# Clinical Cases: TCA Cycle

## Case 1: Wernicke-Korsakoff Syndrome (Thiamine Deficiency)

### Clinical Image
![Wernicke Encephalopathy MRI](case_01_image.jpg)
*Source: [Radiopaedia - Wernicke encephalopathy](https://radiopaedia.org/articles/wernicke-encephalopathy) - CC BY-NC-SA 3.0*

### Case Presentation
A 52-year-old man with a history of alcohol use disorder is brought to the emergency department by police after being found confused and unsteady on the street. He has a disheveled appearance and poor nutritional status. On examination, he is disoriented to time and place, has horizontal nystagmus with bilateral lateral gaze palsy (ophthalmoplegia), and a wide-based ataxic gait. Vital signs show temperature 37.8C and heart rate 110. Laboratory studies reveal mildly elevated lactate at 3.2 mmol/L and macrocytic anemia (MCV 108 fL). Recognizing the classic triad of Wernicke encephalopathy (confusion, ophthalmoplegia, ataxia), the physician immediately administers IV thiamine (vitamin B1) before any glucose-containing fluids, as glucose administration without thiamine can precipitate or worsen Wernicke encephalopathy. MRI brain shows symmetric T2 hyperintensities in the medial thalami and periaqueductal gray, characteristic of Wernicke encephalopathy. With thiamine repletion, his eye findings resolve within days, but he develops Korsakoff syndrome with profound anterograde amnesia and confabulation.

### Key Learning Points
- Thiamine pyrophosphate (TPP) is an essential cofactor for pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, both critical enzymes linking glycolysis to the TCA cycle
- Without thiamine, pyruvate cannot enter the TCA cycle and is instead shunted to lactate; the brain is particularly vulnerable because it cannot use fatty acids and depends on glucose oxidation
- The classic triad of Wernicke encephalopathy is confusion, ophthalmoplegia, and ataxia; Korsakoff syndrome (irreversible memory impairment) develops if treatment is delayed

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## Case 2: Leigh Syndrome (Mitochondrial Disease)

### Clinical Image
![Leigh Syndrome MRI](case_02_image.jpg)
*Source: [Radiopaedia - Leigh syndrome](https://radiopaedia.org/articles/leigh-syndrome) - CC BY-NC-SA 3.0*

### Case Presentation
A 14-month-old boy presents with progressive developmental regression over 3 months. He had been meeting milestones normally until 11 months of age but has since lost the ability to sit independently and has become increasingly hypotonic. His parents also note episodes of vomiting and irritability. Physical examination reveals generalized hypotonia, absent deep tendon reflexes, and dystonic posturing. He has difficulty swallowing and episodes of apnea. Laboratory studies show elevated serum lactate at 6.5 mmol/L and an elevated lactate-to-pyruvate ratio. MRI brain reveals symmetric T2 hyperintense lesions in the basal ganglia (particularly putamen) and brainstem, characteristic of Leigh syndrome. Genetic testing identifies a mutation in the SURF1 gene, which affects cytochrome c oxidase (complex IV) assembly. He is started on supportive care including coenzyme Q10 and thiamine supplementation. The family is counseled about the poor prognosis of this progressive neurodegenerative condition.

### Key Learning Points
- Leigh syndrome results from defects in mitochondrial energy production, including mutations affecting pyruvate dehydrogenase complex, TCA cycle enzymes, or respiratory chain components
- Elevated lactate and elevated lactate-to-pyruvate ratio indicate impaired mitochondrial function; pyruvate accumulates and is converted to lactate when it cannot enter or proceed through the TCA cycle normally
- The characteristic bilateral symmetric lesions in basal ganglia and brainstem reflect the high metabolic demands of these structures and their vulnerability to energy failure

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## Case 3: IDH-Mutant Glioma (Oncometabolite Production)

### Clinical Image
![IDH Mutant Glioma](case_03_image.jpg)
*Source: [Radiopaedia - IDH mutant glioma](https://radiopaedia.org/articles/isocitrate-dehydrogenase-idh-mutation) - CC BY-NC-SA 3.0*

### Case Presentation
A 38-year-old woman presents with new-onset seizures and a 6-month history of progressive headaches. MRI brain reveals a non-enhancing infiltrative mass in the left frontal lobe with T2/FLAIR hyperintensity. Stereotactic biopsy is performed, and histopathology shows a diffuse astrocytoma. Immunohistochemistry for IDH1 R132H mutation is positive. Her neuro-oncologist explains that this mutation, found in approximately 80% of grade 2-3 gliomas, actually confers a better prognosis than IDH-wildtype tumors. The mutant IDH1 enzyme gains a new function: instead of converting isocitrate to alpha-ketoglutarate, it converts alpha-ketoglutarate to 2-hydroxyglutarate (2-HG), an oncometabolite. 2-HG competitively inhibits alpha-ketoglutarate-dependent enzymes including TET2 (involved in DNA demethylation) and histone demethylases, causing epigenetic dysregulation that promotes tumorigenesis. She undergoes maximal safe resection followed by radiation and temozolomide chemotherapy. Targeted IDH inhibitors (vorasidenib) have recently been FDA-approved for such tumors.

### Key Learning Points
- IDH1 and IDH2 mutations are common in gliomas and acute myeloid leukemia; these mutations confer neomorphic enzyme activity that produces the oncometabolite 2-hydroxyglutarate (2-HG)
- 2-HG structurally resembles alpha-ketoglutarate and competitively inhibits alpha-KG-dependent enzymes, including those involved in histone and DNA demethylation, leading to epigenetic dysregulation
- Understanding this biochemical mechanism has led to targeted therapies: IDH inhibitors (ivosidenib for IDH1, enasidenib for IDH2) are approved for AML and gliomas