A 38-year-old woman from Bangalore presents to the emergency department with acute onset weakness of the right foot (inability to dorsiflex the ankle or evert the foot) and foot drop. She reports tripping over a low step 2 days ago and falling directly on the lateral aspect of her right knee. On examination, there is tenderness over the fibular head region. Sensation is preserved over the medial sole of the foot but diminished over the lateral leg and dorsum of the foot. Ankle plantarflexion and toe flexion are intact. What is the most likely diagnosis?

Explanation

## Clinical Presentation Breakdown The patient presents with a classic **common peroneal nerve (CPN) palsy** following direct trauma to the fibular head region. ## Motor Deficits Analysis **Key Point:** The common peroneal nerve innervates muscles of the anterior and lateral compartments of the leg: | Muscle | Action | Status in This Patient | |--------|--------|------------------------| | Tibialis anterior | Dorsiflexion of ankle | **Weak/Paralyzed** | | Extensor digitorum longus | Extension of toes | **Weak/Paralyzed** | | Extensor hallucis longus | Extension of great toe | **Weak/Paralyzed** | | Fibularis longus & brevis | Eversion of foot | **Weak/Paralyzed** | | Gastrocnemius | Plantarflexion | **Intact** | | Flexor digitorum longus | Toe flexion | **Intact** | **Clinical Pearl:** The **foot drop** (inability to dorsiflex) combined with **loss of foot eversion** is pathognomonic for common peroneal nerve injury. The patient can still plantarflex and flex toes (tibial nerve intact). ## Sensory Distribution **High-Yield:** The common peroneal nerve provides sensory innervation via its branches: - **Superficial peroneal nerve:** Lateral leg and dorsum of foot - **Deep peroneal nerve:** First web space (between great and second toe) The patient's sensory loss over the **lateral leg and dorsum of foot** with **preserved medial sole sensation** (tibial nerve territory) confirms CPN involvement. ## Anatomical Vulnerability of the Fibular Head ```mermaid flowchart TD A[Direct Trauma to Fibular Head]:::urgent --> B[Common Peroneal Nerve Compression]:::outcome B --> C[Superficial Peroneal Division Affected]:::action B --> D[Deep Peroneal Division Affected]:::action C --> E[Loss of Foot Eversion]:::outcome D --> F[Loss of Ankle Dorsiflexion + Toe Extension]:::outcome E --> G[Foot Drop Syndrome]:::urgent F --> G G --> H[Patient's Presentation]:::urgent ``` **Mnemonic:** **CPN PALSY = LEAF** - **L**ateral leg sensory loss - **E**version weakness - **A**nterior compartment weakness (dorsiflexion) - **F**oot drop ## Why This Site? The common peroneal nerve is **superficial and vulnerable** as it wraps around the neck of the fibula just below the knee. Direct trauma (fall on lateral knee) compresses the nerve against the bone, causing acute injury. This is the most common site of peroneal nerve compression in the lower limb. **Clinical Pearl:** CPN palsy is the most common lower limb nerve injury, often from: - Direct trauma to fibular head (this case) - Prolonged leg crossing - Tight casts or braces - Prolonged squatting