Continuing Education Activity

A cutaneous horn is usually evident upon physical examination and can be described as a white or yellow exophytic protrusion in the shape of an animal horn. Unlike animal horns that usually contain an osseous cast, cutaneous horns consist solely of cornified proliferative keratinocytes without a bony component. The color, size, shape, and dimensions can vary significantly, so clinical suspicion should be confirmed by histopathological analysis. This activity describes the evaluation and management of a patient with a cutaneous horn and highlights the role of the interprofessional team in the care patients with this condition.

Objectives:

• Identify the epidemiology of a cutaneous horn.
• Review the appropriate points in the evaluation of a cutaneous horn.
• Outline the management options available for a cutaneous horn.
• Summarize interprofessional team strategies for improving care coordination and communication to advance the treatment of cutaneous horns and improve outcomes.

Introduction

A cutaneous horn, or cornu cutaneum, is a relatively uncommon, hyperkeratotic epithelial lesion resembling an animal horn defined as having a height that is more than half of the diameter of its base.[1][2] The first formally documented case originates from the 1588 description of an elderly Welsh woman by the name of Mrs. Margeret Gryffith. Although horns surely predate Mrs. Gryffith’s diagnosis, her disease and subsequent exploitation in circuses profiting from medical oddities led to the generativity of her name and condition in popular culture.[3] The coined term, “horned people,” stuck in colloquial verbiage as Mrs. Gryffith and others were displayed in circuses worldwide as magical beasts. It was not until the mid-seventeenth century that these mislabeled and abusive caricatures were proved incorrect when Danish anatomist Thomas Bartholin described these horny growths as a tissue tumor arising from the surface of the skin.[4] Cutaneous horns are now widely accepted as a reactive cutaneous growth caused by a variety of benign, premalignant, or malignant primary processes.

The lesion is usually evident upon physical examination and can be described as a white or yellow exophytic protrusion in the shape of an animal horn. Unlike animal horns that usually contain an osseous cast, cutaneous horns consist solely of cornified proliferative keratinocytes without a bony component. The color, size, shape, and dimensions can vary significantly, so clinical suspicion should be confirmed by histopathological analysis.[5]

Although prevalence and incidence are not established, epidemiological conclusions have been, including correlations with age, sex, skin color, location, and dimensions.[6][7] Histopathological evidence asserts the cutaneous horn itself is relatively unimportant when compared to its underlying condition.[3] Consequently, understanding the etiology and associated histopathology behind cutaneous horns have been among the primary concerns of clinicians and researchers.

Etiology

The etiology of cutaneous horns varies as it is a secondary manifestation of a benign, premalignant, or malignant primary disease.

The most common cause of benign cutaneous horns is seborrheic or lichenoid keratoses.[7] Other benign causes include infections like human papillomavirus, molluscipoxvirus,[8] rhinosporidiosis,[9] chronic irritation,[8] hemangioma,[8] sarcoidosis,[10] juvenile xanthogranuloma,[11] epidermal nevus,[11] pilomatricoma,[12] angiokeratoma,[11] seborrhoeic keratosis,[6][11] lichenoid keratoses, trichilemmoma, and sebaceous adenoma.[13]

Actinic keratoses are the most common premalignant primary cause of cutaneous horn, while squamous cell carcinoma (SCC) is the most common malignant cause.[7] Additional premalignant and malignant causes include arsenical keratosis, pseudoepitheliomatous keratosis, micaceous balanitis, actinic keratosis, Bowen's disease, verrucous carcinoma, basal cell epithelioma, Kaposi's sarcoma, keratoacanthoma, and carcinoma.[8][14] Furthermore, cutaneous horns have been described in cases involving distant concomitant malignancies, such as renal cell carcinoma.[15]

Although the exact pathogenesis of the cutaneous horn is unknown, it is assumed that cellular aging, photodamage, and subsequent epithelial dysfunction contribute to their manifestation. This is supported by findings of sun exposure and old-age being the most common comorbidities in patients with cutaneous horns.[15] Although not established, it has been postulated that people with fairer skin are preferentially affected.

Epidemiology

The true incidence and prevalence rates of cutaneous horns are not documented in the literature. However, it is well known that they are more commonly found in the elderly population between 60 to 80 years of age. They are also more likely to be premalignant or malignant in geriatric populations. The mean age of patients with a benign cutaneous horn base pathology is 8.9 years less than those with premalignant or malignant base pathologies.[6][7]

The sex distribution of benign lesions is equal among males and females; however, there is a greater incidence of premalignant or malignant cutaneous horns in males.[6] This is possibly attributed to differences in social behaviors.

