Pyruvate, Krebs Cycle, Lactic Acidemia & Mitochondrial Disease

Pyruvate dehydrogenase deficiency · Krebs cycle defects · Lactic acidosis · Respiratory chain disorders · mtDNA vs nuclear DNA · MELAS, MERRF, Leigh syndrome · Coenzyme Q10, riboflavin, supportive care
🧬 Key concepts: Elevated lactate/pyruvate ratio (respiratory chain) vs normal ratio (PDH). Leigh syndrome (basal ganglia lesions). MELAS (stroke-like episodes). Mitochondrial inheritance (maternal). Muscle biopsy (ragged red fibers).

📖 Pyruvate, Krebs cycle, lactic acidemia & mitochondrial disorders

🧪 Pyruvate & Krebs cycle defects
PDH deficiency: lactic acidosis, neurological dysfunction, normal lactate/pyruvate ratio. Krebs: fumarase, succinate dehydrogenase, α-ketoglutarate dehydrogenase deficiency → encephalopathy, cardiomyopathy.
⚡ Respiratory chain disorders
Complex I-V defects. mtDNA (maternal) or nuclear. MELAS (mtDNA 3243), MERRF, Leigh syndrome (SURF1, MT-ATP6). NARP, LHON. Ragged red fibers on muscle biopsy.
🩺 Clinical features
Multisystem: progressive encephalopathy, seizures, myopathy, ptosis, ophthalmoplegia, cardiomyopathy, liver failure, diabetes, deafness, short stature.
💊 Genetic counseling & therapy
mtDNA disorders: maternal inheritance; nuclear: AR/AD. No cure. Supportive: cofactors (riboflavin, CoQ10, thiamine, carnitine). Avoid valproate (mtDNA depletion). Exercise, symptomatic treatment.
⚠️ Red flags for mitochondrial disease: Multisystem involvement (neurologic + cardiac + endocrine), progressive course, maternal inheritance pattern, stroke-like episodes, lactic acidosis, ragged red fibers.

🩺 Diagnostic approach to suspected mitochondrial disease

1
Clinical suspicion – Multisystem involvement: neurological (seizures, ataxia, dementia), myopathy (ptosis, ophthalmoplegia, weakness), cardiac (cardiomyopathy), endocrine (diabetes), hepatic, renal.
2
Initial labs – Blood lactate (fasting and post-prandial), pyruvate, lactate/pyruvate ratio (elevated >20 suggests respiratory chain defect; normal in PDH deficiency). CSF lactate, amino acids.
3
Biochemical & genetic testing – Muscle biopsy (histochemistry, electron microscopy, respiratory chain enzyme assays). mtDNA sequencing (blood, muscle). Nuclear gene panel (exome).
4
Imaging – MRI brain: basal ganglia changes (Leigh), stroke-like lesions (MELAS), leukodystrophy.
📌 Clinical pearl: A child with recurrent stroke-like episodes, seizures, headache, and lactic acidosis suggests MELAS (mitochondrial encephalomyopathy, lactic acidosis, stroke-like episodes).

📋 Stepwise management of mitochondrial & pyruvate disorders

1
Acute management of lactic acidosis – IV fluids, sodium bicarbonate (judicious), thiamine, riboflavin. Avoid medications that impair mitochondrial function (valproate, metformin, aminoglycosides).
2
Pyruvate dehydrogenase (PDH) deficiency – Ketogenic diet (provides alternative fuel). Dichloroacetate (controversial), thiamine supplementation.
3
Respiratory chain disorders – cofactor therapy – Riboflavin (complex I), Coenzyme Q10, idebenone (LHON), creatine, carnitine. Thiamine, vitamin C, vitamin E.
4
Supportive care – Manage seizures, cardiomyopathy, diabetes. Avoid fasting (provoke lactic acidosis). Physical therapy, hearing aids, cochlear implants.
5
Genetic counseling – mtDNA disorders: maternal inheritance (all children of affected mother at risk, sons do not transmit). Nuclear disorders: autosomal recessive/dominant. Prenatal diagnosis available for some.
🚨 Avoid in mitochondrial disease: Valproate (can cause liver failure in mtDNA depletion). Metformin (lactic acidosis). Aminoglycosides (ototoxicity). General anesthesia risk.

🧠 Reflex prompts: pyruvate, Krebs, mitochondrial disorders

🧠 2-year-old with developmental regression, ataxia, bilateral basal ganglia lesions on MRI, elevated lactate. Diagnosis?
Leigh syndrome (subacute necrotizing encephalomyelopathy). SURF1 or mtDNA mutations.
⚡ Adolescent with stroke-like episodes, seizures, migraine, short stature, myopathy, elevated lactate. Diagnosis?
MELAS (mitochondrial encephalomyopathy, lactic acidosis, stroke-like episodes). mtDNA 3243A>G.
👁️ Teenager with progressive external ophthalmoplegia, ptosis, and ragged red fibers on muscle biopsy. Diagnosis?
Chronic progressive external ophthalmoplegia (CPEO). mtDNA deletions.
🧬 A child with lactic acidosis, normal lactate/pyruvate ratio, and no response to dichloroacetate. Most likely?
Pyruvate dehydrogenase (PDH) deficiency. Ketogenic diet.
🩸 What laboratory finding distinguishes respiratory chain defect from PDH deficiency?
Lactate/pyruvate ratio >20 suggests respiratory chain (increased NADH/NAD+). Normal ratio in PDH deficiency.
💊 Which cofactor is used for CoQ10 deficiency?
Coenzyme Q10 supplementation (2-30 mg/kg/day).
🧬 What is the inheritance pattern of MELAS?
Maternal (mtDNA mutation). All children of affected mother inherit, but expressivity varies.
🩺 What is the classic triad of Leigh syndrome on MRI?
Symmetrical basal ganglia (putamen) and brainstem T2 hyperintensities.
💊 Which anticonvulsant should be avoided in suspected mitochondrial disease?
Valproate (risk of hepatotoxicity and worsening of mitochondrial dysfunction).
🧪 What is the diagnostic gold standard for respiratory chain disorders?
Muscle biopsy for histochemistry (ragged red fibers) and respiratory chain enzyme assays.