A 28-year-old male from Bangalore with a known diagnosis of Leigh syndrome (mitochondrial encephalomyopathy due to cytochrome c oxidase deficiency) presents with acute onset lethargy, ataxia, and seizures during a febrile illness. Serum lactate is 9 mmol/L (normal < 2), and arterial pH is 7.28 with HCO3− of 16 mEq/L. Brain MRI shows bilateral putaminal necrosis. What is the most appropriate immediate next step in management?

Explanation

## Pathophysiology of Leigh Syndrome & Lactic Acidosis Leigh syndrome is a progressive mitochondrial disorder characterized by cytochrome c oxidase (Complex IV) deficiency, leading to: - Impaired oxidative phosphorylation - Accumulation of NADH and pyruvate - Shunting of pyruvate to lactate via lactate dehydrogenase (LDH) - Secondary lactic acidosis and neurological crisis **Key Point:** In mitochondrial cytopathies, lactic acidosis occurs because the TCA cycle and electron transport chain cannot efficiently oxidize pyruvate to CO₂. Pyruvate accumulates and is reduced to lactate, causing severe metabolic acidosis. ## Role of Pyruvate Dehydrogenase Complex in TCA Cycle ```mermaid flowchart TD A[Pyruvate]:::outcome --> B{Pyruvate Dehydrogenase<br/>Complex Active?}:::decision B -->|Yes| C[Acetyl-CoA]:::outcome C --> D[TCA Cycle]:::action D --> E[NADH + FADH2]:::outcome E --> F[Oxidative Phosphorylation]:::action F --> G[ATP + H2O]:::outcome B -->|No| H[Pyruvate → Lactate<br/>via LDH]:::urgent H --> I[Lactic Acidosis]:::urgent ``` **High-Yield:** Thiamine (vitamin B1) is a cofactor for pyruvate dehydrogenase complex; DCA (dichloroacetate) is an activator of pyruvate dehydrogenase kinase inhibitor that increases PDH activity. Both reduce pyruvate shunting to lactate. ## Management Hierarchy for Acute Mitochondrial Crisis | Step | Intervention | Rationale | | --- | --- | --- | | **Immediate** | IV dextrose, avoid fasting | Provides glucose without requiring oxidative metabolism; prevents further energy crisis | | **Concurrent** | Treat underlying infection | Fever increases metabolic demand and lactate production | | **Adjunctive** | Thiamine + DCA if lactate > 5 mmol/L | Enhances pyruvate dehydrogenase activity; reduces lactate generation | | **Avoid** | Bicarbonate monotherapy | Does not address root cause; may paradoxically worsen intracellular acidosis | **Clinical Pearl:** In acute mitochondrial decompensation, the priority is supportive care (dextrose, infection control) because these address the acute metabolic stress. DCA is reserved for refractory lactic acidosis (lactate > 5 mmol/L) because its evidence base is limited and it carries risk of peripheral neuropathy with prolonged use. ## Why This Answer is Correct The patient has acute metabolic acidosis secondary to mitochondrial dysfunction during a febrile illness. The most appropriate immediate step is: 1. **IV dextrose** — reduces reliance on oxidative metabolism 2. **Treat the fever** — reduces metabolic demand and lactate production 3. **Avoid fasting** — prevents energy crisis 4. **Reserve DCA** — for persistent lactate > 5 mmol/L, as it is not first-line This stepwise approach addresses the acute trigger (infection) and provides metabolic support without rushing to pharmacological lactate-lowering agents.