Regardless of whether it is benign, premalignant, or malignant, the lesion is most commonly located in areas most vulnerable to ultraviolet radiation, such as the head, neck, and upper extremities.[3][6] Wider-based lesions are more prevalent in these sun-exposed areas of the body.[6]

Pathophysiology

By definition, a cutaneous horn is a well-circumscribed, hyperkeratotic lesion with a height that is more than half of the diameter of its base. It can arise from any part of the skin or mucosa, with its pathophysiology being dependent upon the underlying disease. Those associated with benign disease processes tend to grow slowly over a few months to years.[6] However, diseases that possess a higher mitotic activity rate, such as squamous cell carcinoma, can lead to a rapid growth phase and the development of horns. Due to greater cumulative actinic damage and higher rates of neoplastic degeneration in the elderly population, they are more prone to malignant horns.[7] Lesions with a wider base are more likely to be malignant than those with a narrower base.[6]

Montgomery (1941) classified cutaneous horns into 5 types. These subtypes were distinguished from each other by appearance, histological structure, and causation.[14]

1. Cutaneous horn arising from an epidermoid cyst
2. Mucosal horn arising from a mucous membrane
3. Verrucous cutaneous horn resulting from a wart
4. Papillomatous cutaneous horn developing from the keratinizing stratified squamous epithelium
5. Filiform cutaneous horn arising from normal or hyperkeratotic skin

Contemporarily, the dermatology community has moved away from Montgomery’s classic subtypes into one more focused on the underlying etiology behind the lesion. While patients may still be described as having a Montgomery Type 5 cutaneous horn,[14] histopathological analysis of the underlying lesion and subsequent determination of the underlying pathology is the current model for cutaneous horn classification; namely, benign, pre-malignant, or malignant with a distinct variety of underlying causations defined for each category.

Histopathology

The histopathological analysis will reveal compact, redundant hyperkeratosis with or without orthokeratosis or parakeratosis. Giant horns (> 1cm height) are less suitable for microscopic analysis. Histopathology of the underlying disease will be found beneath the cornified projection.[1]

History and Physical

Patients typically present with a hard, conical projection, most commonly seen over the sun-exposed areas like face, eyelids, forearms, etc. Therefore, a history of poor or improper sun protection behaviors should raise the clinical suspicion for the presence of a horn.[1] However, the lesion may be present in sun-protected areas as well. Pain may be an associated symptom, and the presence of pain has been associated with a higher chance of underlying malignancy.[1] Studies show an overwhelming indication that absolute horn height has less of a contribution, if any, to the primary cause of the disease than factors such as height-to-base ratio, age of the patient, pain, histological analysis, and sun exposure.[1][3][6][10][11][15][16]  

Upon tactile discrimination, the examiner will find a firmly rooted exophytic growth. Absolute widths can vary from a few millimeters to several centimeters in diameter. Absolute heights of cutaneous horns are greater on average than widths by definition, but still within the range of millimeters to several centimeters. Heights greater than 1 cm are rarely encountered; however, giant horns up to 25 cm have been reported.[5] Currently, there is no documented correlation between the lifespan of the cutaneous horn and its classification as a benign, premalignant, or malignant lesion.

Regardless of its classification, horns tend to be evident, keratinous, elongated, yellowish, or white projections with a variety of sizes.[5] These exophytic projections can be described as conical, cylindrical, pointed, or curved like a ram’s horn. The base of these lesions may be seen as flat, nodular, or crateriform. Surrounding erythematous inflammation is rare and indicates underlying malignancy. They may be difficult to distinguish from conditions such as an ectopic nail.[17] Consequently, excision and histopathological analysis are required to confirm the suspicion of the cutaneous horn, but more importantly, to assess the underlying etiology of the lesion.

On dermoscopy, terrace morphology, indicative of benign disease, and base erythema, prognostic for malignant potential, are likely findings. Terrace morphology, defined as structural horizontal contours on the side of the horn, was recorded in the majority of cases of benign keratosis, actinic keratosis, and SCC in situ. Invasive SCC recorded the lowest significant incidence of terrace morphology. Base erythema is defined by a red, erythematous area in the base of the horn compared to the background skin within 5 mm from the horn base boundary. Base erythema occurred in over 55% of the four diagnostic categories studied by Pyne et al., which include: benign keratosis, actinic keratosis, SCC in situ, and invasive SCC. Invasive SCC displayed the greatest incidence (77%) of base erythema. Lastly, horns with a height one-to-two times greater than the base diameter have a higher incidence of malignant potential than those with lower height-to-base diameter ratios.

