscular dystrophy when the child is first examined. However since this disease is so serious its diagnosis must be confirmed by tests.

Creatine kinase (CK) test

CK is an enzyme (protein) that is important for energy production within muscle fibres. If a muscle fibre is damaged by a disease process such as muscular dystrophy, some of the CK leaks out into the blood. Normally there is only a small amount in the blood but in Duchenne muscular dystrophy there may be 10 to 100 times the normal amount. There are very few other disease processes which cause such a high level of CK in the blood.

Electromyography (EMG).

When muscles contract (shorten) there is electricity flowing through the muscle tissue. An abnormal muscle has an abnormal pattern of electricity that can be recognised and recorded using special equipment.An EMG test involves putting a small needle through the skin into a muscle and recording the pattern of electricity in the muscle when it is contracting.

Muscle biopsy

Muscle from patients with muscular dystrophy looks different from normal muscle when it is seen under a microscope. The small piece of muscle that is removed during the biopsy is cut into very thin slices, stained with a series of special dyes to show the different types of muscle fibres, and studied by a pathologist.

Some parents will have been told of the suspicion of the diagnosis of Duchenne muscular dystrophy before any tests have been done. Others will not have been told until either the CK or muscle biopsy (or both) test results are available. Whether parents are told the diagnosis before or after tests have been done depends on many factors and there is no right or wrong way. Most doctors would not wish to tell parents about the possibility of muscular dystrophy until they have a high degree of certainty that they are correct.

Often a doctor will delay disclosing the diagnosis until it is possible for both parents to be present. Although this approach sometimes leads to prolonging anxiety and causing frustration, the information is too serious to convey to one parent alone unless there is no alternative.

Next Section