Muscular Dystrophy is the name given to a group of muscle diseases with certain common characteristics. These characteristics include the development of muscle weakness, wasting and contractures that is usually progressive. The other important characteristic is that the Muscular Dystrophies are all caused by a gene abnormality. This means that Muscular Dystrophies are often inherited. However, for some patients, the gene abnormality was not inherited but arose spontaneously.
Different forms of Muscular Dystrophy
There are MANY forms of Muscular Dystrophy. This is a very important point to remember because each form of Muscular Dystrophy has its own characteristics and course. Some are very severe, while others are very mild. Some occur early in life affecting young children, while others start in adults. Different muscles tend to be affected more severely in different Muscular Dystrophies. Some have complications such as involvement of the heart, while others do not. The other important reason to recognize this variety of Muscular Dystrophies is that they are each cause by a different gene abnormality, and therefore, may have different patterns of inheritance. Hence, the risks of acquiring or transmitting the disease are different among the various Muscular Dystrophies. Therefore, for the patient as well as the doctor, it is very important to know and understand exactly WHICH form of Muscular Dystrophy we are concerned with.
The Common Forms of Muscular Dystrophy
- Duchenne Muscular Dystrophy
This is the commonest form of Muscular Dystrophy occurring in children. It usually only affects boys, because the gene is carried on the X sex chromosome (X-linked recessive inheritance). It occurs about 1 out of every 3000 new born male children. In a third of the patients, the abnormal gene arose spontaneously, and is not inherited. Weakness is noticed as the child begins to walk or run. They have difficulty with stairs and fall down easily. Characteristic signs include walking on tiptoe and having large calf muscles. The disease can affect the heart. The weakness progresses so that the boys may lose the ability to walk by the early teens. There is a milder form of Muscular Dystrophy caused by abnormalities in the same gene causing Duchenne Muscular Dystrophy. This milder form is called Becker Muscular Dystrophy. It has the same pattern of inheritance, although it is not as common as Duchenne Muscular Dystrophy. Weakness develops in affected males in early adult life. The disease is also progressive, although the couse is milder. It can also involve the heart.
- Spinal Muscular Atrophy
This is not a muscle disease in the true sense. However, we are including it in the discussion of Muscular Dystrophy because patients with the disease look very much like patients with Muscular Dystrophy, with problems and treatment needs the are similar. Spinal Muscular Atrophy is actually a disease of the nerve cells that control the muscles and their ability to move and contract. Muscles will only work properly if the nerve cells that control them are working properly. In Spinal Muscular Atrophy, these nerve cells degenerate, resulting in muscle weakness. There are different forms of Spinal Muscular Atrophy, distinguished by the age of onset and the severity of the disease. The most severe form affect infants within a few weeks of life, with severe weakness, loss of muscle tone (floppy), and difficulty feeding and breathing. These infants usually do not survive their first year of life. The milder forms occur in early or later childhood, and these children, although disabled, may and often survive into adulthood. In the more severe forms, recurrent pneumonia can be a serious problem. Like the Muscular Dystrophies, the disease is also caused by a specific gene abnormality. It is inherited in a different way from Duchenne Muscular Dystrophy as the gene is not associated with the sex chromosomes (autosomal recessive inheritance).
- Myotonic Dystrophy
In Western societies, this is the commonest form of muscular dystrophy among adults. It usually begins in early or mid adult life, although it can start at any age group, including infancy. This disease, in addition to progressive weakness, has several special features. It is associated with a phenomenon called myotonia, in which the muscles have difficulty relaxing after contraction. The patients tend to develop cataracts early. The heart can be involved. The pattern of muscle weakness is also unusual, involving the face, forearm and lower leg muscles. The disease is inherited in a pattern different from both Duchenne Muscular Dystrophy or Spinal Muscular Atrophy (autosomal dominant inheritance).
- Limb Girdle Muscular Dystrophy
[abc diag2] In Limb Girdle Muscular Dystrophy, the muscles of the shoulder and the hip are selectively affected. Limb Girdle Muscular Dystrophy can start in early childhood and look like Duchenne Muscular Dystrophy. Or it can start in adult life. We now know that instead of one disease, there are actually many different diseases, causing what is labelled as Limb Girdle Muscular Dystrophy and caused by different genes. It is important to separate this diagnosis from Duchenne Muscular Dystrophy in children and young adults, as the inheritance and disease course are very different (autosomal recessive or dominant inheritance).
- Facioscapulohumeral Dystrophy
This form of Muscular Dystrophy is not uncommon. It usually affects adults and can be very variable in its severity. Indeed, there are patients walking around who are so mildly affected that they do not realise that they have this disease. The disease has a characteristic pattern of muscle weakness. The face is weak so that the patients may not be able to whistle or close their eyes in sleep. The shoulder muscles are also affected but again in a selective pattern, involving the upper are muscles. The scapula may stick out at the back. There may also be weakness of the lower leg muscles. Most patients are not seriously disabled, until late in life (autosomal dominant inheritance).
- Congenital Myopathy
This refers to a large number of different muscle diseases which are caused by gene abnormalities and which are similar to Muscular Dystrophies. These diseases usually start in infancy or early childhood. They may be mild and non-progressive, but can also be severe. Some of the different forms of Congenital Myopathy include: Nemaline Myopathy. Central Core Disease, Myotubular Myopathy. Their pattern of inheritance is very variable (all three forms may occur depending on which specific congenital myopathy).
By Dr Yee Woon Chee