Evaluation

A clinical diagnosis is feasible in most settings, but many clinicians may pursue a histopathological investigation as a diagnostic adjunct. In cases in which a premalignant or malignant etiology is considered in the differential diagnosis, a biopsy is recommended in order to elicit the horn’s potentially carcinogenic implications. A deep biopsy or total excision is the recommended method to obtain a sufficient sample of the suspicious lesion.[4] It is important to preserve the entire base of the lesion as the most superficial component of the horn consists of retained, dead keratinocytes. Obtaining an improper sample would not reveal the underlying carcinogenic cause for the horn and may lead to a false-negative diagnosis.

Further workup of the disease is dependent upon the histopathological findings of the primary disease process. Imaging is not required or recommended, and lab testing is unnecessary unless the primary pathology warrants further investigation.

Treatment / Management

Cutaneous horns can be treated surgically, medically, or via laser ablation. Diagnosis of the cutaneous horn, however, must be made after histological assessment due to its ability to appear like other conditions such as an ectopic nail.[17] This assessment also serves as the most accurate method to demonstrate the underlying etiology of the condition, which is a necessary step due to premalignant or malignant concerns.

Historically, cutaneous horns have been treated using simple detachment and cauterization of the base.[5] However, the current standard of care for cutaneous horns is complete excisional biopsy. Depending upon the histopathological diagnosis, the further plan of management can be decided. For benign lesions, observation is warranted, or the lesion may be excised for aesthetic reasons and subsequently monitored periodically. Wide local excision is the preferred treatment for premalignant or malignant cases. Margins should be implemented depending on the diagnosed underlying premalignant or malignant condition. These margins should be consistent with the most updated guidelines for that condition.[18]

Ablative lasers like carbon dioxide or neodymium-doped yttrium aluminum garnet may be preferred for aesthetic considerations, as well as electrocautery.[4][19] Cryotherapy is not a recommended treatment. Despite these alternatives, deep biopsy or total excision remain the treatments of choice, ensuring that the base of the horn is preserved for histological preparations.[4]

Patients with cutaneous horns caused by underlying squamous cell carcinoma should also be evaluated for metastasis. For patients with basal cell carcinoma or squamous cell carcinoma, follow-up evaluations are necessary for the first three years after diagnosis. Given the other causes for the cutaneous horn, standard follow up for that entity is sufficient.

Differential Diagnosis

• Nongenital warts
• Perforating pilomatricoma
• Ectopic nail
• Seborrheic keratosis
• Actinic keratosis
• Squamous cell carcinoma
• Keratoacanthoma
• Bowen disease
• Melanoma
• Trichilemmal carcinoma
• Sebaceous adenoma

Diagnosis is confirmed via clinical presentation, and histopathological analysis is crucial to determine underlying etiology.

Prognosis

Prognosis is dependent upon the underlying primary disease process. These diseases can be completely benign and painless, where removal is indicated for cosmetic purposes only. However, there is a considerable chance there may be a premalignant or malignant cause that needs to be addressed. Yu et al. described a 38.9% rate at which the underlying lesions to a patient’s cutaneous horn is malignant or premalignant. Therefore, a valid prognosis can only be achieved after histopathological analysis of the underlying condition. In one study, actinic keratoses (AK) were found in 83.84% of premalignant cohort cases, while SCC (in situ or invasive) was discovered in 93.75% of malignant cases. This suggests AK and SCC may be the most common primary disease processes in cutaneous horns with carcinogenic potential.[7]

Complications

Patients with a cutaneous horn typically present with a hard, conical projection, usually seen over the sun-exposed areas like face, eyelids, forearms, etc. Aside from presenting as a cosmetic issue, cutaneous horns may also be associated with pain and rapid growth. More importantly, there is a considerable chance there could be a premalignant or malignant complication causing the cutaneous horn. Each underlying pathology may have complications, which is why it is crucial to identify the underlying cause of a patient's horn.

Deterrence and Patient Education

As the sun-exposed areas are more commonly affected, counseling regarding sun protection measures is paramount. Sun-protective clothing, sunscreen, shade seeking, and avoiding peak ultraviolet radiation hours are essential recommendations for patients with a history of prolonged sun exposure. Furthermore, counseling patients on how to perform monthly self skin checks as well as consistent checkups are routine practices to ensure patient education and long-term wellness.

Enhancing Healthcare Team Outcomes

Cutaneous horns are best managed with an interprofessional team approach. Primary consultation or referral should be with a dermatologist, but clinically benign cases may be addressed in the primary care setting. Nursing support staff are essential for a thorough workup. Surgical excision, followed by histopathology, is essential for the diagnosis. As the underlying causes of cutaneous horns are varied, the further management plan should be decided based on the histopathological report.