Exercise Intolerance & Diagnostic Approach
Metabolic myopathies are a clinically and genetically diverse group of disorders caused by defects in cellular energy metabolism—specifically in glycogenolysis, glycolysis, fatty acid oxidation, or mitochondrial oxidative phosphorylation. They account for a significant proportion of patients presenting with exercise intolerance, exertional myalgias, cramps, and recurrent rhabdomyolysis. These disorders can be broadly classified into dynamic (episodic) phenotypes dominated by exercise-triggered symptoms, static (fixed) phenotypes with progressive proximal weakness, or combined presentations with features of both. A systematic diagnostic approach integrating clinical pattern recognition, biochemical screening, provocative exercise testing, genetic analysis, and muscle biopsy is essential for reaching the correct diagnosis—particularly because several metabolic myopathies are treatable.
Bottom Line
- Clinical phenotypes: Dynamic (exercise intolerance, myalgias, cramps, recurrent rhabdomyolysis), static (progressive proximal weakness), or combined; the pattern of triggers and timing guides differential diagnosis
- Key history clues: “Second wind” phenomenon (McArdle disease), fasting-triggered episodes (fatty acid oxidation defects), carbohydrate-triggered cramps (GSD VII), cold triggers (CPT II), medication-related rhabdomyolysis (statins, anesthetics)
- Rhabdomyolysis evaluation: CK >10,000 U/L (exertional) or >5,000 U/L (nonexertional) defines the syndrome; acute management centers on aggressive IV hydration and renal monitoring; recurrent or unprovoked episodes warrant metabolic and genetic workup
- Tiered diagnostic testing: Tier 1 (blood and urine biochemistry) → Tier 2 (forearm exercise test, cycle ergometry) → Tier 3 (genetic testing—targeted panels or exome sequencing) → Tier 4 (muscle biopsy with histochemical stains)
- Forearm exercise test: Non-ischemic protocol preferred; absent lactate rise with normal ammonia rise indicates a glycolytic defect; absent ammonia rise with normal lactate rise suggests myoadenylate deaminase deficiency
- Treatable conditions: Multiple acyl-CoA dehydrogenase deficiency (riboflavin-responsive), primary carnitine deficiency (carnitine supplementation), Pompe disease (enzyme replacement therapy)—these must not be missed
Clinical Phenotypes of Metabolic Myopathies
Dynamic (Episodic) Phenotype
Patients with dynamic metabolic myopathy present with exercise intolerance, exertional myalgias, muscle cramps or contractures, and recurrent episodes of rhabdomyolysis. Between episodes, the neurologic examination is typically normal. The type, intensity, and duration of exercise that provokes symptoms provide critical diagnostic clues:
- Glycogen storage diseases (GSDs): Symptoms triggered by high-intensity, short-duration exercise (sprinting, weight lifting); muscle contractures are electrically silent on EMG, distinguishing them from true cramps
- Fatty acid oxidation defects (FAODs): Symptoms provoked by prolonged, low-to-moderate intensity exercise (jogging, hiking), fasting, cold exposure, intercurrent illness, or metabolic stress
- Mitochondrial myopathies: Exercise intolerance and early fatigue with endurance activities; exertional dyspnea may be disproportionate to weakness; rhabdomyolysis is less common than in GSDs and FAODs
Static (Fixed) Phenotype
Some metabolic myopathies present primarily with progressive proximal weakness, often mimicking limb-girdle muscular dystrophy. Late-onset Pompe disease (acid alpha-glucosidase deficiency) is the prototypical example, with proximal limb-girdle weakness and early respiratory involvement. Late-onset multiple acyl-CoA dehydrogenase deficiency (MADD) can present with subacute progressive weakness, dysphagia, and respiratory failure. Debrancher enzyme deficiency (GSD III) causes progressive distal and proximal weakness with hepatomegaly.
Combined Phenotype
Many metabolic myopathies demonstrate both dynamic and static features over time. Patients with McArdle disease (GSD V) may develop fixed proximal weakness after decades of episodic symptoms. CPT II deficiency can manifest with both exercise-triggered rhabdomyolysis and slowly progressive limb weakness, particularly in late-onset forms. Recognizing the combined phenotype prevents premature diagnostic closure.
