Schwannomas are solitary, slow-growing, and encapsulated benign soft tissue neoplasms composed of differentiated neoplastic Schwann cells [1]. These soft tissue tumors are thought to arise sporadically [2]. A melanotic, or melanocytic, schwannoma is a rare variant of a schwannoma that typically involves spinal nerve roots [1,2]. However, it can present at other anatomic locations. Melanotic schwannomas share particular ultra-structural features of Schwann cells. These features include a well-developed basal lamina, plentiful pinocytic vesicles, and interdigitating cytoplasmic precessess [3]. The most common location of a melanotic schwannomas is either the posterior spinal nerve roots or the paraspinal sympathetic chain [1,4]. Please see Table 1 for a comparison of a schwannoma and a melanotic schwannoma.

Table 1: Comparison of schwannoma to melanotic schwannoma.

This rare pigmented nerve sheath tumor was first described in 1932 by Millar [4]. Macroscopically, it appears brown or black due to the presence of pigmented melanin [4]. Microscopically, it shows characteristics of uncommon variants of nerve sheath tumors with melanocytic differentiation [4]. It is believed that melanotic schwannomas have the ability to synthesize melanin potentially because both Schwann cells and melanocytes are derived from migrating neural crest cells [1]. Following resection of this tumor, local recurrence is rare [5]. Complete resection of this tumor is recommended to prevent recurrence.

A 78 year old Caucasian female presented to a physician’s office with a complaint of onchyomycosis. Incidentally, she was examined and found to have a mass on the plantar aspect of her right foot. She denied pain or other complaints regarding the mass. The mass had been present for approximately one year. It did not prompt the patient to seek medical attention. The mass was minimally painful, and did not inhibit her activities of daily living. The mass had been very slowly increasing in size. Noninvasive imaging of this mass included plain radiographs, followed by Magnetic Resonance Imaging. The x-rays showed an increase in soft tissue density and volume at the plantar aspect of the right forefoot and midfoot with no bone involvement (Figures 1-3).

Figure 1: X-ray DP view right foot.

Figure 2: X-ray oblique view right foot.

Figure 3: X-ray lateral view right foot.

Magnetic Resonance Imaging revealed a heterogeneous macrolobulated mass measuring 3.0 × 8.3 × 5.3 cm. Malignancy was not ruled out (Figures 4-6). Neither osseous changes nor periosteal reactions were noted on the MRI.

Figure 4: MRI sagittal view right foot.

Figure 5: MRI coronal view right foot.

Figure 6: MRI axial view right foot.

A surgical biopsy procedure was then performed. During this procedure, a fibrous, brown-colored, smooth mass was identified. When a portion of this mass was excised, it exuded a moderate amount of brown, granular discharge. Two portions of the mass, from two separate locations, and its discharge, were sent as specimens for pathologic analysis. The pathology results revealed the mass to be a benign melanotic schwannoma.

During the post-operative course, the patient developed an infection at the surgical site. This infection was treated with incision, drainage and debridement. The patient was treated with antibiotics orally and the mass was subsequently removed in total during a followup procedure. Fluid from the mass removal was sent as specimen and ultimately revealed no malignant cells.

The patient then developed a non-healing wound at the surgical site. An additional surgical procedure, wound excision and rotational flap closure, and local wound care healed the wound within four months of the initial surgical procedure. At her subsequent appointments, the patient did not report any issues or additional complications following the removal of the pedal melanotic schwannoma.

Melanotic schwannomas are a distinguished entity from conventional schwannomas. The gray-brown pigment occasionally seen in conventional schwannoma represents lipofuscin opposed to true melanin [1]. Although nerve roots and the paraspinal sympathetic chain are the most frequent locations of melantoic schwannomas, they may present at various anatomic locations [1,2]. However, their presence in the foot is extremely rare with very few cases published in the literature. The appearance of this tumor on clinical exam and on surgical evaluation may mirror a malignant lesion. Wide excision of a melanotic schwannoma is recommended to prevent growth and reoccurrence of the lesion.

Melanotic schwannoma is a benign rare variant of a conventional schwannoma that has the ultrastructural features of Schwann cells. Additionally, it is composed of melanin-producing cells. Comparable to a schwannoma, it is a slow-growing space occupying lesion. The presence of this tumor is exceedingly rare in the foot. An x-ray of the mass is a first line imaging study. It can reveal if there is any osseous involvement associated with the mass. An advanced imaging study, such as an MRI, is needed to reveal the extent and characteristics of the mass. These characteristics aid with diagnosis of the lesion. However, imaging studies performed on a melanotic schwannoma may be inconclusive and can mirror malignant masses. The ideal diagnosis for this tumor includes biopsy, if advanced imaging studies are inconclusive. If advanced imaging studies are conclusive or after the results of the biopsy return, complete wide excision of the tumor is recommended for treatment.