Working Diagnosis:
Common peroneal nerve palsy secondary to an atraumatic peroneus longus strain with associated hematoma.

Treatment:
Following initial Sports Medicine clinic consultation, the athlete's parents wanted to take athlete out of state to be with his football team, but not to play. This was strongly discouraged, and a repeat MRI was advised to both better characterize the mass due to the original study's image quality and help ascertain if procedural or surgical intervention was indicated. A lumbar spine MRI was recommended to rule out a double crush phenomenon, as was an EMG to localize the lesion. Against medical advice, the athlete traveled with his parents out of state to a football game and followed up in one week. He was given an ankle foot orthotic brace to provide stabilization. When he returned, a second MRI of the right leg was completed and revealed that the mass decreased in size.

Outcome:
Surgery was not recommended since the hematoma was self-resolving. He was followed closely, and with each subsequent visit, he exhibited gradually improving strength. 2 months post injury, he was able to dorsiflex toes 2-5 and provide some resistance, but he remained unable to move his great toe. Sensation was also still decreased at the time.

At a follow up visit 6 months after the injury, repeat electromyography was completed and revealed a resolving right peroneal mononeuropathy at the knee. Peroneal sensory response and motor response were absent from the extensor digitorum brevis. Peroneal motor response was normal from the anterior tibialis. At the extensor hallucis longus, re-innervation from a proximal to distal distribution was confirmed. Overall, there was a significant improvement compared to previous study.

As the patient's nerve function is returning, a full recovery is anticipated.

Author's Comments:
Foot drops are caused by weakness in dorsiflexors, evertors, and invertors.

A common peroneal nerve palsy may be responsible for a foot drop, as may a sciatic nerve palsy or lumbar radiculopathy. The common peroneal nerve travels between the peroneus longus muscle and fibular head before dividing into the superficial and deep peroneal nerves.

The deep peroneal nerve travels to the anterior compartment of the leg and supplies the tibialis anterior, extensor hallucis longus, extensor digitorum longus, and peroneus tertius. The superficial peroneal nerve travels to the lateral compartment of the leg between the peroneus longus and brevis.

Traumatic and non-traumatic causes of a foot drop include metabolic etiologies such as diabetes mellitus, prolonged leg crossing, lithotomy position during urology or gynecology surgery, or lumbar disc herniation. In athletes presenting with a foot drop, clinicians should consider a fibular head fracture, knee dislocation, compartment syndrome, and hematoma. There are two cases of common peroneal nerve palsy in athletes due to a hematoma associated with an atraumatic peroneal longus tear.

Work up of a foot drop should include plain radiographs to rule out a fracture, mass lesion, or arthritis. Ultrasonography or MRI of the leg can be ordered to assess for nerve compression. An MRI of the lumbar spine should be ordered if a radiculopathy or spinal stenosis is suspected. Electromyography can help localize the lesion and confirm the diagnosis.

Nerves regenerate at a rate of about 1 mm per day. Common peroneal nerve palsy recovery begins by 2-3 months. The common peroneal nerve runs 1-2 cm distal to the fibular head before entering the anterior compartment of leg. In general, nerve recovery begins after about 1 month. Surgical decompression is recommended in the setting of rapidly declining nerve function or failure to recover within 3 months.

Editor's Comments:
The common peroneal nerve is a superficial structure that is vulnerable to direct trauma to the lateral knee during sports. In this case, an atraumatic strain of the surrounding peroneal musculature yielded a hematoma that caused a secondary injury to the common peroneal nerve in the days after the incident event.

With the self-resolution of the athlete's hematoma in this case, the mass effect on the common peroneal nerve decreased. However, as the duration of nerve compression increases, so does the chance of longer-lasting injury and delayed recovery.

Careful attention should always be paid to evolving neurological or vascular symptoms following any musculoskeletal injury, and investigated promptly and thoroughly.

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