B e n i g n s c h w an n o m a o f t h e m e d i al d o r

Benign schwannoma of the medial dorsal cutaneous nerve of the foot: A case report by Mark Capuzzi DPM1, Zachery Weyandt DPM1, Dawn Masternick DPM2

The Foot and Ankle Online Journal 13 (2): 6

Schwannomas are benign soft tissue tumors of Schwann cells in the peripheral nerve system that can occur in the foot and ankle with preference for the posterior tibial nerve. A 60-year old female with a histologically diagnosed schwannoma of the medial dorsal cutaneous nerve is described in a location that is atypical according to the literature. These soft tissue masses can easily be misdiagnosed, and MRI studies can be inconclusive in determining possible malignancy. Clinicians should be suspicious and include malignant peripheral nerve tumors in their differential diagnosis with excisional biopsy as a way of definitive diagnosis.

Keywords: schwannoma, foot and ankle tumor, soft tissue mass, medial dorsal cutaneous nerve

This is an Open Access article distributed under the terms of the Creative Commons Attribution License. It permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ©The Foot and Ankle Online Journal (www.faoj.org), 2020. All rights reserved.

Neurilemoma was originally described histologically by Verocay in 1908, with the term schwannoma being coined by Abadie and Argaud some 20 years later [1]. Schwannomas are a variety of peripheral nerve sheath connective tissue masses that arise in the periphery usually of benign nature. This group also includes neurofibromas and malignant peripheral nerve sheath tumors (PNSTs) that are classified based on their histological appearance. Schwannomas are solitary encapsulated tumors histologically described by homogenous arrangement of palisading cells (Verocay bodies) and exhibit hypercellular and hypocellular areas of devoid spindle cells; otherwise known as Antoni A and Antoni B [2]. These lesions can have major clinical impact on the neurocutaneous diseases neurofibromatosis 1 and neurofibromatosis 2 or can be seen as isolated occurrences without systemic involvement [3].

In a retrospective single-center study by Toepher, et al., 25.2% of all foot and ankle tumors were reported

to be benign soft tissue tumors, of which, 16 different variants exist. The most common of which are hemangiomas, pigmented villo-nodular synovitis, superficial fibromatosis, and schwannomas, respectively [4]. Neurinoma/schwannoma account for about 1-10% of all soft tissue tumors in the foot and ankle. It is estimated that 1% of cases have malignant potential [4].

Schwannomas are most frequently seen in the trunk, head, neck, retroperitoneum, brachial plexus, and posterior tibial nerve [5]. Multiple case studies have been published in recent years describing these tumors in the foot and ankle, usually in connection with the posterior tibial nerve, but less commonly also seen in areas of the digits and the forefoot [6,7,8,9].

In this detailed case report, we describe a 60-year old woman with a schwannoma of the medial dorsal cutaneous nerve. The palpable mass arises in the dorsal midfoot associated with the tarsometatarsal

1 - University of Louisville Podiatric Medicine and Surgery Residency/Fellowship, Louisville, Kentucky 2 - Tipton and Unroe Foot and Ankle Care, Kentucky * - Corresponding author: [email protected] ISSN 1941-6806 doi: 10.3827/faoj.2020.1302.0006 faoj.org

http://www.faoj.org/


joint, an area that has only been reported once in the literature and extending distally over the 2nd metatarsal base. The report includes a thorough clinical history, physical examination, diagnostics, as well as a histopathological description.

Case Report

A 60-year old Caucasian female presented to the attending surgeon’s private clinic with a chief complaint of a painful right midfoot mass that began one year previous. She felt the lesion had been increasing in size over time and worsening in severity of pain. She recalls the pain began only in shoe wear, as a rubbing irritation, but upon presenting to the clinic was painful also while walking barefoot. The patient's history revealed trauma to the area of the mass two years ago when a large sign was dropped on her foot that she did not seek medical care for, leaving her foot bruised. She denied any other source of pain and other cutaneous masses as well as any history of puncture wounds or foreign bodies. Her medical history includes tubular adenoma, hemorrhoids, osteopenia, and a non-descript heart murmur. She denied taking any prescription medications, using only calcium supplements daily for her osteopenia.

