Types of Muscular Dystrophy
Metabolic Disorders that Cause Pain and/or Weakness on Exercise
Almost everybody experiences some muscle pain when they exercise more than usual or drive themselves to the limit in training, or suffer an actual 'sports injury'. However some individuals consistently develop muscle pain with 'ordinary' exercise and they may be found to have a 'metabolic disorder' of the muscles. If you have been found to have such a condition this information is designed to give you an understanding of it and why you experience pain with exercise. HOWEVER, you must consult your doctor about your particular needs because the disorders described below vary considerably from person to person.
1. Which conditions are covered in this fact sheet?
Three conditions are covered in this fact sheet:
- Disorders affecting the breakdown of glycogen or glucose into pyruvate, often called glycogen storage disorders (see paragraph 5).
- Defects of fatty acid oxidation, some of which are known as lipid storage myopathies (see paragraph 6).
- Defects of the mitochondrial respiratory chain; these may be known as mitochondrial myopathies (see paragraph 7).
2. Why is pain experienced on exercise?
People who have a metabolic disorder that causes pain and/or weakness when they exercise, have a fault somewhere in the mechanism which breaks up the 'fuels' in our food and converts them into energy. They experience pain when they exercise because the muscles are placing extra demands on this mechanism and, because there is a fault, the mechanism cannot cope with the extra demand and it breaks down.
3. What food do our muscles need to allow us to exercise?
In our diet we all consume carbohydrates (sugars and starches) and fat. Some of the carbohydrates and fat are used immediately for energy, but most is stored for later use. Then, when we need energy, these stored 'fuels' are broken down in the body to provide it. This energy is used in all organs, for instance in the heart to allow the heart to pump blood, and in the brain to allow us to think and control our bodies. Muscles also use a great deal of this energy to allow us to exercise.
When we are sitting down, our energy use is quite low, but when we exercise the muscles demand a great quantity of energy very quickly. Therefore the carbohydrates and fat have to be broken down much more quickly. The type of fuel that is used, carbohydrate or fat, is dependent upon the type and duration of the exercise undertaken. During short duration exercise of high intensity, for example sprinting, the main source of energy comes from carbohydrates. If exercise is more prolonged, for instance a five kilometer jog or a full game of soccer, there is an initial use of carbohydrate with a gradual switch to fat as a major provider of energy. This happens because most of the carbohydrate reserves become depleted and the muscles have to start using stored fat to provide the energy. An understanding of which fuel is required for different types of exercise is very important if we are to recognise the cause of the muscle pain or weakness on exercise, and for the treatment of metabolic disorders.
4. How are carbohydrates and fat converted into the energy we need to exercise?
Carbohydrates and fat are stored in different ways, and are broken down to provide energy by different processes. Carbohydrates are broken down into glucose that is stored as a substance called 'glycogen'. Glycogen is stored in the muscles, liver, heart and brain. When we exercise, particularly when we exercise intensely in short bursts, the muscles call on the glycogen which is converted back into glucose and then into a substance called pyruvate. The pyruvate is then taken into 'mitochondria' (small particles in cells whose major function is to produce energy units known as 'ATP'). If the exercise is strenuous it can overload the system that cannot function fast enough to convert all the pyruvate into energy via the mitochondria, and some of the pyruvate is turned into lactic acid instead.
Fat tends to be more of a storage fuel. It is stored in various places such as under the skin. When we use fat for energy, usually during prolonged exercise, the fat is brought out of the tissue where it is stored, goes into the blood and then into muscle, and finally continues into the 'mitochondria'. There the fats are broken down into carbon dioxide, water and energy (ATP), via a process called 'fatty acid oxidation'.
5. Glycogen storage disorders
People with glycogen storage disorders are unable to process carbohydrates into energy. They have a malfunction somewhere in the chain of chemical reactions by which glycogen should be broken down into pyruvate. This does not happen, and glycogen is therefore stored in excess quantities in muscle and other tissues. These 'unusable stores' may cause problems themselves, but in addition there is an acute lack of pyruvate when it is needed to provide energy. Section A of the diagram (see 8).
What type of exercise will bring on symptoms?
Strenuous exercise, ie. sprinting or running, tends to bring on symptoms. Swimming, for instance, may be painless if you are doing it gently, but painful if you are exerting yourself.
What are the symptoms and what should I do when I experience them?
Symptoms are mainly muscle pain and stiffness. They start during exercise and stop you from continuing. The pain occurs in the working muscles, and therefore is often felt in the legs. Stop exercising when you feel much pain, otherwise you will be damaging your muscles. You should consult your doctor about exercise in the future because some exercise is important to prevent your muscles becoming weak. Each individual is likely to have a type and variety of exercise that they can perform safely. Sometimes you may feel some pain during exercise that then eases off - this is known as the 'second wind phenomenon'. It happens because you started off using carbohydrates for energy and then switched to fat. You can safely carry on with your exercise in these circumstances.
If the pain is particularly bad, an energy crisis may occur. This leads to muscle breakdown, a process called 'rhabdomyolysis'. It can cause the excretion of a muscle protein called myoglobin in the urine (called myoglobinuria) - and may result in kidney failure. You will be warned of this if your urine is noticeably darker than usual (a brownish colour). If this is the case you should see your doctor at once because treatment may be required.
