A 32-year-old man presents with wrist drop and loss of sensation over the dorsum of the first web space following a fracture of the mid-shaft of the humerus. Which investigation would be most appropriate to confirm the diagnosis and assess the severity of radial nerve injury?

## Investigation of Choice for Radial Nerve Injury ### Why EMG/NCS is the Gold Standard **Key Point:** EMG and NCS are the most specific and sensitive investigations for confirming peripheral nerve injury and determining the degree of axonal damage (neuropraxia vs. axonotmesis vs. neurotmesis). **High-Yield:** In radial nerve injury: - **NCS** demonstrates conduction block or reduced amplitude of compound motor action potential (CMAP) across the lesion site - **EMG** shows denervation potentials (fibrillations and positive sharp waves) in affected muscles (extensor carpi radialis, extensor carpi ulnaris, extensor digitorum communis, extensor pollicis longus) - Timing: Denervation potentials appear 2–3 weeks post-injury; early EMG (first 2 weeks) may be falsely negative ### Clinical Correlation **Clinical Pearl:** The combination of clinical signs (wrist drop = loss of wrist extension; sensory loss in first web space = radial nerve sensory distribution) with EMG/NCS findings allows: 1. Confirmation of radial nerve involvement 2. Localization of the lesion (spiral groove in this case, given mid-shaft humeral fracture) 3. Prognosis assessment (neuropraxia recovers in weeks; axonotmesis in months; neurotmesis requires surgery) ### Diagnostic Algorithm ```mermaid flowchart TD A[Clinical suspicion of radial nerve injury]:::outcome A --> B{Acute presentation?}:::decision B -->|Yes, within 2 weeks| C[Clinical exam + EMG/NCS]:::action B -->|No, >2 weeks| D[EMG/NCS for denervation]:::action C --> E[Assess for conduction block]:::action D --> F[Assess for denervation pattern]:::action E --> G[Prognosis: neuropraxia likely]:::outcome F --> H[Prognosis: axonotmesis/neurotmesis]:::outcome ``` **Mnemonic:** **CMAP-D** = Conduction block (Motor Action Potential) or Denervation pattern = EMG/NCS is the answer.