Clinical examination revealed an apparently healthy female in no acute distress. A family history was non-contributory for soft tissue masses and a review of systems was unremarkable. Vascular examination revealed palpable pedal pulses and a brisk capillary refill time to all digits. Neurological examination revealed normal sensory sensations to digits and foot without deficit. No Tinel’s or Valleix’s signs were noted. Biomechanical examination revealed pes planus foot type with normal range of motion of all joints and without structural deformities. Dermatological examination revealed an oval, mobile, semi-compressible, tender to palpation, subcutaneous mass beginning over the 2nd tarsometatarsal joint extending over the base of the 2nd metatarsal. The bulk of the mass was distal to the 2nd tarsometatarsal joint. The mass was non-adherent to underlying structures without signs of infection, drainage, or irritation. There was no open wound in the area or bony exostosis. The mass measured 1.0cm x 1.0cm. The mass did not transilluminate nor have a palpable pulse.

Figure 1 T1 Sagittal and T2 Axial MRI.

Differential diagnoses discussed included: fibroma, neuroma, neurofibroma, lipoma, foreign body, and benign/malignant peripheral nerve sheath tumors. The patient was educated and was presented treatment options for a ganglion cyst during her first appointment. She agreed to receive a corticosteroid injection in the area of the mass. She returned one month later without improvement and received a second injection, with aspiration attempted, revealing no fluid collection. At the 3rd visit, the patient again demonstrated no signs of improvement and worsening pain. An MRI was ordered at this time as there was a lowered clinical suspicion for a ganglion cyst (Figure 1).

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Figure 2 Schwannoma immediately visible in superficial fascia after incision.

The impression of the MRI report read a 6mm x 6mm x 4mm subcutaneous nodular lesion, superficial tear contacting the second extensor tendon anterior to the second TMT joint.

The radiologist suggested statistically the mass was a synovial/ganglion cyst but could not confirm or deny a benign or malignant soft tissue mass on a non-contrast MRI. The patient was educated on the results of her MRI and the inconclusive results. Upon discussion of an additional attempted aspiration versus further work-up including a MRI with contrast, a MRI with contrast was ordered. The second MRI reported a fairly uniform enhancement of the previously noted well-circumscribed subcutaneous nodular lesion along the dorsal aspect of the midfoot. The enhancement aspect of the image favored a non-cystic benign or malignant soft tissue mass in which surgical excision was recommended. Surgical excision was proposed and agreed upon by the patient.

Figure 3 Schwannoma after surgical dissection with distal nerve continuation.

She was taken to the operative room and monitored anesthesia care was exercised in addition to a V-block of local anesthetic just proximal to the soft tissue mass. An approximately 3cm longitudinal incision was made directly over the mass. Careful and meticulous dissection immediately revealed a yellow, firm, and oval mass in the subcutaneous tissue that was encapsulated (Figure 2). The mass was freed from its surrounding structures with ease. Upon further blunt dissection, nerve-like structures were found to be projecting from the mass in a proximal and distal direction with abnormal thickening. The nerve-like projections mimicked the course of the medial dorsal cutaneous nerve as it extends to the 2nd interspace (Figure 3). A measurement of 5mm proximal and distal to the mass was marked and the nerve was transected and removed from the body in its entirety. The specimen was placed in formalin and prepared for histopathological examination (Figure 4).



Figure 4 Pathological specimen of schwannoma without distal and proximal nerve projections.

Upon visual inspection, no further remnants of the mass or abnormal nerve remained and the area was irrigated. Deep structures were approximated with vicryl sutures and skin was closed with prolene. The patient was placed in a soft dressing to the operative foot.

The pathology report grossly described a soft tissue mass of the right foot received in formalin, as yellow-white soft and feathery tissue in multiple fragments. The pathologist described a low power photomicrograph showed a well-circumscribed and encapsulated nodule with no infiltrating borders and no identifiable necrosis. High power photomicrograph showed bland spindle cell proliferation with eosinophilic cytoplasm, indistinct cell borders, and tapered nuclei (Figures 5 and 6). Mitotic figures were inconspicuous. Immunohistochemical stain for S100 was strongly and diffusely positive in the neoplastic cells. Schwannoma was favored by palisading nuclei and encapsulation.

Figure 5 H&E Histopathological image.

Figure 6 S-100 stain positive.

Post-operative protocol for the patient included full weight-bearing as tolerated in a surgical shoe to the operative side. She was evaluated two weeks out from surgery, demonstrating a well healed surgical site without wound dehiscence, infection, or pain. Sutures were removed at this time and the patient returned to normal shoe gear. She was encouraged to complete range of motion exercises at this time. The patient’s follow up at 4 weeks again demonstrating a healed surgical wound in the absence of pain. The patient was satisfied with her surgical results. A phone call was conducted 6 months post-operatively, and the patient denied any pain, wound complications, no neurologic symptoms, and noted she was extremely satisfied with the result.

