Muscle Weakness in Dystrophic Diseases
The muscle weakness which occurs in all types of dystrophy has certain features in common.
It is usually symmetrical. That is, the weakness on the two sides of the body is often very similar, though one side may sometimes seem more affected than the other.
Thus, a right-handed person may be more aware of weakness in that arm, though careful testing by the physician may well reveal a similar degree of weakness in both arms.
The weakness is usually progressive, though this again differs very much from one type of dystrophy to another. Some types begin in infancy or early childhood and may be severe and progress rapidly, but others coming on in later life may be very mild.
The particular group of muscles which is predominantly affected, or the distribution of muscle weakness, is different in different types of dystrophy, and is used to distinguish between these types.
In some, muscle weakness remains localized, for example, to the muscles of the eye and face.
In others the major muscles groups responsible for moving the limbs become mainly involved, and later other muscles may also become affected. But whatever the particular type of dystrophy, the essential feature is muscle wasting and weakness.
There are many different causes of muscle wasting and weakness apart from dystrophy which the physician must first exclude when making the diagnosis.
These other causes are not all genetic. For example, muscle weakness may result from a disease which affects the nerve supply to the muscle.
A common cause for this in the past was poliomyelitis Opens in new window, but there are also some inherited diseases where this may occur. Muscular dystrophy Opens in new window however does not result from a disease of the muscle’s nerve supply. It is a disease which primarily affects muscle tissue, though in some dystrophies other tissues and organs may also be affected.
There is often quite a lot of variation between different affected people with the same type of dystrophy, even within the same family, and some may be concerned because they are more severely affected than a relative of the same age.
We are just beginning to understand how this can occur in myotonic dystrophy Opens in new window, for example, but it has to be admitted that in other dystrophies satisfactory explanation to offer for such variation in families is still being sought at present.
On the other hand, in some types of dystrophy, such as Duchenne muscular dystrophy Opens in new window, the course of the disease is fairly uniform and does not usually vary a great deal.
Though weakness is progressive, it nevertheless often shows periods of apparent arrest. This slowing is often most obvious either in the very early stages or, more often, when a person first starts needing a wheelchair.
At present, there is no way of completely arresting the course of the disease, but with physiotherapy and other measures it is possible to prevent or slow down certain complications from arising, such as contractures.
These are usually the result of prolonged immobilization, when joints can become fixed and can no longer be moved freely.
The course and severity of muscle weakness, its prognosis, depend very much on the particular type of dystrophy from which a person suffers. A precise diagnosis is therefore essential. This is possible from a careful examination by a clinician specialized in the disease and by relevant laboratory tests.
Weakness is not usually associated with pain and the muscles are not tender to touch. There are usually few complaints apart from those which result from weakness of the affected muscles.
Sometimes, especially in those types of dystrophy which mainly affect the major limb muscles, cramps and stiffness are quite common. However, severe cramps and stiffness are very unusual and, especially if the muscles are also tender, may indicate some other cause for the muscle weakness.
Sometimes in certain types of dystrophy, some muscles actually seem to be enlarged rather than wasted. In athletes muscle enlargement is of course common.
In dystrophy, however, the enlarged muscles are weak, and their enlargement has therefore been referred to as pseudohypertrophy Opens in new window (which means false hypertrophy). The calf muscles are most often affected in this way but other muscles may become similarly enlarged.
The cause of pseudohypertrophy is not entirely clear but is probably largely due to the muscle tissue becoming replaced by fat. Duchenne muscular dystrophy, in which this is a prominent feature, was often referred to in the past as ‘pseudohypertrophic muscular dystrophy’. But pseudohypertrophy can occur in several other different types of dystrophy as well.
- Adapted from: Muscular Dystrophy: The Facts, edited by Alan E. H. Emery