ConditionBones, joints & muscles
Metabolic Myopathy
A metabolic myopathy is an inherited disorder in which muscles cannot make or use energy normally, because an enzyme for breaking down sugar (glycogen) or fat, or the mitochondria, does not work properly. The typical result is exercise intolerance: early fatigue, painful cramps, muscle breakdown with dark cola-colored urine, or slowly progressive weakness. Symptoms often start in childhood or the teens. Diagnosis relies on blood tests, genetic testing and sometimes a muscle biopsy. Most types are managed with tailored exercise, diet and avoiding triggers.
Also called: Metabolic muscle disease, inherited metabolic muscle disorder, disorders of muscle energy metabolism
At a glance
- What it is
- A genetic defect in how muscles produce energy from glycogen, fat or mitochondria.
- Main types
- Glycogen storage myopathies (such as McArdle disease), fatty acid oxidation disorders (such as CPT2 deficiency) and mitochondrial myopathies.
- Key symptoms
- Exercise intolerance, cramps, muscle pain, episodes of rhabdomyolysis with dark urine, or progressive weakness.
- Diagnosis
- Creatine kinase (CK), acylcarnitine profile, forearm exercise test, genetic testing, muscle biopsy.
- Treatment
- No cure for most. Diet, adapted exercise and avoiding triggers. Enzyme replacement for Pompe disease.
When to get help
Call 911 or go to the ER
- Dark brown or cola-colored urine with severe muscle pain or weakness after exercise, fasting or illness: go to the ER, as rhabdomyolysis can cause kidney failure
- Muscle weakness affecting breathing or swallowing
See a doctor within a day or two
- Passing very little urine after an episode of muscle breakdown
- Severe muscle pain and swelling after exertion that does not ease with rest
- A known fatty acid oxidation disorder with fever, vomiting or inability to eat
Book a routine appointment
- Cramps, pain or exhaustion with exercise that classmates or peers do not get
- Repeated episodes of muscle pain after long exercise, fasting or cold
- Raised CK on a blood test, or slowly worsening muscle weakness
- A family history of a metabolic muscle disease, to discuss genetic counseling
How muscles get energy
Muscles run on ATP, made from three main fuels:
- Glycogen (stored sugar) for short, intense bursts such as sprinting or lifting
- Fatty acids for long, steady activity, and during fasting
- Mitochondria, the cell's power plants, which turn both fuels into ATP using oxygen
A defect in any of these pathways leaves the muscle short of energy under particular conditions. The pattern of when symptoms happen is the biggest diagnostic clue.
Types of metabolic myopathy
| Glycogen storage myopathies | Fatty acid oxidation disorders | Mitochondrial myopathies | |
|---|---|---|---|
| Examples | McArdle disease (GSD V), Tarui disease (GSD VII), Pompe disease (GSD II) | CPT2 deficiency, VLCAD deficiency, primary carnitine deficiency | MELAS, CPEO, mitochondrial DNA deletions, many others |
| Symptoms triggered by | Short, intense exercise | Prolonged exercise, fasting, fever, cold, stress | Any sustained exertion |
| Typical features | Cramps and pain within minutes. "Second wind" after resting briefly (McArdle) | Pain and dark urine hours after exertion, often normal between attacks | Fatigue, weakness, drooping eyelids, eye movement problems, other organs affected |
| CK between attacks | Usually raised | Often normal | Normal or mildly raised |
Pompe disease (acid maltase deficiency) differs from the others: it usually causes slowly progressive weakness of the hips and breathing muscles rather than exercise attacks.
Other causes of metabolic muscle disease
Some doctors also include acquired endocrine and metabolic causes of myopathy, such as thyroid disease, Cushing syndrome, low vitamin D and electrolyte problems, as well as drug causes like statins. These are usually treatable once found.
Symptoms
- Exercise intolerance: getting tired, breathless or sore much sooner than others
- Muscle cramps, often painful contractures that lock the muscle
- Muscle pain during or after exertion
- Episodes of rhabdomyolysis: muscle breakdown that releases myoglobin, turning urine dark red, brown or cola-colored
- Fixed or progressive muscle weakness, especially in Pompe disease and mitochondrial disorders
- In mitochondrial disease: hearing loss, diabetes, heart problems, seizures, short stature or stroke-like episodes
Many people are told for years that they are simply unfit or lazy before a diagnosis is made.
Causes and inheritance
Metabolic myopathies are caused by changes in genes for specific enzymes. Most glycogen and fat disorders are autosomal recessive: both parents are healthy carriers, and each child has a 1 in 4 chance of being affected. Mitochondrial myopathies may follow recessive inheritance or be passed only through the mother (mitochondrial DNA).
