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We have developed a list of answers to your questions on McArdle’s. We aim for each answer to be succinct, but where necessary we will direct you to other pages, sites or resources.


Bear in mind that our answers have to be in general terms for the typical person with McArdle’s. Please read our medical disclaimer.


Please do let us know what further questions which you would like us to provide answers for.

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How is McArdle disease diagnosed?


Once McArdle’s is suspected the usual way is genetic testing (a DNA test). Before this you may have a CK blood test, a forearm exercise test to look for a failure of lactate to rise, or an exercise test for “second-wind”. Only rarely these days is a muscle biopsy performed.

How many people have McArdle disease?

We believe that approx. 4,000 to 5,000 people are diagnosed around the world. However, the number still undiagnosed may be very much greater. If the current best estimate of 1 in every 100,000 people being affected is correct, there may be about 75,000 cases worldwide. However, very many of those may never get diagnosed.

What are the types of DNA/genetic testing used in McArdle’s?

There are effectively five levels of relevant genetic testing.

  • The hot-spot test looks at just two points on the PYGM gene for the most common mutations in McArdle’s.

  • Full sequencing of the PYGM gene looks for any mutations anywhere on the gene responsible for McArdle’s.

  • A gene panel test looks for mutations anywhere on a set of genes responsible for a range of conditions. There are different panels available covering typically from 20 to 100 conditions in one go. These panels can be for such groups of conditions as those causing rhabdomyolysis, metabolic myopathies, glycogen storage diseases, etc.

  • Whole Exome Sequencing (WES) looks at the part of the genome which is thought to be most important, and reports on any mutations in the exome.

  • Whole Genome Sequencing (WGS) looks at the entire genome and reports on any mutations. There is a vast amount of data and analyzing it can be quite a challenge.

What are the different names for McArdle disease?

  • Myophosphorylase deficiency.

This is the most accurate name, a deficiency (usually a complete absence) of an enzyme which is necessary in the conversion of muscle glycogen (stored carbohydrate) to glucose.

Other names frequently used are:

  • Muscle phosphorylase deficiency. (Myo = muscle.)

  • Muscle glycogen phosphorylase deficiency.

  • Glycogen Storage Disease Type V (or Type 5).

  • GSDV (or GSD5).

  • Glycogenosis Type V (or Type 5).

  • McArdle Syndrome (used before the basis of the condition was fully understood).

How is McArdle disease inherited?

The inheritance pattern is autosomal recessive. This means that the person inherits two copies of the PYGM gene with a causative mutation in it, one from their father and one from their mother. The parents will normally be carriers, having no symptoms themselves, in which case there is a 25% risk of each child inheriting the condition. If one parent actually has the condition and the other parent is a carrier, then the risk is 50%. In the very unlikely event of both parents actually having the condition, all children will also have the condition. 

In very rare instances, perhaps billions to one against, there can be a de-novo (new) causative mutation occur in that pregnancy and the child can get the condition with only one parent being a carrier.​



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What is the baseline level of Creatine Kinase (CK) in people with McArdle’s?


Several papers have reported that the average basal level (i.e. baseline, with no particular injury) is approx. 2,400 IU/L. That compares to the usually quoted normal range of up to about 200 IU/L. (Be aware that some countries and labs report in different units.)

Are liver enzymes raised in McArdle disease?

The enzymes ALT and AST are routinely moderately raised in people with McArdle’s. Although called liver enzymes, they are also released when muscle is damaged. If other liver enzymes are raised these should be investigated.

What causes the dark red/brown urine?

This may show up when you either do a lot of damage in a large muscle group, or less damage but in many muscles at the same time. It is called myoglobinuria (myo = muscle, globin = a red colored protein, uria = urine). (Myoglobin should not be confused with haemoglobin, which is also red-colored, in blood.) Myoglobin is released from muscle into the blood when the muscle is damaged. Large amounts of myoglobin reaching the kidneys can cause damage and block them. If that happens it causes acute renal failure (a.k.a. acute kidney injury – AKI, or acute renal failure – ARF) and is a medical emergency. It is a very good idea to learn to manage activity well, so as to avoid such episodes of severe injury.



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When do I need to seek urgent medical attention


Please go to the top page on our Medical menu. This provides guidance on when to seek medical attention, including a flowchart to help you make your decision.

What is rhabdomyolysis?

  • Rhabdomyolysis is a medical term for muscle breakdown. When muscle is damaged it releases breakdown products into the bloodstream. This can be caused by a range of medical conditions including McArdle disease, but can also be caused by excessive exercise, crush injuries, severe dehydration, drug use, etc.

  • Rhabdomyolysis is often defined as a plasma Creatine Kinase (CK) level of 3,000 IU/L or above. The average baseline CK in people with McArdle’s is about 2,400 IU/L, so many of us must be over the 3,000 level almost permanently, thus almost continuously in rhabdomyolysis.

