This fact sheet deals only with the Muscular Dystrophies (MD). The Muscular Dystrophy Association deals with a wide variety of disorders which affect muscles, nerves which control muscles and the interactions between the two. Please contact the Association for further information.
What are the Muscular Dystrophies?
The MDs are a group of muscle diseases which have three features in common: they are hereditary; they are progressive; and each produces a characteristic selective pattern of weakness of muscle groups.
At what age do signs of Muscular Dystrophy appear?
At any age but most often in young people. The Duchenne type is usually first noticed in toddlers, the Becker type in childhood or adolescence, others in adolescence or early adulthood.
Does the rate of progression vary?
Some MDs progress more rapidly than others. Also people with the same sort of MD may have different rates of progression even within a family.
How are Muscular Dystrophies diagnosed?
Usually the characteristic pattern of weakness and the resulting disability suggest the diagnosis. Often there are typical effects on posture and gait, that is on how an affected person stands and walks.
When the muscles of the face are affected, the facial expression is altered in a recognisable way.
Enlargement of calf muscles and sometimes other muscles is typical of some MDs.
Myotonic MD is associated with delayed muscle relaxation, called myotonia. This is noticed especially when trying to relax after gripping tightly. Affected people sometimes describe this as “stiffness”.
Cramps occur in some MDs, especially Becker type MD. Otherwise, pain is not usually a feature of MDs.
A diagnosis of MD has genetic implications for family members. It is therefore important that the diagnosis be made as soon as possible, so that genetic advice can be offered. In the case of Duchenne type MD, if the diagnosis is suspected, a blood test, serum creatine kinase (SCK), will show whether the diagnosis is possible or not. Affected boys always have very high levels of SCK and a normal or slightly increased level rules out the diagnosis.
A suspicion of any type of MD is investigated by diagnostic tests. These include blood tests (eg SCK), electromyography and muscle biopsy. Electromyography consists of observing the electrical activity generated in muscle when it contracts, through a needle inserted in the muscle.
Muscle biopsy is usually the confirmatory test. A small piece of muscle is removed for examination under a microscope and sometimes also for chemical examination. Often samples are taken from more than one muscle (eg thigh and upper arm). It is sometimes possible to get a satisfactory specimen through a needle, using local anaesthetic, which makes it a very minor procedure. Sometimes a general anaesthetic and a surgical incision are necessary.
Genetic Testing
It is now possible to detect the abnormal gene for a number of the MDs and other inherited muscle diseases. This can be done by examining the DNA extracted from a blood sample. In the future it is expected that this form of confirmation of the diagnosis will be possible for each of the MDs, allowing more precise genetic counselling.
What does hereditary mean?
An hereditary characteristic is one which is determined by the genetic material which is put together when a sperm and an egg join at conception. That is true of both normal characteristics and hereditary diseases.
The genetic material from the sperm and the egg produces inherited characteristics by directing the body to make certain proteins. Normal variations in those proteins produce the normal variability that we see in people. Inability to make an essential protein properly, because of a defect in the genetic material, may cause an hereditary disease like MD.
What is a Gene?
There are about 100,000 different proteins whose manufacture is determined by the genetic material. A bit of the genetic material which is concerned with one protein is called a gene, so there are about 100,000 genes in the egg and a similar number in the sperm.
The genes are made of deoxyribonucleic acid, usually abbreviated to DNA. The chemical structure of the DNA of a particular gene contains a code for the corresponding protein.
Here is an example. Normally, humans have a gene which codes for a protein called dystrophin, essential for the integrity of muscle fibres. People who do not have MD have dystrophin in their muscle fibres. Males with Duchenne type MD have something wrong with that gene. Their muscle fibres cannot make dystrophin properly and their muscles cannot function normally and suffer progressive damage.
Is Muscular Dystrophy always inherited?
By definition, all MDs are genetic, but they are not always inherited. In other words there is not always a history of MD in a previous generation of an affected person’s family for a number of reasons. First a person may be affected by a genetic defect which has just occurred in a family for the first time, in one egg or one sperm of his or her parent. This is called a new mutation. Secondly, a parent may have been so slightly affected by the disease that it was unnoticed and may have been undiagnosable even by an experienced physician. Thirdly, one important mode of inheritance, autosomal recessive, very seldom affects more than one generation.
What are the types of Muscular Dystrophies?
Medical experts do not all agree on the best way to classify MDs. Classifications tend to change as knowledge increases. However, there is, generally, agreement about the types of MD which have been recognised for many years and which are seen relatively often.
The following list has been extracted from a document entitled ‘A Simple Classification of Neuromuscular Diseases’, published in 1989 by The World Federation of Neurology.
* Duchenne type
* Becker type
* Emery – Dreifuss type
* Facioscapulohumeral type
* Limb-girdle Muscular Dystrophies
* Autosomal Recessive Childhood Dystrophy resembling Duchenne/Becker
* Ocular type
* Oculopharyngeal type
* Distal type
* Myotonic Dystrophy
