Tibial Nerve Schwannoma: An Unexplained Cause of Lateral Foot Pain - A Rare Case Report and Review.

INTRODUCTION: Schwannoma is a benign tumor that arises from the peripheral nerve sheath. It presents as a discrete, often tender, and palpable nodule associated with neurogenic pain or paresthesia when compressed or traumatized. The growth rate is usually slow, and these lesions seldom exceed 2 cm in diameter. CASE REPORT: We report the case of a schwannoma arising from the tibial nerve located in the left popliteal fossa. The patient presented with the left foot pain in the lateral plantar region without any motor deficit. The pre-operative diagnosis was made with magnetic resonance imaging (MRI) scan. He was subjected for neurolysis and excision biopsy of the lesion. The surgical specimen consisted of encapsulated white-yellow mass with irregular contours, measuring 2 × 3 cm. The cut section revealed cystic degenerations with areas of hemorrhage and necrosis. The patient reported symptom free in the post-operative period and during follow-up. Marginal excision appears to be recommended therapy for this tumorous lesion, without any evidence of recurrence during follow-up.
CONCLUSION: A benign nerve sheath tumor of a peripheral nerve could be a possibility for long-standing neuropathic pain in the foot, ankle, and leg, wherein all other possibilities have been ruled out. The meticulous examination of the entire length of the tibial nerve including sciatic nerve by palpation and percussion was helpful in diagnosis which should be confirmed by MRI scan. The excision biopsy remains the gold standard treatment of choice for schwannoma of the peripheral nerve. Copyright: © Indian Orthopaedic Research Group.

An unexplained lateral foot pain needs a meticulous examination of the entire length of the tibial nerve which should be confirmed by MRI scan and the excision biopsy remains the gold standard treatment of choice for schwannoma of the peripheral nerve.

Introduction

Among all peripheral nerve sheath benign tumors, schwannoma is the most common tumor arising in the Schwann cells of the peripheral nervous system [1]. They are well-encapsulated, slow-growing neoplasms that form within the perineurium and follow an indolent natural course [2]. The schwannomas are most commonly seen between the third and fourth decade of life with no gender predilection and an infrequent rate of malignant transformation [3, 4].

Schwannomas are most commonly seen in head-and-neck region followed by lower extremities, especially in the foot and ankle [5]. They occur more frequently in the posterior tibial nerve and very rarely seen in tibial nerve in popliteal fossa [6, 7]. Knight et al. in their review about benign solitary schwannomas, they documented 8.97% of cases of tibial nerve schwannomas [1]. Solitary schwannomas present as an asymptomatic mass or cause compressive neuropathy due to mass effect and displacement of nerve bundles. A typical presentation is pain and numbness of the plantar foot which can be mistaken due to lumbar radiculopathy. The clinical diagnosis can be missed or delayed due to the indolent course of these tumors, leading to the development of tarsal tunnel syndrome [8, 9, 10]. The management of schwannoma is meticulous resection of mass without any damage to the adjacent nerve due to their nerve sheath origin.

We report a patient with a lower extremity schwannoma affecting the tibial nerve with lateral foot pain after obtaining informed and written consent from the patient.

Case Report

A 32-year-old male presented at our institution with a history of pain over the lateral aspect of the left foot for 2 years of insidious onset. He complained of pins and needle sensation over plantar aspect of the left foot. There was no significant history of trauma. He noticed swelling behind the left knee in the past 6 months which was initially peanut shaped and gradually progressed to the present size.

On examination, there was an oval mass of 2 × 3 × 1 cm, firm, non-pulsatile, non- fluctuant, non-transilluminating in the left popliteal fossa (Fig. 1) which was mobile in both directions. Skin over the swelling was pinchable. The patient was neurologically stable except with decreased sensation over lateral aspect (plantar region) of the left foot. Notably, the left popliteal and dorsalis pedis pulses were felt.

Figure 1

Clinical image of the left popliteal fossa (marked area) showing the extent of the lesion.

Magnetic resonance imaging (MRI) of the left popliteal fossa revealed encapsulated round to oval shaped, eccentrically placed lesion seen within the intermuscular space in the posterior aspect of knee joint. Lesion shows homogenous isointense on T1 and heterogeneous hyperintense signal on T2. There are hypointense foci seen within the hyperintense area on T2 which is suggestive of fascicular sign. The sagittal image showed continuity of mass lesion with the tibia nerve displaying fat split sign, lesion measuring 2 × 3 × 1 cm (Fig. 2a, 2b, 2c).

Figure 2

(a) (sagittal section, T1) and (b) (sagittal section, T2): Magnetic resonance imaging (MRI) of the left popliteal fossa showing tumorous tissue (shown in red arrow) around tibial nerve. (c) MRI of the left popliteal fossa (axial section) T2 W images showing tumorous lesion around tibial nerve (shown in red arrow).