Pattern Recognition: Triggers and Timing
- “Second wind” phenomenon: After 8–10 minutes of moderate exercise, symptoms improve dramatically—highly characteristic of McArdle disease (myophosphorylase deficiency, GSD V); results from increased delivery of bloodborne glucose and free fatty acids for aerobic metabolism
- Fasting or prolonged exercise triggers: Strongly suggests fatty acid oxidation defects, particularly CPT II deficiency (most common FAOD causing rhabdomyolysis in adults) or very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency
- Carbohydrate-rich meal triggers: Characteristic of phosphofructokinase deficiency (GSD VII, Tarui disease)—the “out-of-wind” phenomenon, where glucose paradoxically worsens exercise intolerance by inhibiting free fatty acid utilization
- Cold exposure triggers: CPT II deficiency episodes are frequently precipitated by cold weather, fever, or fasting
- Medication-related: Statins, fibrates, propofol, volatile anesthetics (malignant hyperthermia in RYR1 myopathy), immune checkpoint inhibitors
- Hemolytic features: Jaundice, hyperuricemia, and reticulocytosis suggest phosphofructokinase, phosphoglycerate kinase, or aldolase A deficiency (shared enzyme expression in red blood cells)
Rhabdomyolysis: Definition, Assessment, and Acute Management
Definition and Classification
Rhabdomyolysis is a clinical syndrome of acute muscle injury characterized by myalgia, weakness, and muscle swelling with release of intracellular contents into the circulation. The 2024 European Neuromuscular Centre (ENMC) consensus workshop defined rhabdomyolysis as:
- Exertional rhabdomyolysis: Serum CK >10,000 U/L
- Nonexertional rhabdomyolysis: Serum CK >5,000 U/L
- In patients with underlying neuromuscular disease: CK >5–10× the individual’s baseline level
- Severe rhabdomyolysis: Meets the above criteria plus myoglobinuria or acute kidney injury
Acute Management
The primary goal is prevention of acute kidney injury from myoglobin-induced tubular damage:
- Aggressive IV hydration: Isotonic saline at 200–1000 mL/hour initially, targeting urine output >200–300 mL/hour; adjust based on renal function and fluid tolerance
- Monitor renal function: Serial creatinine, BUN, electrolytes (especially potassium, calcium, phosphorus); CK trending every 6–12 hours
- Electrolyte management: Hyperkalemia (from muscle necrosis) may require emergent treatment; hypocalcemia is common early and should not be corrected unless symptomatic (calcium deposits in damaged muscle); hyperphosphatemia
- Urine alkalinization: Sodium bicarbonate infusion (target urine pH >6.5) remains debated; may reduce myoglobin precipitation in renal tubules but limited evidence of outcome benefit
- Renal replacement therapy: For refractory hyperkalemia, severe metabolic acidosis, or oligoanuric renal failure
- Compartment syndrome: Monitor for compartment pressures in affected limbs; surgical fasciotomy if clinical suspicion is high
When to Suspect a Genetic Cause of Rhabdomyolysis (RHABDO Criteria)
- R — Recurrent episodes of exertional rhabdomyolysis
- H — HyperCKemia persists ≥8 weeks after the event
- A — Accustomed physical exercise (not extreme or unusual activity) triggered the episode
- B — Blood CK >50× the upper limit of normal
- D — Drugs, medications, supplements, and other exogenous factors cannot sufficiently explain severity
- O — Other family members affected, or other exertional symptoms (cramps, myalgias, exercise intolerance)
- Meeting any one of these criteria should prompt consideration of genetic testing
Diagnostic Testing Algorithm
Tier 1: Blood and Urine Biochemistry
Initial screening tests should be obtained both during and between episodes of rhabdomyolysis. Timing relative to the acute event significantly affects interpretation:
| Test | Significance | Key Interpretation |
|---|---|---|
| Serum CK | Nonspecific marker of muscle injury | Does not indicate specific metabolic myopathy category; monitor trend (half-life ~36 hours); persistent elevation ≥8 weeks post-episode suggests underlying myopathy |
