Ectopic acanthosis nigricans at a post-syndactyly surgical site: An unusual reactive epidermal phenomenon

INTRODUCTION

Syndactyly is one of the most common congenital anomalies of the hand, affecting approximately one in every 2000–3000 live births and accounting for about 20% of congenital hand deformities. Surgical correction aims to restore independent finger movement and preserve web depth. Full-thickness skin grafts are generally preferred over split-thickness grafts because they provide better texture and reduce contracture rates, although pigmentary and textural changes occasionally occur postoperatively.¹

Acanthosis nigricans (AN) is characterized by hyperkeratotic, velvety plaques, most commonly located in the neck, axillae, and groin. It is often associated with insulin resistance, endocrinopathies, or, rarely, internal malignancy.² The pathogenesis involves keratinocyte proliferation stimulated by epidermal growth factor (EGF), transforming growth factor-alpha (TGF-α), and activation of EGF receptors (EGFRs).²

Ectopic or localized AN developing over non-flexural sites such as grafted skin is extremely uncommon. To date, only a handful of such cases have been reported worldwide, most following surgical reconstruction or skin grafting procedures.^3,4 We present a rare Indian case of bilateral ectopic AN developing several years after syndactyly-release surgery and discuss the clinical, dermoscopic, and histopathologic characteristics along with possible pathogenetic mechanisms.

CASE REPORT

A 20-year-old woman presented with progressive darkening and thickening of the skin over both hands for several years. She had undergone bilateral syndactyly-release surgery at 10 months of age for fusion of the middle and ring fingers, during which full-thickness skin grafts from the medial thigh were used. The post-operative course was uneventful, with normal function maintained. However, several years later, she noted the development of brownish-black thickened plaques over both grafted areas. Clinical examination revealed well-demarcated hyperpigmented, velvety plaques on the lateral and medial aspects of the third and fourth fingers, extending into the interdigital web spaces bilaterally [Figure 1]. Sparse terminal hairs were visible over the plaques. There was no associated itching, pain, or functional limitation. No similar lesions were observed over typical sites of AN such as the neck, axillae, or groin. Her body mass index was 22 kg/m², and systemic examination was normal. Routine hematological and biochemical investigations, including fasting glucose, serum insulin, lipid profile, thyroid hormones, luteinizing hormone, follicle-stimulating hormone, and prolactin, were all within normal limits, excluding metabolic or endocrine causes. Polarized dermoscopy (DermLite DL4, ×10) revealed accentuated linear crista cutis and sulci cutis with scattered brown and black dots corresponding to follicular and pigmentary openings, features consistent with dermoscopic findings in AN [Figure 2].⁵ Histopathological examination of a 4-mm punch biopsy demonstrated hyperkeratosis, papillomatosis, moderate acanthosis, and increased basal layer pigmentation with mild perivascular and peri-adnexal lymphocytic infiltration [Figure 3]. No evidence of epidermal dysplasia or malignancy was seen. A final diagnosis of ectopic AN localized to post-syndactyly graft sites was made based on clinicopathological and dermoscopic correlation. The patient was reassured about the benign nature of the condition. She was advised topical keratolytic and depigmenting therapy using 10% urea and 15% azelaic acid creams. Cosmetic laser options, including Q-switched Nd: YAG for pigment reduction and long-pulsed Nd: YAG for hair removal, were discussed.⁶ At six-month follow-up, the lesions remained stable, with mild improvement in pigmentation and texture.

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Figure 1:

Palmar view showing hyperpigmented, velvety plaques with sparse terminal hairs over the interdigital web spaces of both hands.

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Figure 2:

Dermoscopy demonstrating linear crista cutis and sulci cutis with scattered brown dots (Polarized, ×10).

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Figure 3:

Photomicrograph showing epidermal hyperkeratosis, papillomatosis, basal pigmentation ( black arrow), and mild periadnexal inflammation (Hematoxylin & eosin , 400×).

