Educational scope notice: This is a study note for medical students, not medical advice, diagnosis, or treatment guidance. Clinical management should follow local protocols and current guidelines.
Metabolic myopathies weaken muscle not by breaking its structure but by starving its chemistry. The fibre may look intact at rest yet fail the moment energy demand exceeds what a defective pathway can supply. Pain and fatigue therefore arrive with exertion and ease with rest, a timing that separates these disorders from the fixed weakness of dystrophy and the fatigability of junction disease.
Three pathways, three failure modes
Muscle draws rapid energy from glycogen breakdown, sustained energy from fatty acid oxidation, and supporting flux through nucleotide salvage. Which pathway fails determines which effort provokes symptoms: brief intense bursts implicate glycogen use, prolonged sustained work implicates oxidative phosphorylation, and fasting or long low-intensity effort implicates fat oxidation. Lipid and purine-cycle defects are recognised but covered in less depth here; the glycolytic and mitochondrial patterns below carry most bedside diagnoses.
McArdle disease and the second wind
McArdle disease, glycogen storage disease type V, comes from myophosphorylase deficiency, the enzyme that releases glucose from muscle glycogen. Short intense efforts such as sprinting, stair climbing, or heavy lifting provoke cramps, pain, and fatigue because anaerobic fuel is unavailable.
Its hallmark is the second wind: after several minutes of continued moderate exercise, symptoms ease as blood-borne glucose and fatty acids arrive as alternative fuels that bypass the blocked step. Cramps that improve with continued activity therefore point toward McArdle disease, while cramps that worsen with effort point elsewhere. Management centres on activity planning and avoiding precipitating bursts rather than on any established replacement therapy.
Mitochondrial myopathies
Mitochondrial disease impairs oxidative phosphorylation, so sustained activity rather than brief bursts provokes weakness and fatigue. Because mitochondria serve every energy-hungry tissue, muscle symptoms travel with brain, heart, and eye involvement, producing encephalomyopathies rather than pure myopathies.
Mitochondrial DNA is maternally inherited, and each cell carries hundreds of mitochondria. Mutant and normal genomes commonly coexist in one cell, a state called heteroplasmy, with symptoms emerging only where the mutant share crosses a threshold. This explains why severity varies so widely within a single family.
Recognisable manifestations include chronic progressive external ophthalmoplegia, a frozen-eye weakness of gaze from extraocular muscle failure; lactic acidosis from forced anaerobic metabolism; myoclonus epilepsy; and stroke-like episodes in young patients, the MELAS pattern, where a young person with a stroke-like event and matrilineal history deserves a mitochondrial workup. On biopsy, subsarcolemmal mitochondrial aggregates stain red with trichrome, the classic ragged-red fibres.
Reading the exercise history
| Exercise pattern | Likely pathway | Example |
|---|---|---|
| Cramps in brief intense effort, easing with continued activity | Glycolytic | McArdle disease |
| Weakness and fatigue in prolonged sustained effort | Oxidative phosphorylation | Mitochondrial myopathy |
| Myalgia in fasting or long low-intensity effort | Fatty acid oxidation | Lipid-pathway defects |
This framework narrows the differential before biochemical or genetic testing confirms the defect.
Evidence anchors
- El-Hattab AW, Almannai M, Scaglia F. MELAS. GeneReviews [Internet]. University of Washington, Seattle. Available from: https://www.ncbi.nlm.nih.gov/books/NBK1233/