| Serum acylcarnitine profile | Screens for fatty acid oxidation defects and MADD | Elevated long-chain species (C16, C18) → CPT II or VLCAD deficiency; elevated species across all chain lengths → MADD; best obtained during metabolic stress or fasting |
| Urine organic acids | Screens for organic acidurias and FAODs | Dicarboxylic aciduria in FAODs; elevated tricarboxylic acid cycle intermediates or 3-methylglutaconic acid in mitochondrial disorders |
| Urine myoglobin | Confirms myoglobinuria | Positive dipstick for blood with absent red blood cells on microscopy is suggestive; direct myoglobin assay is more specific |
| Serum lactate and pyruvate | Screening for mitochondrial disorders | Elevated lactate with normal pyruvate (increased lactate:pyruvate ratio >20) suggests mitochondrial dysfunction; avoid tourniquet artifact |
| Dried blood spot acid alpha-glucosidase | Screens for Pompe disease | Low enzyme activity requires confirmatory genetic testing; do not miss this treatable condition |
| GDF-15 and FGF21 | Biomarkers for mitochondrial myopathy | GDF-15 specificity ~86%, sensitivity ~76%; FGF21 specificity ~89%, most elevated in mitochondrial translation defects and mtDNA deletions |
| Serum carnitine (free and total) | Screens for carnitine deficiency | Low free carnitine suggests primary carnitine deficiency (SLC22A5) or secondary depletion in FAODs |
| CBC with differential | Screens for hemolytic features | Reticulocytosis, low haptoglobin, hyperuricemia → consider glycolytic defects (GSD VII, PGK deficiency); Jordan anomaly (lipid-containing vacuoles in leukocytes) → neutral lipid storage disease |
Tier 2: Provocative Exercise Testing
Exercise testing can help localize the metabolic defect and clarify variants of uncertain significance identified on genetic testing. Two main tests are used:
Forearm Exercise Test (Non-Ischemic Protocol — Preferred):
- Preparation: Place an antecubital IV catheter; patient should be rested; draw baseline serum lactate and ammonia
- Exercise protocol: Patient squeezes a hand dynamometer (or rolled towel) rhythmically for 1 minute: 6 cycles of 9 seconds of maximal effort squeeze followed by 1 second of rest
- Blood sampling: Draw lactate and ammonia at 1, 2, 4, 6, and 10 minutes after exercise cessation
- Normal response: Both lactate and ammonia rise to ≥3× baseline values
- Non-ischemic protocol advantages: No tourniquet required; avoids the risk of painful muscle contractures and compartment syndrome that can occur with the ischemic (tourniquet) test; better patient compliance; comparable diagnostic sensitivity
| Result Pattern | Lactate | Ammonia | Interpretation |
|---|---|---|---|
| Normal | ≥3× baseline rise | ≥3× baseline rise | Normal glycolytic pathway and purine nucleotide cycle |
| Glycolytic defect | Absent or blunted rise | Normal or exaggerated rise | Glycogen storage disease (McArdle, GSD VII, other glycolytic enzyme defects) |
| Myoadenylate deaminase deficiency | Normal rise | Absent or blunted rise | AMPD1 deficiency; clinical significance debated—may be an incidental finding |
| Insufficient effort | No rise | No rise | Patient did not exercise adequately; test is uninterpretable and must be repeated |
Cycle Ergometry (Graded Exercise Test):
- Monitors heart rate, oxygen consumption (VO2), and lactate during graded aerobic exercise on a cycle ergometer
- Exaggerated heart rate response relative to workload (low VO2 max with tachycardia) suggests impaired oxidative metabolism
- Failure to increase venous lactate appropriately during high-intensity exercise suggests a glycolytic defect
- Particularly useful for demonstrating the “second wind” phenomenon in McArdle disease: heart rate decreases and exercise tolerance improves after ~8–10 minutes of moderate exercise
Tier 3: Genetic Testing
Genetic testing has increasingly become the first-line diagnostic tool for suspected metabolic myopathies, often preceding or replacing muscle biopsy:
