common peroneal nerve palsy

**Anatomy:** The common peroneal (fibular) nerve (L4–S2) is the lateral branch of the sciatic nerve. It winds around the fibular neck, where it is superficial and vulnerable, then divides into two branches. - Deep peroneal nerve: innervates tibialis anterior, EHL, EDL, and peroneus tertius, so it mediates dorsiflexion and toe extension. It supplies sensation to the first dorsal web space. - Superficial peroneal nerve: innervates peroneus longus and brevis, so it mediates eversion. It supplies sensation to the lateral leg and dorsum of the foot.

**Etiology (compression/trauma at the fibular head/neck):** - Prolonged leg crossing, squatting, or kneeling - Tight casts, splints, or compression stockings - Lithotomy or prolonged bed rest/immobilization, including anesthesia or coma - Weight loss (loss of the protective fat pad) - Fibular head or neck fracture, or knee dislocation (traction injury) - Other causes: diabetes or other neuropathies, vasculitis (mononeuritis multiplex), and Baker cyst or ganglion cyst compression

**Clinical features:** - **Foot drop:** Weak dorsiflexion causes a high-stepping (steppage) gait, and the foot slaps down. - **Weak eversion:** Inversion and plantarflexion are intact because the tibial nerve supplies those movements. - **Sensory loss:** Anterolateral leg and dorsum of the foot, including the first web space. - **Tinel sign:** May be present at the fibular head.

**Differential diagnosis (key board distinction):** - **L5 radiculopathy:** Also causes weak inversion (tibialis posterior) and hip abduction (gluteus medius), and may cause back pain. Sensory loss follows the L5 dermatome. - **Sciatic neuropathy:** Plantarflexion and hamstrings are also affected, and the ankle reflex is reduced. - **Other causes of foot drop:** Peripheral neuropathy and motor neuron disease (e.g., ALS).

**Diagnosis:** Mostly clinical. EMG/NCS shows focal conduction block or slowing across the fibular head and helps localize the lesion and differentiate it from L5 radiculopathy. Imaging (MRI or ultrasound) is used if a mass or structural lesion is suspected.

**Management:** - Remove the cause (avoid leg crossing and pressure; adjust casts). - Use an ankle-foot orthosis (AFO) to prevent foot drop and contracture. - Give physical therapy. - Most compression neuropraxias resolve spontaneously. - Surgical decompression or exploration is indicated for penetrating or traumatic injuries, mass lesions, or no recovery.

**High-yield pearls:** - Nerve most commonly injured in the lower extremity. - Classic vignette: A patient with recent weight loss, a cast, or a habit of crossing legs, or a fibular neck fracture, presents with foot drop and a steppage gait. - Sensation over the sole and plantarflexion are preserved.