A 28-year-old woman is admitted with a stab wound to the lateral aspect of the mid-arm. She has inability to extend the thumb at the interphalangeal joint and loss of sensation in the dorsal first web space. Which investigation would best localize the site of radial nerve injury and guide surgical exploration?

Explanation

## Investigation of Choice for Anatomical Localization of Radial Nerve Injury ### Why MRN is Superior for Surgical Planning **Key Point:** Magnetic Resonance Neurography (MRN) provides high-resolution anatomical visualization of peripheral nerves, allowing precise localization of the injury site and assessment of nerve continuity—critical for surgical decision-making. **High-Yield:** MRN advantages in radial nerve injury: - **Spatial resolution:** Visualizes the nerve in cross-section and longitudinally, identifying the exact location of transection, compression, or swelling - **Soft tissue contrast:** Distinguishes nerve from surrounding structures (muscle, fat, vessels) - **Continuity assessment:** Determines whether the nerve is completely divided (neurotmesis) or intact but compressed/contused - **Timing:** Can be performed acutely (unlike EMG/NCS, which require 2–3 weeks for denervation changes) ### Clinical Context: Stab Wound **Clinical Pearl:** In acute penetrating trauma with suspected nerve injury: - **EMG/NCS** cannot assess denervation for 2–3 weeks (too early in this case) - **MRN** can immediately show nerve discontinuity, guiding urgent surgical repair - The finding of **loss of thumb IP extension** = extensor pollicis longus (EPL) paralysis, confirming posterior interosseous nerve (PIN, a branch of radial nerve) involvement - **First web space sensory loss** = superficial radial nerve (sensory branch) involvement ### Comparison Table: Investigations for Nerve Localization | Investigation | Spatial Resolution | Timing | Continuity Assessment | Surgical Utility | | --- | --- | --- | --- | --- | | **MRN** | Excellent (high-res) | Acute | Yes (shows transection) | **Best for acute surgical planning** | | Ultrasound | Good (operator-dependent) | Acute | Partial | Useful but less detailed | | CT angiography | Fair (nerve not primary target) | Acute | No | Assesses vascular injury only | | NCS/EMG | N/A (functional only) | Delayed (2–3 weeks) | No | Best for prognosis, not localization | **Mnemonic:** **MRN-PLAN** = Magnetic Resonance Neurography for Precise Localization And Nerve continuity assessment.