Discussion

The diagnosis of a Schwannoma, can pose a challenge for a clinician in the office setting. As reported, our patient was originally thought to present with a ganglion cyst which was treated with steroid injections and attempted aspiration. For any benign soft tissue mass, multiple differential diagnosis should be thought of prior to intervention, such as, but not



limited to, ganglion cyst, fibroma, neuroma, neurofibroma, lipoma, and benign/malignant peripheral nerve sheath tumors. Good practice dictates that if the mass is not responding to conservative measures, biopsy with histopathological examination is the standard of care. Excisional biopsy is the accepted technique in cases of the foot and ankle with small lesions (< 2cm) located in the subcutaneous tissue as was the case in our report [10].

Schwannomas’ have been reported in the foot and ankle literature, however, rarely in the dorsal midfoot. To the author's knowledge only one other case has been reported in the location of the tarsometatarsal joint [11]. An area not uncommon to find ganglion cysts corresponding to an underlying joint. Knight, et al., retrospectively review 234 benign solitary schwannomas and describe their peripheral nerve distribution. Of 64 schwannomas involving the lower limb and pelvis, they reported the most commonly involved nerves included the sciatic nerve (n = 15), tibial nerve (n = 21), and common peroneal nerve (n = 15) [12]. Kransdorf, et al., described one of the largest retrospective review studies regarding benign soft tissue lesions. They looked at over 18,000 benign soft tissue lesions, and further categorized them via distribution of specific diagnosis by age, sex and location. Regarding total body distribution, schwannoma prevalence in the foot and ankle was 0.09% (81/895). No definitive predilection for male vs. female has been noted in the literature. There is a wide age range, with the majority of patients being between the ages of 50-70 years old [13].

Topfer, et al., as part of a retrospective study looked at the data of patients that were treated for foot and ankle tumors between June 1997 and December 2015. The primary aim of the study was to describe the prevalence, demography, and anatomical distribution of the tumors. This study presented an analysis of the second largest population of patients, with current literature. Out of 7487 musculoskeletal tumors, 413 cases (5.52%) of tumors of the foot and ankle were included. There were 147 soft tissue tumors (36%), of those 104 (71%) were benign and 43 (29%) were malignant. Benign soft tissue tumors, including all variants, were most commonly located in the ankle and midfoot. Malignant soft tissue tumors were most commonly at the midfoot. Schwannomas specifically were most common in the hindfoot and least common in the midfoot. Of the 104 benign soft

tissue tumors, only 11 were found to be Schwannomas with zero being found in the midfoot and only one in the forefoot [4]. Malignancy should be suspected early in treatment, with regard to the aforementioned literature at hand.

Hao, et al., conducted a literature review of schwannomas that included 46 reported masses. Of the 46 schwannomas, 14/46 were on the ankle, 14/46 plantar aspect of foot, 9/46 heel, 3/46 interdigital spaces, 1/46 dorsal foot over 4th and 5th metatarsal, 5/46 unreported anatomical location. Again, none were found associated with the tarsometatarsal joint or located in the midfoot [7].

Conclusion

To our knowledge this is the first reported case of a schwannoma associated with the dorsal 2nd tarsometatarsal joint, and one of two reported cases overlying the tarsometatarsal joint complex. They can be difficult to diagnose clinically and become more challenging when mimicking a common ganglion cyst location. A detailed medical history is to be performed on all patients, with a high index of suspicion in patients with a familial history or active history of Neurofibromatosis Type 1 or Type 2. Recurrence and failed attempts of injections and/or aspirations should warrant the podiatric physician to have a detailed discussion regarding surgical intervention. MRI can be an effective diagnostic and surgical planning tool; however, surgical excision and histopathological examination is considered to be the gold standard. Particularly in cases where MRI cannot definitively diagnose a mass and is inconclusive in regard to a mass’s malignancy. Schwannomas may resemble other soft tissue tumors of the foot and diagnosis is generally made via histopathological findings status-post excision, as was the case in this report.

Funding Declaration: N/A

Conflict of Interest Declaration: No Conflicts of Interest to Report

Acknowledgements: University of Louisville Department of Pathology

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