Cornu Cutaneum: Case Reports of Patients With a Cutaneous Horn Associated With Either a Verruca Vulgaris or an Inverted Follicular Keratosis and a Review of the Etiologies of Cutaneous Horns

A cutaneous horn, referred to as a cornu cutaneum in Latin, presents as a mound of keratinizing epithelium. The etiology of the cutaneous horn is associated with the lesion at its base. In addition to numerous benign and malignant neoplasms, cutaneous horns may be related to infections and skin conditions. The features of a 22-year-old woman with a cutaneous horn associated with a recalcitrant verruca vulgaris on her left fifth toe are described. In addition, the characteristics of a 57-year-old man with an inverted follicular keratosis-related cutaneous horn on his upper lip are reported. In order of decreasing frequency, a cutaneous horn is most associated with either an actinic keratosis (25%), a squamous cell carcinoma (19%), a seborrheic keratosis (19%-20%), or a verruca vulgaris (18%). Adnexal neoplasms, epithelial lesions, fibrous lesions, granular cell tumors, hamartomas, histiocytic lesions, melanocytic nevus, premalignant keratoses, a subungual lesion, and vascular lesions comprise the benign neoplasms that have been observed at the base of a cutaneous horn. Dermatologic conditions that have been associated with a cutaneous horn include discoid lupus erythematosus (three patients) and one patient with either palmoplantar keratoderma, psoriasis, or sarcoidosis. Human papillomavirus infection presenting as a verruca vulgaris is the most commonly associated infection; pox virus-related molluscum contagiosum is another viral infection that is less often observed associated with a cutaneous horn. Leishmaniasis, rhinosporidiosis, and cutaneous tuberculosis are rare cutaneous horn-related infections. A malignant tumor-associated cutaneous horn is most frequently caused by squamous cell carcinoma; other less common cancers include basal cell carcinoma, sebaceous carcinoma, verrucous carcinoma, and malignant melanoma. A cancer-related cutaneous horn has only been described in two patients with Kaposi sarcoma and one patient with either Merkel cell carcinoma or Paget disease of the breast or metastatic renal cell carcinoma. In summary, a cutaneous horn is potentially related to a tumor, an infection, or a skin disorder; an adequate evaluation of the base of the cutaneous horn is usually required to establish the associated diagnosis.