A 28-year-old man with a family history of progressive neurodegeneration presents with exercise intolerance, myalgia, and recurrent episodes of dark urine after exertion. Serum creatine kinase is 2400 IU/L. Plasma lactate is disproportionately elevated relative to pyruvate (lactate:pyruvate ratio = 35:1, normal <20:1). Which investigation would best confirm a defect in lactate dehydrogenase or impaired NAD+ regeneration in the TCA cycle?

Explanation

## Clinical Context The markedly elevated lactate:pyruvate ratio (35:1 vs normal <20:1) is a hallmark of impaired NAD+ regeneration or lactate dehydrogenase (LDH) dysfunction. The clinical triad of exercise intolerance, myalgia, and myoglobinuria suggests a metabolic myopathy affecting energy production and lactate clearance. ## Investigation of Choice: Forearm Ischaemic Exercise Test **Key Point:** The forearm ischaemic exercise test is the gold standard for diagnosing metabolic myopathies, particularly those affecting lactate metabolism and NAD+ regeneration: 1. Measures lactate and ammonia response to anaerobic muscle work 2. In LDH deficiency or impaired NAD+ regeneration, lactate fails to rise appropriately (paradoxical flat or blunted response) 3. Ammonia typically rises normally (distinguishes from adenylate deaminase deficiency) 4. Directly assesses muscle's ability to regenerate NAD+ and clear lactate **High-Yield:** A **blunted lactate response** with **normal ammonia rise** during forearm exercise is pathognomonic for LDH deficiency or NAD+ regeneration defects. The elevated resting lactate:pyruvate ratio confirms impaired lactate oxidation. **Clinical Pearl:** In glycolytic myopathies (e.g., phosphofructokinase deficiency), lactate rises but ammonia does not. In LDH deficiency, lactate is already elevated at rest and does not rise further with exercise — a distinctive pattern. ## Diagnostic Algorithm for Metabolic Myopathy ```mermaid flowchart TD A[Exercise intolerance + myalgia + elevated CK]:::outcome --> B{Elevated lactate:pyruvate ratio?}:::decision B -->|Yes| C[Impaired NAD+ regeneration or LDH defect]:::outcome C --> D[Forearm ischaemic exercise test]:::action D --> E{Lactate response?}:::decision E -->|Blunted/flat + normal ammonia| F[LDH deficiency or NAD+ regeneration defect]:::outcome E -->|Normal lactate rise + low ammonia| G[Adenylate deaminase deficiency]:::outcome E -->|Absent lactate + low ammonia| H[Glycolytic enzyme defect]:::outcome ``` ## Comparison of Investigations for Metabolic Myopathy | Investigation | Diagnostic Yield for LDH/NAD+ Defect | Specificity | Feasibility | |---|---|---|---| | **Forearm ischaemic exercise test** | **Excellent** | **High** | **Bedside/outpatient** | | Muscle biopsy + COX staining | Good for mitochondrial disease | Lower for glycolytic defects | Invasive; delayed results | | Myoglobin/creatinine | Confirms rhabdomyolysis | Non-specific | Rapid but not diagnostic | | Cardiac MRI | Assesses cardiomyopathy | Not for metabolic diagnosis | Expensive; indirect | **Mnemonic:** **LACTIC** = **L**actate **A**nd **T**ransaminase **I**schaemic **C**ontraction **I**nvestigation (forearm exercise test) [cite:Harrison 21e Ch 383]