biology

Glycogen storage disease type V

Glycogen storage disease type V is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Glycogen storage disease type V rather than just read about it. In short: Glycogen storage disease type V (GSD5, GSD-V), also known as McArdle's disease, is a metabolic disorder, one of the metabolic myopathies, more specifically a muscle glycogen storage disease, caused by a deficiency of myophosphorylase. Its incidence is reported as one in 100,000, roughly the same as glycogen storage disease type I.

Key takeaways

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Reference excerpt

Glycogen storage disease type V (GSD5, GSD-V), also known as McArdle's disease, is a metabolic disorder, one of the metabolic myopathies, more specifically a muscle glycogen storage disease, caused by a deficiency of myophosphorylase. Its incidence is reported as one in 100,000, roughly the same as glycogen storage disease type I. The disease was first reported in 1951 by British physician Brian McArdle of Guy's Hospital, London.

Signs and symptoms

Onset of symptoms and diagnostic delay In the classic phenotype, the onset of this disease is usually noticed in childhood, but often not diagnosed until the third or fourth decade of life, frequently due to misdiagnosis and dismissal of symptoms. The median age of symptom onset is 3 years, with the median diagnostic delay being 29 years. Misdiagnosis is overwhelmingly common, with approximately 90% of patients being misdiagnosed, and approximately 62% receiving multiple misdiagnoses before a correct diagnosis. The prolonged diagnostic delay, misdiagnosis or multiple misdiagnoses, or being given inappropriate exercise advice (such as ignore pain or avoid exercise) severely impacts the quality of life (QoL), physically and mentally.

Ultra-rare phenotypes

Late adult-onset, limb–girdle phenotype There is an ultra-rare adult-onset, limb–girdle phenotype that presents very late in life (70+ years of age) due to a recessive homozygous PYGM mutation (p. Lys42Profs*48) resulting in severe upper and lower limb atrophy, with the possibility of ptosis (drooping eyelids) and camptocormia (stooped posture). As of 2017, there have been two reported cases of this specific homozygous mutation and phenotype. In 1980, a woman also had a limb–girdle phenotype with onset at age 60, histochemical staining showed myophosphorylase deficiency; however the genetic mutation was unknown.

Fatal infantile-onset phenotype There is an ultra-rare, fatal infantile-onset phenotype that results in profound muscle weakness ("floppy baby") and respiratory failure within weeks of birth (perinatal asphyxia). Post-mortem biopsy showed a deficiency of myophosphorylase and abnormal glycogen accumulation in skeletal muscle tissue. This phenotype may also include premature birth and joint contractures. Two reported cases, in 1978 and 1989.

Mild phenotype There is an ultra-rare mild phenotype caused by recessive heterozygous alleles in the PYGM gene, where one allele is a common exon mutation and the other allele is an ultra-rare intronic mutation. It can also be caused by recessive homozygous intronic mutations. These intronic mutations result in a milder phenotype compared to the classic phenotype of McArdle disease. There is residual myophosphorylase activity, between 1-2% residual activity compared to unaffected individuals. This results in greater exercise capacity compared to classic phenotype McArdle individuals, particularly for sustained aerobic activity, but the capacity was still below that of unaffected individuals. In this mild phenotype, since their early teens, they did experience cramping and premature muscle fatigue during sudden vigorous exercise and prolonged isometric exercise; however, due to their less diminished capacity for aerobic activity, they were able to keep up with their peers in sports and everyday activities. As of 2009, there have been 3 reported cases of non-related individuals, a reported Druze family of consanguineous (related) individuals and 9 reported cases in two Finnish families.

Common signs and symptoms The most prominent symptom is that of exercise intolerance which includes:

premature muscle fatigue (particularly for anaerobic activity and high-intensity aerobic activity, which may be described as inability to keep up with peers or reduced stamina); exercise-induced painful cramps; inappropriate rapid heart rate response to exercise; exaggerated cardiorespiratory response to exercise (heavy or rapid breathing with inappropriately rapid heart rate); second wind phenomenon (muscle fatigue and heart rate improve for aerobic activity after approximately 6–10 minutes). Heart rate during exercise is a key indicator as, unlike the symptoms of muscle fatigue and cramping, it is a medical sign (meaning that it is observable and measurable by a third party rather than felt subjectively by the patient). In regularly active individuals with McArdle disease, they may not feel the usual symptoms of muscle fatigue and cramping until they increase their speed to very brisk walking, jogging, or cycling; however, they will still show an inappropriately rapid heart rate response to exercise, with a declining heart rate once second wind has been achieved."In McArdle's, our heart rate tends to increase in what is called an 'inappropriate' response. That is, after the start of exercise it increases much more quickly than would be expected in someone unaffected by McArdle's."

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Illustrations

Worked examples

Example 1 — a first encounter with Glycogen storage disease type V

Start with the simplest possible case. Write down what Glycogen storage disease type V claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Glycogen storage disease type V before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Glycogen storage disease type V ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Glycogen storage disease type V

In research: Glycogen storage disease type V appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Glycogen storage disease type V in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Glycogen storage disease type V is common in secondary-school and first-year university syllabi. It links to neighbouring topics Autosomal recessive disorders, Inborn errors of carbohydrate metabolism, Muscular disorders, so understanding it makes those chapters shorter.
In everyday life: Look for Glycogen storage disease type V outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Glycogen storage disease type V in 20 minutes

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Frequently asked questions

What is Glycogen storage disease type V in simple terms?

Why does Glycogen storage disease type V matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Glycogen storage disease type V?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Glycogen storage disease type V.