  • Rhabdomyolysis does not depend on the appearance of myoglobinuria (dark red/brown urine), which will often not appear until CK levels in the tens of thousands are reached. 

  • McArdle patients who have learnt to manage well are likely to never be aware of rhabdomyolysis or myoglobinuria, unless they get caught out by circumstances forcing them to do something that they would normally avoid (e.g. running from danger).

  • It is often reported in scientific papers that about 50% of people with McArdle’s have never had rhabdomyolysis, but they almost certainly mean they have never noticed myoglobinuria.

  • Rhabdomyolysis should be avoided as much as possible due to the associated risks of acute renal failure and compartment syndrome.

What is the path of recovery from rhabdomyolysis?

Depending on the level of muscle damage caused, after being discharged from medical care you need to have at least a few days rest and not use the damaged muscle very much until it feels fully recovered. For a while you are likely to be at increased risk of a repeat episode.

As your body heals, you will need to re-slowly reintroduce activity and exercise. It may take a few weeks to re-condition the muscles that were damaged.

As long as you seek prompt attention for an episode, especially if you have acute renal failure, there is every reason to expect a full recovery. Kidney function usually returns to normal once the by-products of muscle breakdown have been flushed through.

The muscles are very good at recovering, although less so as we get older. However, repeated episodes of major muscle damage should be avoided as they are believed to cause cumulative damage if repeated over many years.

What is a fixed contracture?

A fixed contracture is a severe type of cramp or spasm, when a muscle or group of muscles contracts and then fails to relax again. The muscle becomes hard and stiff, swells up and becomes extremely painful. 

You must NOT try to force the muscle (e.g. do not try to forcibly uncurl the fingers of a “clawed” hand). This may tear the muscle and cause a lot more damage. Cooling the affected muscle, for example with an ice pack, may slow the development of symptoms and/or assist recovery. 

If the muscle is not quite in a fixed contracture, but just sore and enlarged, some very gentle massage may help, but stop if it is painful.

A muscle in a fixed contracture is at risk of developing compartment syndrome, which requires urgent medical attention.

What is compartment syndrome?

In a very small number of people something called “compartment syndrome” (CS or ACS) has developed. This is caused by muscle swelling which leads to increased pressure within the fascia (strong membrane) around the affected muscle group. If left untreated it can permanently damage the muscle and affect the circulation. Surgical intervention is usually required to relieve the pressure within the fascia.

If a fixed contracture does not start to recover after about 24 hours from onset it should be assessed for compartment syndrome.



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Is it possible to build muscle mass and strength


Contrary to earlier beliefs, it has now been shown that resistance training (weight lifting) in people with McArdle disease is possible, with care and preferably under the supervision of a physiotherapist or personal trainer experienced with McArdle disease.

The principle is to utilise the immediate ATP energy and the phosphocreatine system in the muscle cell to undertake a brief session, lasting around 6 to 10 seconds. You must then rest to recover these systems in about 3 minutes, but during that time you can move on to exercise other muscle groups, then return to the first group. 

Please refer to these papers in our Research section:



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Is McArdle disease progressive?


The condition itself is not progressive.

  • Reduction of aerobic fitness is by far the biggest factor in deterioration - and that is reversible, as seen many times.

  • Loss of muscle mass is probably next and these days we know that can also be reversed in many people.

  • Repeatedly pushing on too hard in the presence of pain, rather than slowing or pausing, will build up muscle damage in the long term and make things worse - thus IamGSD's campaigns for early diagnosis, good information and clinical practice guidelines.

  • Ageing is possibly the next important factor - older bodies are less good at many things, especially repair.

  • With age there may be a greater degree of other conditions which, while they probably don’t directly affect McArdle’s, they may make the effect more noticeable.

Is there some information I can give to my family doctor/GP?


There is a booklet specially created for this purpose, the 20 page “Medical Overview”. We suggest you give a copy to your doctor, and have a copy with you whenever you see any specialist about other medical conditions. It gives them a quick overview of McArdle’s and is structured to direct them to the key points that they need. You and your doctor can read the booklet on-line or download a PDF. Go to the Publications page on the Resources menu.

In addition, we now have Clinical Practice Guidelines for GSD5 and GSD7. These are available in a booklet, which can also be downloaded from the above page.


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Who discovered McArdle disease?


It was Dr Brian McArdle, who published a paper in 1951 describing a man with what we now know of as McArdle disease or Glycogen Storage Disease type 5. 

Is there a treatment for McArdle disease?


The best treatment at present is to improve aerobic fitness of our muscles by regular exercise within our limits. Many people can also benefit from strength training to build muscle mass. Both these approaches should ideally be guided by an experienced  physiotherapist. Guidance is available in the supplementary material to the Clinical Practice Guidelines. There is as yet no medical treatment.

What research is going on into McArdle disease?


This web site has a top menu item for “Research” where you will find details of past and present research projects. Go to our “Current research” page for details of current trials, and a link to the web site “” where you can search for a list of relevant clinical trials.

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