Nevus trichilemmocysticus: a case report and review of the rare type of epidermal nevi

Nevus trichilemmocysticus (NTC) constitutes a broader group of epidermal nevi that are commonly present at birth or during early childhood and marked by hamartomatous lesions with epidermal hyperplasia.1, 2 Epidermal nevi lesions often begin as a linear tan patch or thin plaque presenting along Blaschko lines and typically progress over time, especially with puberty. NTC is an uncommon form of epidermal nevi that was first recognized as a distinct entity in 2007.2 There are not many reports in literature identifying this condition. We describe the case of a 38-year-old female with nodules on the leg and scalp consistent with NTC. A 38-year-old female with no significant past medical history presented with a several-year history of slowly enlarging nodules on her scalp and left lower leg. These nodules were occasionally tender and red. The patient reports having similar nodules on her left knee that were surgically removed and thought they contained “calcium deposits.” The left leg nodules were spread over a rash that had been present since birth. Her family history was non-contributory. She denied fevers, chills, headaches, joint pains, or muscle aches. On physical examination, there were two 5 cm soft, nontender, and mobile nodules on the bilateral parietal scalp. A thin, scaly plaque covered the left posterior upper and lower leg in a blaschkoid pattern along with scattered firm, erythematous nodules and hyperkeratotic papules (Figure 1). Punch biopsies were obtained for the nodules on the proximal and distal left leg. Histopathology of the proximal lesion revealed filiform hyperkeratosis with sections of the skin that demonstrated compact orthokeratosis, mild papillomatosis, and epidermal hyperplasia (Figure 2a). A hyperparakeratotic plug in a dilated follicle showed trichilemmal differentiation. The tissue sample of the distal left leg showed sections with squamous-lined cysts with abrupt trichilemmal type keratinization and a lack of granular cell layer within the cyst lining (Figure 2b). Given the history, physical exam, and histopathological findings, a diagnosis of nevus trichilemmocysticus (NTC) was made. The patient underwent excision of two leg nodules and two pilar cysts on the scalp. Epidermal nevi evolve from genetic mutations or epigenetic changes that alter gene expression in the early postzygotic embryo, resulting in somatic mosaicism that follows the lines of Blaschko.1, 3 Based on a report of 38 Danish family members, trichilemmal cysts may be associated with the TRICY1 gene as an autosomal dominant trait.4 However, solely indicating this gene may oversimplify the more complex disease findings found in NTC.2 Extracutaneous manifestations may further accompany NTC. One patient reported headaches, arthritis during childhood and adolescence, and severe bone deformities due to osteomalacia.2 However, it is uncertain if these bone deformities were affected by NTC or due to her vitamin D deficiency. Mild frontal bossing was also observed in a 10-year-old female.5 They suspect that the facial abnormalities observed in their case do have a connection to NTC given that CNS deformities can be associated with several types of epidermal nevi. Clinicians should be aware of these systemic findings and their association with epidermal nevi syndromes. NTC treatment involves excision of bothersome nodules. CO2 ablation and radiofrequency treatment have been used in some cases to improve cosmetic outcomes.3 Overall, epidermal nevi treatment can be challenging. Topical therapies including cryotherapy, dermabrasion, and electrodessication may allow temporary improvement but are prone to recurrence.1 Emerging research involves identifying genotypes and elucidating the pathogenesis of epidermal nevi; this may provide greater insight into targeted therapies and individualized treatment modalities.