- Targeted gene panels: Commercially available “metabolic myopathy and rhabdomyolysis” panels include genes for glycolytic defects (PYGM, PFKM, PHKA1, PHKB, PGAM2, PGK1, GYS1, ALDOA, GBE1, AGL, GAA, LAMP2, GYG1), fatty acid oxidation defects (CPT2, ACADVL, ACADM, ETFDH, ETFA, ETFB, SLC22A5, SLC25A20, HADHA, HADHB), and mitochondrial nuclear genes
- Whole-exome or whole-genome sequencing: When targeted panels are negative; identifies recently discovered rhabdomyolysis genes (MLIP, OBSCN, MYH1, DTNA) not yet included in commercial panels
- Mitochondrial DNA sequencing: Best performed on muscle tissue due to heteroplasmy; blood-based mtDNA testing may miss tissue-specific pathogenic variants
- Repeat expansion testing: For suspected myotonic dystrophy (DMPK, CNBP) presenting as myalgia or exercise intolerance
- Important caveat: Include muscular dystrophy genes (DMD, DYSF, ANO5, FKRP, CAV3, GMPPB, RYR1) in the panel, as dystrophies can present with a “pseudometabolic” phenotype of recurrent rhabdomyolysis without fixed weakness
Tier 4: Muscle Biopsy With Special Stains
When genetic testing is nondiagnostic or yields variants of uncertain significance, muscle biopsy provides morphologic and biochemical confirmation. Biopsy should be performed on frozen tissue (not formalin-fixed paraffin-embedded) from a mildly weak muscle, ideally ≥8 weeks after the most recent rhabdomyolysis episode:
| Stain/Technique | Target | Diagnostic Significance |
|---|---|---|
| PAS (periodic acid–Schiff) | Glycogen | Subsarcolemmal glycogen accumulation in glycogen storage diseases; absent staining in GSD V (McArdle) with phosphorylase histochemistry |
| Oil Red O | Lipid | Increased lipid droplets in type 1 fibers in FAODs and MADD; lipid storage myopathy pattern |
| Modified Gomori trichrome | Mitochondria, structural | “Ragged red fibers” (subsarcolemmal mitochondrial accumulation) in mitochondrial myopathies |
| SDH (succinate dehydrogenase) | Complex II activity | “Ragged blue fibers”; SDH-positive/COX-negative fibers strongly suggest mitochondrial DNA deletions or depletion |
| COX (cytochrome c oxidase) | Complex IV activity | COX-negative fibers indicate mitochondrial respiratory chain deficiency; combined COX/SDH staining is the gold standard |
| Myophosphorylase histochemistry | Phosphorylase enzyme | Absent staining diagnostic of McArdle disease (GSD V) |
| PFK histochemistry | Phosphofructokinase enzyme | Absent staining diagnostic of Tarui disease (GSD VII) |
| Electron microscopy | Ultrastructure | Paracrystalline mitochondrial inclusions; glycogen or lipid storage; structural abnormalities |
Differential Diagnosis: Glycogen Storage vs. Lipid Storage vs. Mitochondrial Myopathies
| Feature | Glycogen Storage Diseases | Fatty Acid Oxidation Defects | Mitochondrial Myopathies |
|---|---|---|---|
| Typical trigger | High-intensity, short-duration exercise (sprinting, lifting) | Prolonged exercise, fasting, cold, illness, fever | Endurance exercise; nonspecific fatigue |
| Symptom timing | Early in exercise (first minutes) | Late in exercise or hours after prolonged activity | Early fatigue; exercise intolerance throughout |
| Second wind | Present (McArdle disease) | Absent | Absent |
| Rhabdomyolysis | Common; triggered by anaerobic exercise | Common; triggered by fasting + exercise; can be life-threatening in children | Less common; usually milder |
| Interictal CK | Often elevated (especially McArdle) | Usually normal between episodes | Normal or mildly elevated |
| Fixed weakness | Develops in later decades (McArdle, GSD III) | Uncommon; reported in late-onset CPT II and VLCAD | Common; progressive proximal and/or external ophthalmoplegia |
| Multisystem features | Hepatomegaly (GSD III, IV); hemolysis (GSD VII, PGK) | Hepatic, cardiac (severe infantile forms); cardiomyopathy (VLCAD) | CNS (stroke-like episodes, seizures), deafness, cardiac, endocrine, retinal |
| Forearm exercise test | Absent lactate rise; normal ammonia rise | Normal (glycolytic pathway intact) | Normal or exaggerated lactate rise |
