Working Diagnosis:
Deep peroneal nerve palsy with isolated extensor hallucis longus dysfunction

Treatment:
He was prescribed formal physical therapy for strengthening of his ankle and foot with massage therapy.

Outcome:
He was allowed to return to sport with pain as his guide. He continued to have slow improvement of his extensor hallucis longus dysfunction and was lost to follow-up prior to full resolution of symptoms.

Author's Comments:
Common peroneal nerve palsies account for about 15% of acute peripheral nerve injuries. Frequent causes of peroneal neuropathy are fractures, severe knee dislocation, avulsion of the biceps tendon or tears of the iliotibial tract, surgery, systemic diseases, nerve tumors, and synovial and ganglion cysts. Entrapment of the peroneal nerve is not a frequent condition. The common peroneal nerve courses around the fibular neck where it becomes superficial. This then branches into the deep and superficial peroneal nerves. The deep peroneal nerve courses into the anterior compartment while the superficial branch remains in the lateral compartment.
An uncommon cause of nerve entrapment is a hematoma following a traumatic injury at the fibular neck. Ultrasound and MRI are great imaging modalities for these type of injuries. The hematoma was not seen on ultrasound due to the exam being limited to the ankle and foot.
Conservative management is indicated which includes avoiding aggravating activities, avoiding tight leggings, socks or compression stockings. Many times, these hematomas must be surgically removed to allow healing of the nerves. In this case, the patient showed slow improvement with conservative measures, but improvement nonetheless.
Much of the literature on peroneal nerve palsies have shown that the extensor hallucis longus has a tendency to recover more slowly than the rest of the anterior compartment.
It’s important to rule out anterior compartment syndrome, as was done in our case, as well as abnormal growths as management would be different than our case. With severe inversion ankle sprains, traction-type injuries on the peroneal nerves may also lead to prolonged recovery. Hematoma formation after inversion ankle sprain, as in our case, has not been cited in the literature as reported hematomas in the literature have occurred from direct blows or were related to previous surgeries

Editor's Comments:
In certain cases ultrasound examination may be helpful in identifying and possibly aspirating the hematoma and relieving pressure on adjacent structures. In this case, it appeared that the hematoma was not seen on ultrasound.

References:
1) Girolami et al., Common peroneal nerve palsy due to hematoma at the fibular neck. J Knee Surg 2013;26(Suppl 1):S132–S135.
2) Moorman CD, Pontious J., Compression peroneal nerve palsy causing isolated extensor hallucis longus dysfunction. J Foot Ankle Surg. 2009 Jul-Aug;48(4):466-8.

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