Makindo Medical Notes

About

The Common Peroneal Nerve (CPN), also known as the common fibular nerve, is a branch of the sciatic nerve and contains axons from the L4, L5, S1, and S2 nerve roots. It plays a crucial role in the motor and sensory innervation of the lower leg and foot. Injury to the CPN can result in significant functional impairment, including foot drop.

Anatomy

• Origin:
  • The CPN branches off from the sciatic nerve in the popliteal fossa, typically at the upper angle of the fossa.
  • The sciatic nerve bifurcates into the tibial nerve (medial) and the common peroneal nerve (lateral).
• Course:
  • The CPN runs laterally along the lateral border of the popliteal fossa.
  • Wraps around the neck of the fibula, passing superficially where it is susceptible to injury.
  • Divides into two terminal branches:
    • Superficial Peroneal Nerve: Supplies muscles in the lateral compartment of the leg and provides sensory innervation to the dorsum of the foot.
    • Deep Peroneal Nerve: Supplies muscles in the anterior compartment of the leg and provides sensory innervation to the web space between the first and second toes.
• Branches:
  • Articular Branches: To the knee joint.
  • Cutaneous Branches: Lateral sural cutaneous nerve supplying the skin of the upper lateral leg.
  • Muscular Branches: To short head of biceps femoris before bifurcation.

Function

• Motor Innervation:
  • Anterior compartment muscles (via deep peroneal nerve):
    • Tibialis anterior
    • Extensor hallucis longus
    • Extensor digitorum longus
    • Peroneus tertius
  • Lateral compartment muscles (via superficial peroneal nerve):
    • Peroneus longus
    • Peroneus brevis
• Sensory Innervation:
  • Skin over the upper lateral leg (lateral sural cutaneous nerve).
  • Dorsum of the foot (superficial peroneal nerve), except the first web space.
  • First web space between the first and second toes (deep peroneal nerve).

Clinical Presentation of Common Peroneal Nerve Injury

Injury or compression of the common peroneal nerve can lead to characteristic signs and symptoms:

• Motor Deficits:
  • Foot Drop: Weakness of dorsiflexion due to paralysis of tibialis anterior and other dorsiflexor muscles.
  • Weakness of foot eversion due to paralysis of peroneus longus and brevis.
  • Preserved inversion and plantarflexion since tibial nerve innervation remains intact (tibialis posterior and gastrocnemius/soleus muscles).
• Gait Abnormalities:
  • High-Stepping Gait: To prevent the foot from dragging, patients lift the knee higher than normal during the swing phase.
  • Foot Slap: Uncontrolled foot drop during heel strike due to lack of dorsiflexion control.
• Sensory Deficits:
  • Reduced sensation over the lower lateral part of the leg.
  • Numbness or paresthesia on the dorsum of the foot.
  • Loss of sensation in the first web space (deep peroneal nerve involvement).
• Reflexes:
  • Tendon reflexes are generally intact, as the Achilles tendon reflex (S1) is mediated by the tibial nerve.

Etiology of Common Peroneal Nerve Injury

• Compression Injury:
  • Prolonged bed rest or immobility with legs externally rotated.
  • Compression from tight plaster casts or splints.
  • Habitual leg crossing.
  • Sitting or squatting in certain positions (e.g., yoga, kneeling).
• Trauma:
  • Fibular neck fractures.
  • Knee dislocations or ligament injuries affecting the fibular head.
• Entrapment Neuropathy:
  • Compression by ganglion cysts, tumors, or masses near the fibular head.
• Systemic Conditions:
  • Diabetes mellitus causing peripheral neuropathy.
  • Weight loss leading to reduced protective fat padding over the nerve.
• Iatrogenic Causes:
  • Surgical procedures involving the knee or fibula.
  • Incorrect positioning during anesthesia or surgery.

Differential Diagnosis

• L5 Radiculopathy:
  • Disc prolapse affecting the L5 nerve root can mimic CPN injury.
  • Associated with back pain radiating down the leg.
  • Weakness in foot inversion (tibialis posterior) may be present.
  • Reduced ankle reflexes may occur.
• Peripheral Neuropathy:
  • Generalized conditions affecting multiple nerves (e.g., diabetes, Charcot-Marie-Tooth disease).
  • Symmetric sensory loss and weakness.
• Motor Neuron Disease:
  • Progressive weakness without sensory deficits.
• Myopathies:
  • Muscle diseases causing weakness but usually with proximal muscle involvement.
• Myasthenia Gravis:
  • Typically affects ocular and bulbar muscles, but can cause generalized weakness.

Investigations

• Blood Tests:
  • Full Blood Count (FBC)
  • Urea and Electrolytes (U&E)
  • Liver Function Tests (LFTs)
  • Erythrocyte Sedimentation Rate (ESR)
  • Thyroid Function Tests (TFTs)
  • Blood glucose levels to assess for diabetes.
• Electrophysiological Studies:
  • Electromyography (EMG) to assess muscle activity.
  • Nerve Conduction Studies (NCS) to evaluate the integrity of the nerve.
  • Helps localize the lesion and differentiate between axonal loss and demyelination.
• Imaging:
  • MRI Spine: To rule out lumbar disc prolapse or spinal pathology causing L5 radiculopathy.
  • Ultrasound: May detect structural lesions compressing the nerve.
  • MRI of Knee/Leg: If a mass or localized pathology is suspected.

Management

Management depends on the underlying cause and severity of the nerve injury:

• Conservative Management:
  • Most cases due to compression recover spontaneously over weeks to months.
  • Physiotherapy: To maintain muscle strength and prevent contractures.
  • Ankle-Foot Orthosis (AFO): Splints to support the foot and improve gait.
  • Avoid Pressure: Modify activities to reduce pressure on the fibular head (e.g., avoid leg crossing, prolonged kneeling).
• Medical Management:
  • Treat underlying systemic conditions (e.g., optimize diabetes control).
  • Analgesics for neuropathic pain if present.
• Surgical Intervention:
  • Indicated if there is no improvement with conservative measures or if there is a structural lesion compressing the nerve.
  • Nerve Decompression: Surgical release of entrapment sites.
  • Nerve Repair: Suturing or grafting if the nerve is transected.
  • Nerve Transfer or Tendon Transfer: To restore function in cases of severe or irreversible nerve damage.
• Rehabilitation:
  • Ongoing physiotherapy for muscle strengthening and gait training.
  • Occupational therapy to aid in activities of daily living.

Prognosis

The prognosis of common peroneal nerve injury varies:

• Compression neuropathies often have good recovery if the cause is identified and managed promptly.
• Axonal injuries may take longer to recover and may not achieve full function.
• Early intervention and rehabilitation improve outcomes.

Prevention

• Avoid prolonged pressure on the lateral aspect of the knee.
• Proper positioning during surgery and immobilization.
• Use of protective padding when engaging in activities that put pressure on the fibular head.
• Management of systemic conditions like diabetes to prevent neuropathy.

References

• Moore KL, Dalley AF, Agur AM. Clinically Oriented Anatomy. 7th ed. Lippincott Williams & Wilkins; 2013.
• Stewart JD. Foot drop: where, why and what to do? Pract Neurol. 2008;8(3):158-169.
• Kane PM, Oware A. Common peroneal neuropathy: a clinical and electrophysiological review. Clin Neurophysiol Pract. 2019;4:112-123.
• Campbell WW. Evaluation and management of peripheral nerve injury. Clin Neurophysiol. 2008;119(9):1951-1965.