Introduction

Granuloma annulare (GA) is an inflammatory, non-infectious granulomatous skin disease characterized by annular plagues or nodular lesions, primarily involving the dermis and subcutaneous tissue.¹ A cutaneous horn is a cone-shaped hyperkeratotic lesion, most commonly observed in sun-exposed areas.² However, its development on the lower extremities remains rare. Clinically, cutaneous horns are diagnosed based on underlying lesions, which may include viral warts, keratoacanthoma, seborrheic keratosis, actinic keratosis, squamous cell carcinoma, or basal cell carcinoma.³ To date, few cases of cutaneous horn complicating GA on the calf have been reported. Here, we describe a unique case of a cutaneous horn arising from GA on the calf.

Case Report

A 65-year-old male presented with a worsening skin lesion that had persisted for 52 years. The initial annular lesion on the flexor aspect of the right calf developed during adolescence, expanding slowly without medical intervention. Six months before evaluation, a hyperkeratotic lesion emerged on the annular lesion, increasing in size gradually. The patient reported no subjective symptoms. A former train driver with a 30-year career, he had no family history of similar dermatological conditions.

Physical examination revealed a large, golden-yellow, cone-shaped cutaneous horn (1.5 cm in length, 0.5 cm base width) localized on the annular lesion of the right calf flexor surface. The horn had smooth margins, and the surrounding skin exhibited a 3.0-cm-diameter annular plague with dark red-brown pigmentation (Figure 1). No similar lesions were noted elsewhere.

Figure 1 Cutaneous horn arising from granuloma annulare on the right calf flexor surface.

The horn was surgically excised, and a biopsy of the annular plague was performed. Histopathology of the plague showed abundant mucin in dermal collagen spaces, dense histiocyte infiltration, and focal mild collagen degeneration (Figure 2). The horn’s epidermis displayed hill-like protuberances with hyperkeratosis and parakeratosis, forming columnar lesions (Figure 3). No malignant features were observed. After a 3-year follow-up, there was no recurrence, and the annular plague significantly regressed.

Figure 2 (a) Superficial dermis of GA lesion showing mucin deposition and histiocytes in collagen gaps; (b) Histiocyte-lymphocyte infiltration and focal mild collagen degeneration. H&E stain, original magnification (a) ×200, (b) ×100.

Figure 3 Cutaneous horn demonstrating lamellar parakeratosis columns. H&E stain, original magnification ×50.

Discussion

Cutaneous horns are clinical diagnoses encompassing various benign and malignant underlying lesions. Literally defined as overgrowths of the skin’s superficial layer,⁴ they are more prevalent in Caucasians, rare in Asians, and extremely uncommon in Africans—likely due to pigmented skin photoprotection. Etiologic factors include chronic sun exposure and immunodeficiency.⁵

Typically occurring in individuals over 50 years without gender predilection, cutaneous horns usually involve sun-exposed areas.⁶ Notably, our patient’s horn developed in a non-exposed site (calf flexor), a location with few reported cases. While most horns grow slowly over years to decades, this case exhibited rapid progression (6 months), highlighting clinical variability in growth kinetics.⁷

GA is an idiopathic benign granulomatous disorder characterized by annular dermal plagues or plaques, commonly affecting the lateral/dorsal hands and feet. It often presents symmetrically in extremities and the trunk, typically asymptomatic but occasionally pruritic.⁸ Etiologies include immune dysregulation, heredity, viral infection, drugs, stress, insect bites, and trauma. Similar to cutaneous horns, GA may correlate with sun exposure. Pathologically, focal collagen degeneration and palisading granulomas are diagnostic, consistent with our case. Most GAs resolve within 2 years, though rare cases persist for decades—unusually, our patient’s lesion lasted over 50 years.

The co-occurrence of a cutaneous horn on long-standing GA is unique. Cutaneous horns rarely involve non-sun-exposed sites, and their association with GA is scarcely documented. The pathogenesis here remains unclear, but prolonged GA may have contributed to horn development. Histopathological studies of 222 cutaneous horn cases showed 58.56% were premalignant/malignant, with actinic keratosis (83.84%) and squamous cell carcinoma (93.75%) as leading underlying pathologies.⁹ Given the high prevalence of premalignant/malignant lesions beneath cutaneous horns, surgical excision with histopathological evaluation is imperative.⁹

Conclusion

This case emphasizes the need for heightened vigilance in long-standing GA, underscoring the importance of comprehensive diagnostic workup for cutaneous horns, regardless of anatomical location.