Rare giant schwannoma of superficial peroneal nerve on the dorsum of foot

Abstract

Schwannoma is a benign tumour of Schwann cells originating from myelin sheath of axons of the nerves. It is usually seen in the vestibular region, very rarely found in extremity regions. Schwannomas are termed ‘Giant’ when they measure more than five centimetres in a minimum of one dimension. Giant schwannomas in the foot and ankle region are extremely uncommon. Only eight cases of such a condition are reported. This is a case report of a giant schwannoma found arising from the superficial peroneal nerve in the dorsolateral aspect of the foot. This swelling was excised completely and the diagnosis was confirmed by histopathology.

Background

Schwannomas are benign, well-encapsulated nerve sheath tumours composed of Schwann cells. These are rare and slow-progressing tumours originating from myelinated nerves.¹ Malignant transformation is extremely rare.¹ Ninety per cent of the schwannomas are solitary.² Multiple tumours are usually seen in syndromic associations like neurofibromatosis type 2, Carney complex and schwannomatosis.³

Schwannomas are the most frequently encountered nerve sheath tumours.⁴ Vestibular schwannomas account for 60% of benign schwannomas.⁵ Other common locations are upper limbs, head, neck, pelvis, and extensor aspects of extremities, posterior mediastinum, retroperitoneum, spine, sacrum, mandible, and gastrointestinal tract.^{6 7} The foot and ankle region is a very rare location for these tumours.⁸ Giant schwannomas are those which measure more than 5 cm in one dimension. Only eight such cases within the foot and ankle area are reported in literature.^{9 10} Here, we report a case of a giant schwannoma seen in the dorsolateral aspect of the midfoot.

Case presentation

A male patient in his late 60s presented to our outpatient department with swelling in the right foot since 3 years which was insidious in onset, gradually increased in size and was associated with dull aching pain. He also had numbness over the swelling and over the third and fourth toes. The patient did not give history of weight loss, loss of appetite, fever and malaise. The patient was able to walk with some limitation in the range of motion of ankle and subtalar joint due to the size of the swelling.

On examination, the patient was found to have an ovoid swelling (figure 1) of 6×4 cm on the dorsolateral area of the right foot with well-defined borders and engorged veins on its surface. Swelling was firm to touch, not adherent to the skin and present in the subcutaneous layer. It was not mobile, warm, tender or pulsatile. It was found attached to the superficial peroneal nerve. The patient had a dorsiflexion of 30° and plantar flexion of 25° at the ankle joint. Inversion and eversion at the subtalar joint were 20° each. Further movements were restricted due to the size of the swelling.

Figure 1.

Clinical image showing a swelling on the dorsolateral aspect of the right midfoot measuring 6×4 cm.

Investigations

Plain radiographs of the foot and ankle were taken. They were found to be normal. MRI done showed a well-circumscribed, lobulated subcutaneous lesion in the dorsolateral midfoot with focal matrix calcification with hypointense central core with hyperintense peripheral areas (figure 2). Focal oedema with degenerative changes in the proximal cuboid bone was seen. There were no signs suggestive of osteomyelitis, infective or inflammatory arthritis.

Figure 2.

MRI showing a well-circumscribed, lobulated with focal matrix calcification with hypointense central core with hyperintense peripheral areas.

All preoperative blood investigations were found within normal limits.

Differential diagnosis

Such firm soft tissue swellings in the foot and ankle region are uncommon. We considered a ganglion cyst, as the particular region had a number of surrounding tendons and synovial structures. Synovial sarcoma was another consideration; however, the radiological findings did not match. Due to the close proximity of the superficial peroneal nerve and the lateral plantar nerve, nerve sheath tumours were considered with high suspicion. The presence of neurological symptoms of tingling and numbness also pointed towards a tumour involving a nerve.

Treatment

In the operating room, under pneumatic tourniquet of pressure of 350 mm Hg, a dorsolateral longitudinal incision was taken over the swelling. It was localised to the subcutaneous layer. The swelling appeared glistening white, capsulated and attached to the sheath of the superficial peroneal nerve (figure 3). Swelling was excised completely without rupture of its walls. It measured 6×3×5.3 cm. The swelling was sent for histopathological examination. The surgical site was closed in layers.

Figure 3.

Intraoperative image showing swelling attached to the superficial peroneal nerve.

The biopsy revealed a benign, well-encapsulated tumour composed of spindle cells arranged in alternate hypercellular (Antoni A) and hypocellular (Antoni B) areas with individual cells possessing spindled nuclei with bipolar cytoplasmic process with no mitotic activity and no tumour necrosis (figure 4). Extensive areas showed dilated vascular structures accompanied by haemorrhage and foci with haemosiderin deposits with no features of malignancy. These findings pointed towards a benign schwannoma.

Figure 4.