Under spinal anesthesia, the patient underwent neurolysis of tibial nerve and excision biopsy of tumorous lesion (Fig. 3a, b) by incising the epineurium of tibial nerve since the tumorous lesion was eccentrically placed. As there was no disruption of tibial nerve continuity, no nerve repair was done. Gross resected specimen of tumorous mass of 2 × 3 × 1 cm and the cut specimen showing areas of hemorrhage, necrosis, and cystic spaces interspersed along the tumorous lesion (Fig. 4a, b). Histopathological examination (×40) of the lesion showed biphasic tumor consisting of compact hypercellular Antoni A areas and myxoid hypocellular Antoni B areas and nuclear palisading around fibrillary process of Verocay bodies without any neural elements seen (Fig. 5). Immunohistochemistry (IHC) staining of the resected lesion revealed strong positivity for S-100 staining (Fig. 6).

Figure 3

(a and b) Intraoperative images showing the dissected tibial nerve schwannoma in the left popliteal fossa and complete excision of schwannoma from fascicles of tibial nerve, respectively.

Figure 4

(a and b) Gross resected specimen of tumorous mass of 2 × 3 cm and the cut specimen showing areas of hemorrhage, necrosis, and cystic spaces interspersed along the tumorous lesion.

Figure 5

Histopathological examination (×40) of the lesion showing biphasic tumor consisting of compact hypercellular Antoni A areas and myxoid hypocellular Antoni B areas and nuclear palisading around fibrillary process of Verocay bodies without any neural elements seen.

Figure 6

Histopathological examination staining of the lesion showing strongly positive for S-100 staining.

The patient was regularly followed up at timely intervals for 9 months since the excision of the lesion. The follow-up period was uneventful. The patient had improved sensation over lateral aspect (plantar region) of the left foot and without any recurrence of tumorous lesion during the follow-up period.

Discussion

Schwannoma is also known as neurilemomas, neurocytomas, peripheral gliomas, neurinomas, and neurolemmomas. Being the most common peripheral nerve tumor, Schwannoma is usually an asymptomatic, slow-growing, solitary, eccentric, firm, well-circumscribed, and encapsulated tumor [11, 12]. The peripheral location and non-involvement of the main course of the nerve are seen in our patient where the mass was displacing the proximal and distal branches of the tibial nerve at popliteal fossa.

Schwannoma runs a varied symptomatology from asymptomatic phase to mild-to-severe phase of nerve involvement [13]. In our case, symptoms occurred as a result of nerve compression caused by the growth of the tumorous mass. Even though schwannoma occurs at any age, it is commonly seen between the second and fifth decade of life, with no gender or racial predilection [14]. Most of the schwannoma lesions are solitary but multiple schwannomas are associated with neurofibromatosis [15]. Although schwannoma is benign, the malignant potential of the tumor has been elicited when they are associated with von Recklinghausen disease [16]. Schwannomas are most commonly seen in head and neck (25–45%) [17], mediastinum (20%) [18], upper extremity (19%) [19], retroperitoneum (5–10%) [20], pelvis (<0.5%) [21], and lower extremities (13.5–17.5%) [22]. Various studies in the literature reported that schwannomas are common in the anterior and flexor areas of the upper extremities and the posterior aspects of the lower extremities [12, 13]. The review of lower extremity schwannoma from 2013 to 2020 is tabulated in [Table 1] [10, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33].

Table 1

Review of lower extremity schwannoma from 2013 to 2020

The clinical diagnosis can be missed or delayed due to the indolent course of these tumors, leading to the development of tarsal tunnel syndrome [8, 9, 10]. The delay in diagnosis of schwannoma is due to the relative slow growth potential, mobility of the mass, and affection toward the surrounding soft tissues. The use of imaging modalities to diagnose peripheral nerve sheath tumors is of prime importance. Among all imaging modalities, MRI enhanced the preoperative diagnosis of nerve sheath tumors and provided information regarding the gross characteristics, size, location, and relationship of the tumorous mass as well as infiltration with surrounding structures [34]. In our case, MRI revealed encapsulated round to oval lesion seen within the intermuscular space in the posterior aspect of knee joint which is homogenous isointense on T1 and heterogeneous hyperintense signal on T2-weighted images. The management of tumor remains easy when the tumor mass is located eccentrically along the perineurium. It is important to note that schwannomas have to be differentiated from neurofibroma and malignant peripheral nerve sheath tumor as they all have similar radiological features with subtle differences which are tabulated in [Table 2].

Table 2

Differential diagnosis of schwannoma

In our case, the patient underwent neurolysis of tibial nerve at popliteal fossa and excision biopsy of tumorous lesion. The cut section of the excised mass showed round, smooth, well-defined, heterogeneous mass with cystic degeneration with areas of hemorrhage and necrosis. The histopathological examination (40×) of the biopsied lesion revealed the features of schwannoma which was confirmed by S-100-positive IHC staining of the resected lesion. The patient reported symptom free interval during the immediate post-operative period. No recurrence was noted during the follow-up period of 9 months.

Conclusion

A benign nerve sheath tumor of a peripheral nerve could be a possibility for long-standing neuropathic pain in the foot, ankle, and leg where all other possibilities are ruled out. The meticulous examination of the entire length of the tibial nerve including sciatic nerve by palpation and percussion was helpful in diagnosis which should be confirmed by MRI scan. The excision biopsy remains the gold standard treatment of choice for schwannoma of the peripheral nerve.

The meticulous examination of the entire length of the tibial nerve including sciatic nerve by palpation and percussion was helpful in diagnosis which should be confirmed by MRI scan. The excision biopsy remains the gold standard treatment of choice for schwannoma of the peripheral nerve.