| Key screening labs | CK, uric acid, reticulocyte count, haptoglobin | Acylcarnitine profile, urine organic acids, free carnitine | Lactate:pyruvate ratio, GDF-15, FGF21, amino acids (elevated alanine) |
| Biopsy hallmark | PAS-positive glycogen deposits; absent enzyme histochemistry | Oil Red O–positive lipid accumulation in type 1 fibers | Ragged red fibers (Gomori trichrome); COX-negative fibers |
| Inheritance | Autosomal recessive (most); X-linked (PHKA1, PGK1) | Autosomal recessive | Maternal (mtDNA); autosomal (nuclear genes) |
| Prototypical disorder | McArdle disease (PYGM) | CPT II deficiency (CPT2) | MELAS; chronic PEO |
Flowchart Approach: When to Suspect Each Category
A clinical flowchart helps guide the initial differential based on presentation:
Clinical Decision Pathway
- Step 1 — Dynamic vs. static presentation:
- Episodic exercise intolerance, cramps, rhabdomyolysis → dynamic metabolic myopathy (proceed to Step 2)
- Progressive proximal weakness ± respiratory involvement → consider Pompe disease (dried blood spot), MADD (acylcarnitine profile), or GSD III; also consider muscular dystrophy or inflammatory myopathy
- Step 2 — Identify the trigger pattern:
- High-intensity exercise with early onset ± second wind → glycogen storage disease (check forearm exercise test; myophosphorylase, PFK histochemistry on biopsy)
- Prolonged exercise, fasting, cold, or illness → fatty acid oxidation defect (check acylcarnitine profile, urine organic acids, free carnitine)
- Nonspecific fatigue, exercise intolerance ± multisystem features → mitochondrial myopathy (check lactate:pyruvate ratio, GDF-15, FGF21)
- Step 3 — Genetic testing: Metabolic myopathy and rhabdomyolysis gene panel (include dystrophy genes); if negative, proceed to whole-exome or whole-genome sequencing
- Step 4 — Muscle biopsy: If genetic testing is negative or yields variants of uncertain significance; perform ≥8 weeks after acute rhabdomyolysis; frozen tissue with full histochemical panel
Mimics of Metabolic Myopathy
Several conditions can mimic the exercise intolerance and myalgias of metabolic myopathy and should be considered in the differential diagnosis:
- Functional/deconditioning: Most common cause of exercise intolerance in the general population; normal CK, normal examination, no rhabdomyolysis; gradual exercise reconditioning improves symptoms
- Hypothyroidism: Causes fatigue, myalgias, and mild CK elevation; check TSH in all patients with unexplained exercise intolerance; may cause proximal weakness and myoedema (mounding of muscle on percussion)
- Vitamin D deficiency: Associated with proximal weakness, myalgias, and fatigue; CK may be mildly elevated; common and easily treatable
- Statin myopathy: Spectrum from asymptomatic CK elevation to rhabdomyolysis; onset typically weeks to months after starting therapy; can unmask underlying genetic susceptibility (CPT II, VLCAD, RYR1, MADD); check for immune-mediated necrotizing myopathy (anti-HMGCR antibodies) if symptoms persist after statin discontinuation
- Muscular dystrophies with pseudometabolic presentation: Becker muscular dystrophy, dysferlinopathy (DYSF), anoctaminopathy (ANO5), FKRP-related LGMD R9—can present with exertional rhabdomyolysis and normal or near-normal strength between episodes; persistently elevated interictal CK is a clue
- RYR1-related myopathies: Can cause malignant hyperthermia, exertional rhabdomyolysis, and late-onset periodic paralysis-like episodes; family history of malignant hyperthermia is an important clue
- Chronic fatigue syndrome/fibromyalgia: Widespread pain and fatigue without objective weakness; normal CK; normal provocative testing
Specific Treatable Metabolic Myopathies
Do Not Miss These Treatable Conditions
- Multiple acyl-CoA dehydrogenase deficiency (MADD/glutaric aciduria type II): Caused by autosomal recessive variants in ETFDH (most common in late-onset form), ETFA, or ETFB; presents with progressive proximal weakness, lipid storage on biopsy, and elevated acylcarnitines across all chain lengths; dramatically responsive to riboflavin (100–400 mg/day) and coenzyme Q10; recently linked to sertraline use as a phenocopy