Histopathological image showing a benign, well-encapsulated tumour composed of spindle cells arranged in alternate hypercellular (Antoni A) and hypocellular (Antoni B) areas with individual cells possessing spindled nuclei with bipolar cytoplasmic process with no mitotic activity and no tumour necrosis, with extensive areas showing dilated vascular structures accompanied by haemorrhage and foci with haemosiderin deposits and no features of malignancy, suggestive of a benign schwannoma.

Outcome and follow-up

Postoperatively, the patient was mobilised full weight-bearing from postoperative day 1. The surgical site healed well in 2 weeks and the patient returned to daily activities.

At 1-year follow-up, the patient had full range of motion at ankle joint. He had no tingling and numbness. No recurrence was seen. The patient was counselled and put on long-term follow-up to monitor the possibility of recurrence.

Discussion

Schwannomas account for 5% of all benign soft-tissue neoplasms, according to the study done by Kransdorf comprising of 18 677 benign tumours of the soft tissues.¹¹ Schwannomas are most often found on the trunk, head and neck but are infrequently present on the foot. Moreover, these are extremely rarely seen on the dorsal aspect of the foot.⁶ The most common locations in this region are the heel, ankle and plantar aspect of the foot (78.3%).⁸

Between 1997 and 2014, Toepfer et al, at a tumour institute, found 11 (10.5%) tumours, which included neurofibroma and schwannoma, out of the 104 soft tissue benign tumours of the foot and ankle.¹² In a study at Rizzoli Institute, only 14 schwannomas were treated out of 189 benign tumours of the foot from 1990 to 2007.¹³ This shows the low incidence of schwannomas among foot tumours.

Among the schwannomas, those found in the foot region are rare. Kehoe et al found only 12 (11.5%) tumours out of 104 peripheral nerve tumours to be present on the foot in a 32-year span.¹⁴ In a study done by Odom et al, 557 schwannomas were observed and only 19 (2.93%) were seen to be involving the foot.¹⁵ Hao et al reported seven cases of foot and ankle schwannoma from their institute in 3 years in 2019.⁸

Plexiform schwannoma is an entity distinct from conventional schwannoma. Very few cases in the foot and ankle region have been reported in literature.^16–18 These tumours are typically found in the dermis and subcutaneous tissue. On histopathology, they can be clearly differentiated from conventional schwannomas based on the absence of Antoni B regions and multiple plexiform-like interlacing nodules and fascicles connected to each other.¹⁹ However, the tumour, in our case, showed clearly defined Antoni B regions and hence is a conventional schwannoma.

There are various other tumours which can occur in the area of the foot and ankle like ganglion cyst, spindle-cell lipoma, neuroma, solitary fibrous tumour, neurofibroma and fibroma.⁸ A high index of suspicion is required for the diagnosis of schwannoma. Predominant neurological symptoms like numbness and tingling might point towards a nerve involvement; however, compression of a nerve by any tumour may mimic the same. So clinically, it is difficult to diagnose a schwannoma.

MRI is the radiological investigation of choice for such swellings. Zhang et al²⁰ described a criteria for diagnosis of schwannomas on an MRI. Presence of two of the following four features suggests towards the diagnosis.

1. Split fat sign.

2. Bright rim sign.

3. Absence of lobular shape.

4. Absence of extensive oedema.

However, high degree of clinical suspicion and good communication between the surgical and radiology teams are required for this diagnosis. Target sign is also described as a part of MRI findings in schwannomas, which consists of a hypointense region inside a hyperintense periphery on T2-weighted images.²¹ MRI of our patient shows a target lesion along with all four of these signs. However, histopathological examination is the gold standard for diagnosis.

The average dimension of schwannoma in the foot and ankle is 1.5–2 cm.²² Despite schwannomas being of various sizes, most of them are smaller than 5 cm, whereas those larger than 5 cm are termed giant schwannomas.⁹ Only eight such cases in foot and ankle are reported, out of which only three were on the dorsum of the foot but none on the dorsolateral aspect.⁸ So our case is one of the first reports of a giant schwannoma in the region of the midfoot arising from the superficial peroneal nerve.

Histopathological features of schwannoma of the foot and ankle are the same as the ones at other sites, and these are described to compose of an Antoni A zone which has hypercellular areas with palisaded spindle cells in short bundles and an Antoni B zone which has hypocellular regions with vascularisation in myxoid matrix. Immunohistochemistry reveals increased S100 expression which is confirmatory towards diagnosis.¹ In our study, the histopathology was highly suggestive of schwannoma and did not require immunohistochemistry for confirmation.

Learning points.

• Schwannomas are benign tumours which are extremely rare in the superficial peroneal nerve, with no cases reported to date arising from this nerve in the region of the foot.

• Giant schwannomas are also very rare in this region.

• MRI allows for an early diagnosis. It can also provide valuable information about the size and extent of the lesion.

• Schwannomas are histologically composed of palisaded spindle cells forming Antoni A and B zones.

• Surgical excision is the primary treatment option. Long-term studies and follow-up are however required to know about the recurrence of these tumours.

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