Tarui disease (GSD VII) is a rare autosomal recessive metabolic myopathy caused by mutations in the PFKM gene (chromosome 1), resulting in deficiency of the muscle isoform of phosphofructokinase (PFK) — the rate-limiting enzyme of glycolysis that catalyzes conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. [1-2] Incidence is estimated at <1 per 1,000,000 live births, with higher prevalence in Japanese and Ashkenazi Jewish populations. [3-4] Four clinical subtypes exist: classical (most common), severe infantile, late-onset, and hemolytic. [5-6]
1. History
- Exercise intolerance is the hallmark — onset typically in childhood or second/third decade [1][7]
- Myalgia, muscle cramps, and stiffness provoked by short-duration, high-intensity, or isometric exercise [1][8]
- "Out-of-wind" phenomenon: worsening of exercise intolerance after carbohydrate-rich meals or glucose ingestion — a distinguishing feature from McArdle disease [1][9]
- Absence of "second wind" phenomenon (unlike McArdle disease, where symptoms improve after brief rest) [1]
- Nausea and vomiting commonly accompany exercise-induced crises [3-4]
- Dark or cola-colored urine after exertion (myoglobinuria) [5][8]
- Better exercise tolerance in the fasting state or with increased circulating free fatty acids [7][9]
- Ask about symptom onset age, frequency of episodes, relationship to meals, and family history of consanguinity
2. Alarm Features
- Myoglobinuria → risk of acute kidney injury/renal failure [5][10]
- Severe rhabdomyolysis with markedly elevated CK, hyperkalemia, compartment syndrome
- Severe infantile form: hypotonia at birth, cardiomyopathy, respiratory failure — typically fatal within the first year [5]
- Progressive fixed weakness in late-onset form, especially after age 50 [3][7]
- Jaundice or signs of hemolytic crisis [5]
- Seizures reported in association with acute rhabdomyolysis episodes [10]
3. Medications
- No specific pharmacologic therapy exists for GSD VII [4][11]
- Avoid pre-exercise glucose/carbohydrate supplementation — worsens symptoms via the "out-of-wind" mechanism (glucose inhibits lipolysis, depriving muscle of alternative fatty acid fuel) [1][12-13]
- Avoid myotoxic drugs: cyclosporine and amiodarone have been reported to worsen myopathy significantly in GSD VII [14]
- Statins and other potentially myotoxic medications should be used with extreme caution
- Allopurinol may be considered for management of hyperuricemia/gout
- Creatine monohydrate has shown mild benefit in McArdle disease but is not established for GSD VII [12]
4. Diet
- Avoid high-carbohydrate meals before exercise — this is the single most important dietary intervention [7][9][13]
- Ketogenic diet (KD) implemented as a modified Atkins diet has shown long-term benefit in a 5-year case study: improved exercise tolerance, normalized ammonia levels, improved oxygen uptake and ventilatory parameters [11]
- High-fat diet provides alternative fuel (free fatty acids and ketones) that bypasses the glycolytic block [7][11]
- Adequate hydration is critical during and after exercise to reduce rhabdomyolysis and myoglobinuria risk
- Fasting state may paradoxically improve exercise performance by promoting lipolysis [7][9]
- KD should be implemented only under medical and nutritional supervision [11]
The following figure from Haller & Lewis (NEJM, 1991) demonstrates the profound impact of substrate availability on exercise capacity — glucose infusion markedly impairs maximal work intensity and oxygen uptake compared to fasting or lipid infusion conditions:
5. Review of Systems
- Musculoskeletal: exercise intolerance, myalgia, cramps, muscle stiffness, fixed weakness (late-onset)
- Hematologic: jaundice, pallor (compensated hemolytic anemia)
- Renal: dark urine (myoglobinuria), decreased urine output
- GI: nausea, vomiting during exercise-induced crises
- Cardiac: palpitations, dyspnea (rare hypertrophic cardiomyopathy) [8]
- Rheumatologic: gout symptoms (hyperuricemia)
6. Collateral History and Family History
- Autosomal recessive inheritance — both parents are carriers [2][5]
- Inquire about consanguinity, which increases risk
- Family members with exercise intolerance, unexplained anemia, or gout
