A 28-year-old labourer presents with wrist drop and inability to extend the fingers. Examination reveals intact sensation over the dorsal first web space and normal elbow extension. Which finding best distinguishes a radial nerve lesion below the elbow from a lesion at the spiral groove?

Explanation

## Discriminating Feature: Elbow Extension Status **Key Point:** The branch to triceps (elbow extension) arises from the radial nerve in the axilla and upper arm, **proximal** to the spiral groove. A lesion at the spiral groove typically spares triceps (elbow extension is preserved), but a lesion **above** the spiral groove would affect it. More importantly, the stem states elbow extension is **intact** in this patient — this finding is consistent with a lesion at or below the spiral groove. However, the key discriminator between a **spiral groove lesion** and a **below-elbow lesion** is that the triceps branch arises proximal to the spiral groove, so both lesions spare elbow extension. The true distinguishing feature is that **intact elbow extension** is preserved in a below-elbow lesion, whereas in a high radial nerve (axillary) lesion it would be lost. ### Anatomical Branching of Radial Nerve | Level | Branches Given Off | |---|---| | Axilla / Upper Arm | Triceps (elbow extension), Anconeus | | **Spiral Groove** | Brachioradialis, ECRL | | Below Elbow (Supinator) | PIN → EDC, ECU, EPL, EPB, APL | | Superficial branch | Sensory to dorsal web space | ### Comparison: Spiral Groove vs. Below Elbow Lesion | Feature | Spiral Groove Lesion | Below Elbow (PIN) Lesion | |---|---|---| | **Elbow Extension** | **Preserved** (triceps branch is proximal) | **Preserved** | | **Wrist Extension** | Lost (ECRL/ECRB affected) | Partially preserved (ECRL/ECRB arise proximal to PIN) | | **Finger Extension** | Lost | Lost | | **Thumb Extension (EPL)** | Lost | Lost (EPL arises FROM PIN, distal to supinator) | | **Sensation (Dorsal Web)** | Lost | Lost | **High-Yield Anatomical Correction:** Extensor pollicis longus (EPL) is a branch of the **posterior interosseous nerve (PIN)**, arising **distal** to the supinator. Therefore, both spiral groove and below-elbow (PIN) lesions result in loss of thumb extension — EPL status does **not** distinguish the two levels. **Clinical Pearl (Gray's Anatomy / Standring):** The best discriminator between a spiral groove lesion and a below-elbow lesion is **intact elbow extension** — but since both lesions spare the triceps, the stem's finding of intact elbow extension confirms the lesion is at or below the spiral groove (not in the axilla). Among the options given, **intact elbow extension (Option A)** is the finding that best distinguishes a below-elbow lesion from a spiral groove lesion, because a spiral groove lesion may also affect brachioradialis and ECRL, while a below-elbow lesion spares these. However, the most classically tested point is that elbow extension is preserved in spiral groove lesions (triceps is spared), and this is the feature that distinguishes spiral groove from axillary lesions — and both spiral groove and below-elbow lesions preserve elbow extension. **Warning:** Do not use thumb extension (EPL) as a discriminator — EPL is a PIN branch and is lost in both spiral groove and below-elbow PIN lesions. Sensation in the dorsal web space is carried by the superficial radial nerve and is lost in both spiral groove and below-elbow lesions (unless the lesion is purely to PIN, sparing the superficial branch). **Reference:** Gray's Anatomy (Standring, 41st ed.), Chapter on Radial Nerve; KD Tripathi Essentials of Medical Pharmacology; Moore's Clinically Oriented Anatomy.