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DISCUSSION

AN is a heterogeneous condition classified into benign, obesity-related, syndromic, drug-induced, and malignant forms.² The ectopic or localized variant occurs in non-flexural areas without systemic association. Post-grafting cases represent an even rarer subset, possibly due to epidermal– dermal interaction alterations following transplantation.

Wu and Cunningham (2008) first reported ectopic AN in a 12-year-old boy with oculodentodigital dysplasia who developed velvety hyperpigmentation at a groin-derived graft site following syndactyly repair.³ More recently, Burke et al. (2022) described “ectopic AN” in a child who developed similar plaques on inguinal skin grafted to the hands for post-burn contracture release.⁴ Our case appears to be the first reported adult Indian case, occurring bilaterally years after syndactyly correction.

The proposed pathogenesis involves reactive epidermal hyperplasia due to mismatched donor and recipient site characteristics. Thigh skin, being thicker and containing pilosebaceous units and a higher melanocyte density, when transplanted onto palmar regions, may show persistent hyperpigmentation and hypertrichosis.^3,6 In addition, increased expression of growth factors such as EGF and TGF-α may activate the EGFR pathway, inducing papillomatosis and acanthosis similar to classical AN.² This supports the hypothesis that ectopic AN represents a localized reactive process rather than a metabolic or malignant one.

Donor dominance may also play a contributory role in such cases. It is well recognized that skin grafts often retain several morphologic and physiologic characteristics of the donor site, including pigmentation potential, hair-bearing nature, and adnexal structure density.^1,6 The medial thigh, from which the graft was harvested, likely imparted its intrinsic attributes of thicker epidermis, melanocyte activity, and follicular density to the recipient palmar region. This persistence of donor-site characteristics – referred to as donor dominance – may explain the presence of localized pigmentation and hypertrichosis manifesting as ectopic AN.

Furthermore, the thickness of the graft may influence the degree of epidermal proliferation. Full-thickness grafts, which include the entire epidermis and dermis, retain adnexal elements and dermal signaling pathways more completely than split-thickness grafts. This greater structural preservation enhances local growth-factor communication, promoting keratinocyte proliferation and hyperpigmentation¹. In contrast, split-thickness grafts, which lack the full dermal component, tend to remodel more completely to the recipient site phenotype and exhibit less reactive hyperplasia. Therefore, both donor dominance and graft thickness likely acted synergistically in this patient to induce the reactive epidermal changes consistent with ectopic AN.

Dermoscopy provides non-invasive confirmation of AN by demonstrating accentuated crista-sulcus patterns, brown dots, and papillomatous projections.⁵ Histopathology typically reveals epidermal hyperkeratosis, papillomatosis, and basal hyperpigmentation, as observed in our case. Differential diagnoses include post-inflammatory hyperpigmentation (flat, non-velvety), confluent and reticulated papillomatosis (reticulated pattern over trunk), epidermal nevus (since birth, linear arrangement), and seborrheic keratosis (stuck-on papules in older adults). Dermoscopy assists in distinguishing these entities.

Treatment is primarily cosmetic and supportive. Topical keratolytic and depigmenting agents such as glycolic acid or azelaic acid may help lighten pigmentation, while laser-based therapies, including Q-switched Nd: YAG and long-pulsed Nd: YAG lasers, can further improve pigment and hair changes.⁶ Long-term prognosis is excellent, and no malignant transformation has been reported.

CONCLUSION

Ectopic AN is a rare, benign reactive epidermal phenomenon that may arise in skin grafts following reconstructive surgery. It likely represents localized epidermal hyperplasia driven by epidermal–dermal mismatch and altered growth-factor signaling. Recognition of this entity prevents misdiagnosis as metabolic or malignant AN and avoids unnecessary investigations. A multidisciplinary approach between dermatologists and surgeons ensures accurate diagnosis and appropriate patient reassurance.