Diagnosis: Palisaded and encapsulated neuroma
• PEN is also known as solitary circumscribed neuroma, and is a benign proliferation of normal components of peripheral nerve.
• PEN is most commonly found on the face, but also commonly occurs on the neck, and oral cavity. The usual clinical appearance is that of a small skin colored or white papule that is asymptomatic. PEN has also been reported infrequently on the trunk, extremities (including fingers and palms), as well as the penis.
• PEN is a dermal nodule composed of multiple neural fascicles comprised mostly of Schwann cells. The Schwann cells can be highlighted with stains for S-100 and collagen IV. Axons of peripheral nerve fibers are also included in the lesion. The axons can be highlighted with a stain for neurofilament. The fascicles are often separated from each other with clefts. Rarely, PEN can demonstrate a plexiform or multinodular architecture.
• The connective tissue encapsulation of a PEN may or may not be present. The encapsulation can include perineurium and epithelial membrane antigen positive cells. One way distinction can be made from traumatic neuroma (TN) is that in TN the individual fascicles are surrounded by a capsule of perineurial cells, while with PEN, the perineurial cells encase the entire lesion.
• PEN is most common in adults, but has also been reported in the pediatric population. There is no sex predilection.
• While most lesions of PEN occur as a single lesion, cases have been reported where multiple lesions have been demonstrated on the same patient.
