Palisaded Encapsulated Neuroma
The palisaded encapsulated neuroma (PEN) is an uncommon benign tumor that is derived from nerve tissue. It is also known as solitary circumscribed neuroma of the skin. Most of the tumors occur on the head and neck.
Clinical Findings: The lesions of PEN most often manifest on the head and neck region of patients in the fourth and fifth decades of life. They afflict men and women equally and have no race predilection. They are firm, dome-shaped papules or dermal nodules. They are almost always solitary in nature. The overlying epidermis is unaffected and is flesh colored. These benign tumors tend to grow slowly over a period of years until they reach a size (often <1 cm in diameter) that makes them worrisome to the patient. They are commonly misdiagnosed as compound nevi or basal cell carcinomas, and it is not until they are biopsied that the true diagnosis is made. These tumors have a propensity to develop on the eyelid margin and at the interface between keratinized skin and the mucous membranes. Many are seen and removed by ophthalmologists. Most of these tumors are completely asymptomatic. On occasion, they are tender. This tumor is not associated with any underlying neural or systemic symptoms. In contrast, traumatic neuromas occur at sites of trauma, especially at amputation stump sites, and are caused by hypertrophy and proliferation of the damaged nerve ending. These tumors are solid, hard dermal nodules that cause pain on palpation.
Pathogenesis: The PEN tumor is derived from neural tissue. The Schwann cell is believed to be the cell type of origin for this growth. The proliferation of Schwann cells forms the tumor lobule. The exact mech- anism or signal that causes this proliferation has not yet been discovered. Schwann cell origin is important to recognize and helps differentiate this tumor from other neurally derived tumors. The capsule is derived from perineural cells and collagen bundles. The capsule is believed to occur as a reaction to the underlying Schwann cell proliferation.
Histology: The PEN has a clear and well-demarcated capsule lining that is derived from collagen and peri- neural cells. The tumor is located entirely within the dermis, and the overlying epidermis is normal in appearance. There is no inflammatory infiltrate. The tumor is composed of spindle-shaped cells that form a tight, interweaving pattern. Immunohistochemical staining is often used to help differentiate these tumors form other neurally derived tumors such as schwannomas, neurofibromas, and traumatic neuromas. Neurofibromas do not have a true capsule circumventing the tumor. The capsule stains with epithelial membrane antigen (EMA). This stain helps indicate the location of the perineural capsular cell components. The tumor proper stains with S100, vimentin, and type IV collagen. This staining pattern has been described for Schwann cells, so a positive result helps to determine the derivation of this tumor. Schwannomas are differentiated by their characteristic Antoni A and B regions and their subcutaneous location. Traumatic neuromas are not encapsulated and are composed of all the individual components that make up the previously normal traumatized nerve tissue.
Treatment: Complete excision is diagnostic and curative. The tumors rarely recur after elliptical excision. They have no malignant potential, and patients can be reassured that they do not have any possibility of an underlying neural syndrome. Traumatic neuromas can be cured by surgical removal. There is a small risk of recurrence. Pain control is also critical in the management of traumatic neuromas.