Pigmented neurofibromas, also known as melanotic neurofibromas, are uncommon (accounting for <1% of neurofibromas), and are presumed to arise from the Schwann cells of peripheral nerves or the neuroectodermal layer. They can occur on their own or be associated with neurofibromatosis. The lesions ranged from 1–50 cm in diameter; most were located on the head or neck.4
There were only a few reports concerning this phenomenon. It is characterised histologically by the association of a benign pigmented tumour producing melanin and the presence of Schwann cells and nervous cells, a prominent storiform pattern and a pure neural differentiation.5
Many neoplasms are diagnosed inadvertently at the time of an incisional biopsy for presumed other classical neurofibromas.
The diagnosis of pigmented neurofibroma is mainly confirmed by histopathology, and immunohistochemical stains can help to establish the diagnosis. The tumour cells are fusiform or full and rounded, the nuclei are irregular spindle shaped and dark stained with fine dispersed chromatin, the nucleoli inconspicuous and mitotic figures absent.6
Scattered or clustered epithelioid, spindled and dendritic pigmented cells are distributed widely within the neurofibromatous tissue, but are most prominent in the reticular dermis and subcutis.7
Electron microscopy can detect some pigmented cells containing numerous, electron-dense, melanin granules in the cytoplasm. Immunohistochemically, the tumour cells are positive for S-100, vimentin, HMB45 and negative for EMA and CK. But in this case, the tumour cells were negative for HMB45. This may be due to these cells having mature melanosomes, because adult melanocytes are generally non-reactive for this marker.8
Though pigmented neurofibromas are rare and have a lower potentially life-threatening chance of malignant transformation than malignant peripheral nerve sheath tumour, among others,9
they can be especially difficult to distinguish from pigmented dermatofibrosarcoma protuberans (Bednár tumour), melanotic schwannoma, or cellular blue nevus because of the clinical and histological similarities. Tissue biopsy is necessary to confirm the diagnosis. Diagnosis of pigmented neurofibroma can also be aided by immunohistochemical stains. The immunoreactivity for S-100 protein demonstrates that the tumour arose from the neural crest. Complete excision is recommended while a confirmed diagnosis is made. The management of these patients involved periodic and long-term follow-up to avoid the recurrence of the tumour with (or without) malignant transformation.
This case was presented to make ophthalmologists aware of pigmented neurofibroma, although it is very rare and difficult to distinguish.
- Tissue biopsy is necessary to confirm the diagnosis of pigmented neurofibroma.
- Complete surgical resection is the recommended treatment.
- The histopathological and immunohistochemical features are very important for diagnosis.