- Ethnic background: higher prevalence in Ashkenazi Jewish and Japanese populations [3-4]
- Siblings should be screened if index case is identified
- Assess for co-existing myoadenylate deaminase (AMPD) deficiency, which can exacerbate the phenotype [3]
7. Risk Factors
- Ashkenazi Jewish or Japanese ancestry [3-4]
- Parental consanguinity
- Family history of metabolic myopathy or unexplained hemolytic anemia
- High-carbohydrate diet worsens clinical manifestations
- High-intensity or isometric exercise triggers acute episodes [1][7]
8. Differential Diagnosis
- McArdle disease (GSD V) — most important mimic; distinguished by presence of second-wind phenomenon, absence of hemolytic anemia, and improvement with pre-exercise carbohydrate [1][3]
- Phosphoglycerate kinase (PGK) deficiency — the only other glycogenosis with combined myopathy and hemolytic anemia [3]
- Other glycolytic/glycogenolytic defects: GSD IX (phosphorylase kinase), GSD X (phosphoglycerate mutase), GSD XI (LDH-A) [13]
- Fatty acid oxidation disorders: CPT II deficiency, VLCAD deficiency — distinguished by symptoms during prolonged exercise/fasting rather than short intense exercise [15]
- Late-onset Pompe disease (GSD II) — progressive proximal weakness with respiratory insufficiency [15]
- Inflammatory myopathies (polymyositis, dermatomyositis) — if fixed weakness predominates
- Mitochondrial myopathies — elevated lactate at rest, multisystem involvement
9. Past Medical History
- Prior episodes of rhabdomyolysis or myoglobinuria
- History of unexplained anemia or jaundice
- Gout or hyperuricemia
- Renal impairment from recurrent myoglobinuria
- Cardiac history (rare hypertrophic cardiomyopathy) [8]
- Liver disease (relevant if considering myotoxic drugs) [14]
- Surgical history (anesthesia risk with metabolic myopathies)
10. Physical Exam
- Often normal between episodes, especially in patients <50 years [7]
- Vital signs: generally normal; tachycardia during acute rhabdomyolysis
- Muscle exam: usually no fixed weakness in classical form; proximal weakness may develop in late-onset form [5]
- Jaundice/scleral icterus: from compensated hemolysis [5][9]
- Muscle tenderness during or after acute episodes
- No muscle hypertrophy (unlike some cases of McArdle disease)
- Cardiac exam: rarely, signs of hypertrophic cardiomyopathy [8]
- Urine inspection: dark/cola-colored during myoglobinuria episodes
11. Lab Studies
- Creatine kinase (CK): elevated at baseline in most patients; markedly elevated during rhabdomyolysis [2][7]
- Reticulocyte count: elevated (compensated hemolysis) [3][7]
- Indirect bilirubin: elevated [3][9]
- Uric acid: elevated (hyperuricemia from myogenic purine degradation) [3][8]
- Urinalysis: myoglobinuria during acute episodes
- CBC: anemia may be absent due to compensation; look for reticulocytosis [7]
- BMP/renal function: monitor for AKI during rhabdomyolysis
- Haptoglobin: decreased (hemolysis marker)
- LDH: elevated
- Lactate and ammonia (during exercise testing): absent/blunted lactate rise with exaggerated ammonia response [1][13]
12. Imaging
- Muscle MRI: may show fatty replacement of muscle in advanced/late-onset cases; pattern includes anterior and posterior thigh involvement with selective sparing of medial compartment [14]
- Imaging is generally not required for diagnosis in classical presentation
- Echocardiography: indicated to screen for hypertrophic cardiomyopathy, particularly in infantile form or patients with cardiac symptoms [8]
- Renal ultrasound if concern for myoglobinuria-related kidney injury
13. Special Tests
- Nonischemic forearm exercise test (NIFT): absent or blunted lactate rise with normal/exaggerated ammonia elevation — highly suggestive of glycolytic/glycogenolytic defect [1][13][16]
- Cycle ergometry with lactate/ammonia monitoring: a unique late lactate rise (10–30 min post-exercise) may help distinguish GSD VII from McArdle disease [17]
- Muscle biopsy:
- PAS-positive vacuoles (glycogen accumulation) [3][7]
- Polyglucosan (amylopectin-like material) alongside normal glycogen — a feature relatively unique to GSD VII [3]
- Negative PFK histochemical stain (though false negatives can occur) [3][6]
