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.
