Treating Schwannoma Of The Tibial Nerve In The Presence Of Tarsal Tunnel Syndrome

Emphasizing diagnostic pearls, these authors describe the treatment of a rare lower extremity schwannoma tumor in a 68-year-old patient with tarsal tunnel syndrome.

A schwannoma, also commonly called neurilemoma, is the most common tumor arising from the peripheral nerve sheath. It arises from the Schwann cell and malignant transformation is exceptionally rare.¹

Its incidence in the foot and ankle is rare. Schwannoma presents as a discrete, often tender nodule associated with neurogenic pain or paresthesias that are compressed or traumatized.² Few published reports focus on solitary lesions of the tibial nerve. This report describes the unique case of tibial nerve schwannoma with clinical symptoms of tarsal tunnel syndrome.

A 68-year-old female presented with a 10-month history of progressive pain and swelling to the posteromedial left ankle. Her symptoms had recently worsened with an increased sensation of burning, tingling and numbness to the plantar-lateral aspect of the associated foot. The patient did not express concern related to weakness of the foot or ankle. She did not mention any recent history of injury or trauma to the foot or ankle. The patient admitted to the severity of the symptoms forcing her to remain non-weightbearing on the affected limb.

On physical examination, a small mobile, palpable mass was visible posterior to left distal 1/3 of the tibia approximately 8-10 cm proximal to distal malleolus. A positive Tinel’s sign was present at the tibial nerve with paresthesia and a burning sensation radiating to plantar-lateral aspect of the foot. The Semmes-Weinstein monofilament exam was intact to the left foot with the exception of the plantar-lateral section. Manual testing revealed no deficit in motor function to the left foot or ankle in comparison to the contralateral limb.

Diagnosis of tibial schwannomas may be delayed for many years. Ghaly described a case of a patient suffering for 10 years with a tibial schwannoma before getting a correct diagnosis.³ Nawabi suggested two reasons for a delay in diagnosis: a deep-seated swelling that may escape detection by palpation or the presence of neuropathic pain, which may mislead the clinician to a diagnosis of radiculopathy or entrapment neuropathy.⁴

Plain radiograph studies of the patient’s foot and ankle failed to demonstrate any significant pathology. Nerve conduction studies of the lower extremities demonstrated normal sensory nerve conductions, normal F waves and symmetric bilateral tibial H-reflexes. Electromyography (EMG) showed electrophysiologic evidence of mild chronic bilateral lumbar radiculopathy, most notably at the level of L4/L5. We ordered magnetic resonance imaging (MRI) of the bilateral lower extremities to assess the soft tissue mass and the MRI revealed a well-encapsulated, spherical, heterogeneous mass adjacent to the tibial nerve of the left lower limb. In general, schwannomas appear hypointense on T1-weighted images and hyperintense on T2-weighted images.⁵

Ultrasound imaging is also an effective diagnostic tool to help in diagnosing such lesions.⁶ 

Keys To Effective Surgical Excision

Surgical excision of schwannomas is the ideal treatment. With the patient under general anesthesia, we performed careful surgical dissection, which led to the identification of a well-encapsulated, firm lesion with tibial nerve involvement. The lesion appeared smooth with a glistening surface and a gray-white appearance. The lesion involved the entire tibial nerve and resection of the nerve segment in toto would be unfavorable to the patient’s outcome. We carefully separated the lesion from tibial nerve fascicles using blunt tenotomy scissors and carefully retracted the fascicles using vessel loops.

After freeing the lesion from its attachment, we sent the lesion to pathology for final identification. Decompression fasciotomy of the tarsal tunnel and posterior compartments then facilitated the release of any adhesions and possible areas of entrapment of the tibial nerve. We then wrapped the affected portion of the tibial nerve with a nerve wrap to help reduce fibrosis and adhesion. We reapproximated the subcutaneous layer with absorbable suture and closed the skin.

The patient had weekly postoperative follow-up in the clinic. The pathology report confirmed the suspected diagnosis of neurilemoma. The patient had permission for full weightbearing postoperatively in a walking boot. At the two-week evaluation, we removed the sutures and evaluated the patient clinically. The patient related improved sensation to the plantar lateral foot and decreased pain. She had another exam at the four-week mark with great improvement in pain, swelling and numbness. Overall, the patient related satisfaction in the surgical outcome. Of course, she will need close follow-up for long-term results.

What You Should Know About Schwannoma Tumors

Schwannomas are benign nerve sheath tumors that can arise in the lower extremity. Lesions can grow slowly and act as space-occupying lesions pressing on associated nerves and causing entrapment-like symptoms. This case study represents a solitary lesion involving the tibial nerve with symptoms consistent with tarsal tunnel syndrome or entrapment neuritis. Magnetic resonance imaging is a great tool to aid in diagnosis of these soft tissue tumors.

Surgical planning includes resection of the lesion using an atraumatic technique to salvage nerve tissue. A nerve wrap helped decrease postoperative fibrosis and adhesions of the involved nerve tissue. In our case, we preserved the tibial nerve without major transection of the nerve fibers. If nerve damage has occurred, the surgeon may consider nerve repair or even nerve grafting. Nerve repair should result in appropriately aligned coaptation of nerve fascicles with the use of intraoperative nerve stimulation. When the gap or defect at nerve injury site is too great, grafting is an option to aid in repair.

In conclusion, while schwannomas are rare, they can arise in the lower extremity and should prompt a differential diagnosis when a mass presents in the tarsal tunnel region.