- Specimen must be flash-frozen immediately — PFK is notoriously labile [3]
- Genetic testing (PFKM gene): definitive diagnosis; whole exome sequencing (WES) or targeted gene panels [2][7]
- 31P-MRS: increased phosphocreatine/ATP ratio at rest, ATP depletion during exercise, absence of intracellular acidification [4]
14. ECG
- Generally normal in classical form
- Indicated if cardiac symptoms or suspicion of cardiomyopathy
- One case reported paroxysmal atrial fibrillation associated with hypertrophic cardiomyopathy [8]
- ECG screening reasonable in all confirmed cases given rare cardiac involvement
15. Assessment
Tarui disease is a rare but clinically significant metabolic myopathy with a complete glycolytic block at the PFK step. [13] The classical form is the most common and generally compatible with a normal lifespan if complications (rhabdomyolysis, renal failure) are avoided. [5][7] Key distinguishing features from McArdle disease include compensated hemolytic anemia, absence of second wind, and the pathognomonic "out-of-wind" phenomenon with carbohydrate ingestion. [1][3] The severe infantile form carries a poor prognosis with death typically in the first year. [5] Late-onset forms may be misdiagnosed as inflammatory myopathy or other neuromuscular conditions for years. [14]
Complications to consider:
- Acute kidney injury from myoglobinuria
- Gout from chronic hyperuricemia
- Progressive myopathy (late-onset)
- Rare hypertrophic cardiomyopathy
- Drug-induced worsening (cyclosporine, amiodarone) [14]
16. Treatment Plan
- No specific pharmacologic cure exists [4][11]
- Lifestyle modification is the cornerstone:
- Avoid high-intensity isometric exercise; engage in gradual, progressive aerobic exercise [4][13]
- Pause and rest at onset of symptoms — pain and fatigue may improve once fatty acid oxidation increases [4]
- Dietary management:
- Avoid pre-exercise carbohydrates [12-13]
- Consider ketogenic or modified Atkins diet under medical supervision [11]
- High-fat, low-carbohydrate diet to promote fatty acid utilization
- Acute rhabdomyolysis management:
- Aggressive IV fluid resuscitation
- Monitor and correct electrolytes (potassium, calcium, phosphorus)
- Monitor renal function and urine output
- Alkalinization of urine if severe myoglobinuria
- Hyperuricemia: allopurinol if symptomatic gout or significantly elevated uric acid
- Genetic counseling for patient and family [2][5]
- Avoid myotoxic medications (cyclosporine, amiodarone, statins with caution) [14]
17. Disposition
- Admit if:
- Significant rhabdomyolysis (CK >5× ULN with myoglobinuria)
- Acute kidney injury or oliguria
- Severe electrolyte derangements (hyperkalemia)
- Hemolytic crisis
- Severe infantile form with respiratory distress or cardiomyopathy
- Discharge if:
- Mild exercise-induced symptoms with normal renal function
- CK trending down, adequate oral hydration
- No myoglobinuria
- Specialist consultation: neurology (neuromuscular), genetics/metabolic medicine, nephrology (if AKI), cardiology (if cardiomyopathy suspected), dietitian (for KD implementation)
18. Follow Up / Return Precautions
- Follow-up with neuromuscular specialist and metabolic medicine within 2–4 weeks of diagnosis or acute episode
- Periodic monitoring: CK, renal function, uric acid, CBC with reticulocyte count, bilirubin
- Echocardiography at baseline and periodically to screen for cardiomyopathy [8]
- Return immediately for: dark/cola-colored urine, decreased urine output, severe muscle pain unresponsive to rest, chest pain, palpitations, or progressive weakness
- Patient counseling:
- Avoid strenuous/isometric exercise; titrate activity to tolerance
- Do not eat high-carbohydrate meals before physical activity
- Carry a medical alert identification and emergency letter explaining the condition [18]
- Hydrate well before, during, and after exercise
- Expected course: classical form is generally stable with appropriate lifestyle modification; late-onset form may develop progressive weakness after age 50 [5][7]
- Genetic counseling: 25% recurrence risk for siblings; carrier testing available for family members [5]
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