Diagnosis
- History of triggers: what type of exercise, fasting, illness or cold brings symptoms on.
- Creatine kinase (CK): a muscle enzyme. It rises during attacks and often stays high in McArdle disease.
- Urine myoglobin and kidney function during an attack.
- Acylcarnitine profile and carnitine levels: blood tests that point to fatty acid oxidation disorders.
- Forearm exercise test: lactate and ammonia are measured during hand-grip exercise. Lactate fails to rise in glycogen disorders.
- Lactate levels and cardiopulmonary exercise testing, useful in mitochondrial disease.
- Genetic testing: gene panels or exome sequencing now often confirm the diagnosis without a biopsy.
- Muscle biopsy: shows stored glycogen or fat, missing enzymes, or abnormal mitochondria ("ragged red fibers").
A doctor will also check for look-alikes such as inflammatory myopathy, muscular dystrophies and thyroid disease.
Treatment and management
General measures
- Regular moderate aerobic exercise: improves fitness and reduces symptoms in McArdle disease and many mitochondrial disorders. Build up slowly and avoid sudden, maximal or heavy isometric effort.
- Warm up gradually, and in McArdle disease use the "second wind" by starting slowly.
- Avoid triggers: fasting, prolonged intense exercise and cold in fat oxidation disorders.
- Stay hydrated and stop exercise at the first sign of cramps or unusual pain.
- Emergency letter or medical alert explaining the condition and the need for IV glucose and fluids during attacks.
Type-specific treatment
- McArdle disease: a small sugary drink before planned exercise can help. A diet adequate in complex carbohydrates is recommended.
- Fatty acid oxidation disorders: frequent meals, avoiding fasting, extra carbohydrate during illness and exercise. Some people use medium-chain triglyceride oil or triheptanoin under specialist supervision. Carnitine replacement for primary carnitine deficiency.
- Pompe disease: enzyme replacement therapy given by IV infusion.
- Mitochondrial myopathies: exercise training, coenzyme Q10 and other supplements in some cases, and management of heart, hearing and diabetes complications.
Medicines to discuss with your doctor
Some medicines can worsen certain metabolic myopathies, including statins, valproate (in mitochondrial disease) and some anesthetics. Tell all doctors and anesthetists about your diagnosis before surgery.
Outlook
Most people with McArdle disease or CPT2 deficiency live a normal lifespan by adapting exercise and avoiding triggers. The main danger is kidney injury from severe rhabdomyolysis, which is preventable with prompt treatment. Mitochondrial myopathies vary widely, from mild exercise intolerance to severe multi-system disease. Early-onset Pompe disease is serious, but enzyme replacement has improved survival.
Frequently asked questions
What are the symptoms of metabolic myopathy?
The main symptoms are exercise intolerance, early fatigue, muscle cramps and pain during or after exercise, and episodes of dark urine from muscle breakdown. Some types cause slowly worsening weakness instead, and mitochondrial forms can affect the eyes, hearing, heart and brain.
What is the most common metabolic myopathy?
McArdle disease and CPT2 deficiency are among the most commonly diagnosed metabolic myopathies in adults, though each is still rare. Mitochondrial disorders, taken together, are also relatively common among inherited muscle diseases.
Can exercise help metabolic myopathy?
Yes, when done correctly. Regular moderate aerobic exercise improves fitness and tolerance in McArdle disease and many mitochondrial myopathies. Intense, sudden or prolonged exertion should be avoided, and fasting before exercise is risky in fatty acid oxidation disorders.
How is metabolic myopathy diagnosed?
Doctors combine your history of exercise triggers with blood tests (CK, lactate, acylcarnitines), a forearm exercise test and, increasingly, genetic testing. A muscle biopsy is used when genetic tests are inconclusive.
Why does my urine turn dark after exercise?
Dark brown or cola-colored urine after exercise can mean myoglobin from muscle breakdown (rhabdomyolysis), which can damage the kidneys. Seek urgent care. Repeated episodes, especially after modest exercise, fasting or illness, should be investigated for a metabolic myopathy.
Sources
This guide was written from current patient and clinical guidance, including:
- Muscular Dystrophy Association: Metabolic Diseases of Muscle
- National Organization for Rare Disorders (NORD): Carnitine palmitoyltransferase II deficiency; McArdle disease
- MedlinePlus Genetics: CPT II deficiency; Glycogen storage disease type V
- Merck Manual Professional Version: Inherited muscular disorders
- American Association of Neuromuscular and Electrodiagnostic Medicine: Metabolic myopathies