Most individuals are perfectly well between attacks but a few develop mild, permanent muscle weakness. This tends to be in the larger muscles; in the shoulder, hips, legs, and arms.
Does diet help?
There is no proof that a change in diet will help control these conditions.
What are the names of some of the glycogen storage disorders?
Two of the names of the glycogen storage disorders that cause pain on exercise are McArdle's disease and phosphofructokinase deficiency (PFK).
6. Disorders of "fatty acid oxidation" including lipid storage myopathies
Individuals affected by these disorders are unable to process fat into energy; the breakdown occurs at the point of fatty acid oxidation'. Section B of the diagram (see 8).
What type of exercise brings on symptoms?
Generally, symptoms occur during prolonged exercise, for instance when you are playing a full game of soccer or going for a long run. The symptoms vary from person to person. Some individuals find they can sprint and weight-train with no ill effects. Others can play half a game of soccer, only developing symptoms during the second half of the game. The important thing is to find your own limits and stay within them. Sometimes you will experience symptoms when you are feeling unwell or fasting. Fasting is another occasion when we use up the body's fat stores to provide energy.
What are the symptoms and what should I do if I experience them?
You may feel muscle pain, which can be an ache or very intense, both during and after exercise. (Sometimes the pain only starts a couple of hours after you stop exercising). When you feel pain you should stop exercising and should try to avoid continuing that form of exercise for such a long period in the future.
Does a specific diet help?
Diet is extremely important in alleviating the symptoms of these conditions. You should definitely see a dietician and take expert advice. If you are unwell, frequent glucose drinks should be taken to try and ensure that you never entirely rely on fat for energy.
The effects of some of these conditions in some individuals, are potentially serious. It can lead to severe muscle weakness that rapidly progresses to weakness of respiratory muscles and can be life threatening. Diet can be an effective method of treatment and prevention, and it is important to remember this. Discuss your diet with your doctor.
Are children who have these conditions affected in the same way?
Youngsters rarely develop muscle pain on exercise but may develop severe muscle weakness at times of illness or stress when the child is not eating, especially after a cold or other viral infection. When this happens you should consult your doctor immediately.
What are the names of some of the disorders of fatty-acid oxidation?
The names of some of the disorders of fatty-acid oxidation are carnitine palmityl transferase (CPT) deficiency, acyl-cortdehydrogenase deficiency and 3-hydroxy acylcortdehydrogenase deficiency.
7. Defects of the mitochondrial respiratory chain
These disorders imply that there is a breakdown in the final stage of the chain that converts carbohydrates and fats into energy. The point where pyruvate (from carbohydrates) or fatty acid (from fats) are converted into energy (ATP) in the mitochondria. Section C of the diagram (see 8).
You may experience pain on exercise and if this happens, you should stop exercising and try and avoid such exercise in the future. The MDF has a separate fact sheet for Mitochondrial Myopathies. Please contact any of our offices to obtain a copy.
8. Diagram of how carbohydrates and fat is converted into energy for exercise
9. Has research been conducted on these disorders in South Africa?
Yes. Many centres in South Africa conduct research on these conditions. Amongst others, metabolic disorders are investigated in depth at the department of Biochemistry, Potchefstroom University for Higher Christian Education. An extensive mitochondrial myopathy research programme is conducted in the departments of Human Genetics and Paediatrics at the University of Pretoria.
10. The role of the Muscular Dystrophy Foundation (MDF) in South Africa
The MDF supports individuals affected by muscular dystrophy and their families by offering emotional support, information - including a series of fact sheets, referrals to genetic counselling and other clinics, formation of support groups, assistance with special equipment, when possible, as well as financial support for research projects in muscular dystrophy in South Africa. Creating public awareness for muscular dystrophy is also an important aspect of our work, since the MDF relies solely on contributions from its members and other donors to provide an on-going support service. Through our newsletter members are kept informed of all the activities and receive national and international research updates. Please contact any office of the MDF if you require information about any of our activities or programmes.
11. Support group or contact person
Please contact the MDF for further information.
12. Where can we find assistance?
Please contact your local MDF office for further information.
13. Where can we find assistance?
Special Equipment: Phone either ILC or DIC for information on where to get special equipment.
MDF Website: Please visit our MDF website (www.mdsa.org.za) for muscular dystrophy news updates.
14. Please note
The treatments and drugs mentioned in this fact sheet are for information purposes ONLY. Please consult your physician or other health care specialist for information regarding the use of any of the above. The MDF encourages duplication of this fact sheet, under the following condition: that it is duplicated in its entirety - including the MDF logo and full text. Only individuals authorised by the MDF may make changes to this fact sheet (the information "updated by" and "last update" should be completed). Alterations to this fact sheet by any other party are strictly prohibited.
This fact sheet was adapted from the following source(s): Fact sheet(s) of the Muscular Dystrophy Group of Great Britain and Northern Ireland.
Compiled by: MDF-Gauteng Branch
Updated by: MDF-Gauteng Branch
Approved and Released by: National Office of the MDF
Last update: 30 May 2000
