Striking a Nerve: Deep Intraneural Neurofibroma, a Rare Benign Peripheral Nerve Sheath Tumor Variant

Summary: A 16-year-old skin type 5 boy with autism spectrum disorder was referred to dermatology clinic for 5 months of an insidiously enlarging left lower back 5mm firm dermal papule with 1.5cm ill-defined overlying hyperpigmentation. The papule was largely asymptomatic but the patient endorsed occasional tenderness with direct external pressure on the area. An excisional biopsy was performed with histopathology revealing a discrete deep dermal spindled neoplasm encapsulated by normal-appearing nerve sheath tissue. Special staining demonstrated tumoral positivity with S-100, negative tumoral EMA, and positive capsular EMA suggesting a diagnosis of intraneural neurofibroma. The patient and his mother subsequently denied any additional similar lesions or other cutaneous findings which might suggest a syndromic phakomatosis. We present this case to raise awareness of this rare entity with distinct histologic findings and to highlight the ambiguous state of the literature concerning nomenclature of intraneural neoplasms. We propose the new term “deep intraneural neurofibroma” to distinguish this type of benign peripheral nerve sheath tumor from more common neurofibroma variants and from other lesions arising within peripheral nerves, particularly intraneural perineurioma and intraneural hybrid nerve sheath tumors.