- Primary carnitine deficiency (SLC22A5): Systemic carnitine transporter defect; presents with cardiomyopathy, weakness, and hypoketotic hypoglycemia; very low free carnitine on plasma testing; L-carnitine supplementation is life-saving
- Pompe disease (acid alpha-glucosidase deficiency): Late-onset form presents with limb-girdle weakness and respiratory insufficiency; enzyme replacement therapy (alglucosidase alfa, avalglucosidase alfa) slows progression; screen with dried blood spot assay
- McArdle disease (GSD V): While not curable, the “second wind” can be harnessed therapeutically; gentle warm-up before activity, oral sucrose before planned exercise, and supervised aerobic conditioning improve exercise tolerance significantly
Electrodiagnostic and Imaging Considerations
Electrodiagnostic testing is part of the routine evaluation of patients with suspected metabolic myopathy, although findings may be nonspecific:
- Needle EMG: May be normal between episodes; myopathic motor unit potentials (short duration, low amplitude, polyphasic with rapid recruitment) in patients with fixed weakness; electrical myotonia (waning discharges) can be seen in Pompe disease and acid maltase deficiency without clinical myotonia
- Nerve conduction studies: Typically normal; associated peripheral neuropathy may occur in MADD (ETFDH), debrancher enzyme deficiency (AGL), or mitochondrial disorders
- Repetitive nerve stimulation: Normal in metabolic myopathies; helps exclude neuromuscular junction disorders
- Muscle MRI: Can identify patterns of fatty infiltration and guide biopsy site selection; edema on STIR sequences may indicate active muscle injury; longitudinal imaging tracks disease progression
References
- Kushlaf H. Muscle channelopathies and rhabdomyolysis. Continuum (Minneap Minn) 2025;31(5):1409–1436.
- Milone M. A pattern recognition approach to myopathy. Continuum (Minneap Minn) 2025;31(5):1244–1269.
- Kruijt N, Laforet P, Vissing J, et al. 276th ENMC International Workshop: recommendations on optimal diagnostic pathway and management strategy for patients with acute rhabdomyolysis worldwide. Neuromuscul Disord 2025;50:105344.
- Tarnopolsky MA. A diagnostic algorithm for metabolic myopathies. Curr Neurol Neurosci Rep 2010;10(2):118–126.
- Scalco RS, Gardiner AR, Pitceathly RD, et al. Rhabdomyolysis: a genetic perspective. Orphanet J Rare Dis 2015;10:51.
- Bosch X, Poch E, Grau JM. Rhabdomyolysis and acute kidney injury. N Engl J Med 2009;361(1):62–72.
- Haller RG, Vissing J. Spontaneous “second wind” and glucose-induced second “second wind” in McArdle disease: oxidative mechanisms. Arch Neurol 2002;59(9):1395–1402.
- Poulsen NS, Madsen KL, Hornsyld TM, et al. Growth and differentiation factor 15 as a biomarker for mitochondrial myopathy. Mitochondrion 2020;50:35–44.
- Lehtonen JM, Forsstrom S, Bottani E, et al. FGF21 is a biomarker for mitochondrial translation and mtDNA maintenance disorders. Neurology 2016;87(22):2290–2299.
- Hargreaves M, Spriet LL. Skeletal muscle energy metabolism during exercise. Nat Metab 2020;2(9):817–828.
- Stahl K, Rastelli E, Schoser B. A systematic review on the definition of rhabdomyolysis. J Neurol 2020;267(4):877–882.
- Skolka MP, Milone M, Litchy WJ, et al. The utility of electrodiagnostic testing in unprovoked rhabdomyolysis in the era of next-generation sequencing. Muscle Nerve 2024;70(2):180–186.
- Prasun P. Multiple acyl-CoA dehydrogenase deficiency. In: Adam MP, Feldman J, Mirzaa GM, et al, eds. GeneReviews. University of Washington, Seattle; 1993. Updated 2020.
- Scalco RS, Snoeck M, Quinlivan R, et al. Exertional rhabdomyolysis: physiological response or manifestation of an underlying myopathy? BMJ Open Sport Exerc Med 2016;2(1):e000151.
- Chin HL, Lai PS, Tay SKH. A clinical approach to diagnosis and management of mitochondrial myopathies. Neurotherapeutics 2024;21(1):e00304.