A 35-year-old woman presents with winging of the scapula and inability to abduct the arm beyond 15 degrees following a difficult childbirth with shoulder dystocia. Neurological examination shows weakness of the supraspinatus and infraspinatus muscles. Which investigation would be most appropriate to confirm involvement of the suprascapular nerve and rule out concurrent C5 root injury?

Explanation

## Investigation of Choice for Suprascapular Nerve Injury ### Why NCS of Suprascapular Nerve is Optimal **Key Point:** NCS with CMAP recording from supraspinatus is the most specific investigation to confirm suprascapular nerve conduction block or slowing, thereby localizing the lesion to the suprascapular nerve rather than the proximal C5 root. **High-Yield:** Suprascapular nerve NCS: - Directly assesses conduction velocity and amplitude across the suprascapular notch - Differentiates suprascapular nerve lesion from C5 radiculopathy: - **Suprascapular nerve lesion:** Abnormal suprascapular NCS; normal C5 root conduction (assessed via upper trunk or Erb's point stimulation) - **C5 radiculopathy:** Abnormal conduction at root level; may show normal or mildly abnormal suprascapular NCS depending on severity - Provides objective, quantifiable evidence of nerve dysfunction ### Clinical Context: Birth Injury with Shoulder Dystocia **Clinical Pearl:** Shoulder dystocia can cause traction injuries to the upper trunk (C5–C6) or isolated suprascapular nerve compression at the suprascapular notch. The clinical presentation (supraspinatus/infraspinatus weakness) is compatible with both, so electrophysiological localization is essential. ### Comparison of Investigations for Nerve Localization | Investigation | Localizes Lesion | Functional Assessment | Differentiates C5 Root from Suprascapular Nerve | Practical Utility | |---|---|---|---|---| | **Suprascapular NCS** | Yes (suprascapular notch) | Excellent | Yes (direct nerve assessment) | **Gold standard** | | Cervical MRI | Visualizes root | None | Possible (anatomical) | Complementary; shows root avulsion | | Ultrasound (notch) | Possible (anatomical) | None | No | Emerging; not standard | | Plain radiograph | No | No | No | Detects fractures only | **Mnemonic:** **NCS-CMAP LOCALIZES** — Nerve Conduction Study with Compound Muscle Action Potential recording directly Localizes the site of suprascapular nerve lesion and differentiates it from root pathology. ### Why EMG Complements NCS Combined EMG (showing denervation in supraspinatus/infraspinatus but not in other C5-innervated muscles like deltoid or biceps) further confirms isolated suprascapular nerve injury.