Rupioid Porokeratosis on the Lower Leg: An Unprecedented Case Report

Abstract

Verrucous porokeratosis (VP) is a rare variant that typically presents as solitary hyperkeratotic plaques on acral sites, and atypical morphology or unusual location is exceptionally uncommon. We report a rare case of VP on the right lower leg with a striking rupioid (oyster-shell-like) morphology in a patient with long-standing diabetes mellitus and a history of rectal cancer resection and the patient received oral isotretinoin combined with topical corticosteroids and microcirculation-targeting agents, which was well tolerated and led to clinical improvement, which expanding the clinicopathological spectrum of porokeratosis.

Introduction

Porokeratosis represents a heterogeneous group of disorders of keratinization unified by the distinctive histopathological hallmark of the cornoid lamella. The clinical spectrum encompasses several subtypes, among which verrucous porokeratosis (VP) is considered a rare variant.¹ Classically, VP presents as solitary or localized, well-demarcated, hyperkeratotic, verrucous or papillomatous plaques. While some earlier reports described a predilection for acral sites such as the palms and soles, more recent literature has increasingly emphasized the association of VP with genitogluteal involvement.^2,3 While typical presentations are widely recognized, deviations from this classic phenotype are exceedingly uncommon.

We herein report an unprecedented case of VP exhibiting a striking rupioid morphology resembling oyster shells, arising on the right lower leg which not previously associated with this variant. Furthermore, the patient had a significant medical history of long-standing diabetes mellitus and prior resection for rectal cancer. To the best of our knowledge, this is the first reported case of rupioid porokeratosis occurring on the lower leg. This case expands the clinicopathological spectrum of porokeratosis and underscores the importance of considering this entity in the differential diagnosis of atypical hyperkeratotic lesions, even at unusual locations, particularly in patients with underlying metabolic or immunologic alterations.

Case Presentation

An 85-year-old male of Chinese Han ethnicity presented to our dermatology with a six-month history of multiple painful nodules on his right lower limb, which was initially presenting as a solitary nodule, subsequently increased in number with partial coalescence into plaques thereafter. The patient has a history of type 2 diabetes mellitus, managed with oral hypoglycemic agents. Notably, he underwent curative surgical resection for a rectal adenocarcinoma one year ago and received no adjuvant in the postoperative period. No family history of similar skin lesions. Physical examination revealed multiple 2-to-15mm papules, firm nodules and confluent plaques overlaid with scant white scales, yellow purulent crusts and abundant dark gray, oyster-shell-like thick crusts on the right lower limb (Figure 1A). Microbiological investigations were not performed as the clinical and histopathological findings were consistent with porokeratosis rather than an infectious process. An incisional biopsy was taken from one representative intact nodule for definitive histopathological diagnosis, and the specimen included the peripheral raised ridge of the lesion. Multiple sections were examined, and cornoid lamellae were consistently identified at the ridge, confirming the classic histopathological relationship between the cornoid lamella and the clinical border of the lesion. Syphilis serology and HIV testing were negative.

Figure 1.

(A) Multiple 2-to-15mm oyster-shell-like papules, firm nodules and confluent plaques overlaid with scant white scales and yellow purulent crusts on the right lower limb. (B) Histopathological examination of a biopsy taken from the same representative lesion shown in Figure 1A reveals characteristic cornoid lamellae inserting into the epidermis with absence of the underlying granular layer. The cornoid lamella appears as a thin, vertical column of parakeratotic cells extending through the epidermis, with focal dyskeratosis of adjacent keratinocytes. Extravasated erythrocytes are present within dermal papillae, along with a superficial band-like mononuclear lymphocytic infiltrate and focal neutrophilic aggregates. (Hematoxylin-eosin staining; original magnification, ×20). (C) Higher magnification of the cornoid lamella demonstrates a column of parakeratosis with abrupt loss of the underlying granular layer and dyskeratosis within the adjacent epidermis. (Hematoxylin-eosin staining; original magnification, ×50). (D) Clinical appearance of the same lesion after six months of treatment, showing marked reduction in lesion thickness, crusting, and erythema. (Same patient, same lesion; different viewing angle.).

Histopathological examination revealed epidermal hyperplasia, hyperkeratosis, and acanthosis. Characteristic cornoid lamellae were observed inserting into the epidermis, with absence of the underlying granular layer. The granular layer was globally diminished, with focal dyskeratotic cells scattered within the spinous layer. Additionally, extravasated erythrocytes within dermal papillae and a predominantly superficial, band-like inflammatory infiltrate composed of mononuclear lymphocytic with focal aggregates of neutrophils was present in the superficial dermis. No significant basal cell liquefactive degeneration observed (Figure 1B). Consequently, the combined clinical and histopathological features led to the definitive diagnosis of verrucous porokeratosis.

Discussion

Verrucous porokeratosis has been described in the literature with considerable variation in its clinical presentation and anatomical distribution. While some authors have emphasized its occurrence on acral sites, particularly the palms and soles,^1,2 others have used the term synonymously with genitogluteal porokeratosis, highlighting a predilection for intertriginous and anogenital areas.^3,4 Despite the terminological controversy, involvement of the lower leg remains exceptionally rare regardless of the classification system adopted. To the best of our knowledge, only few reports have documented porokeratosis at this site, and none presented with a rupioid appearance. It should be noted that the classic “butterfly” distribution in the genitogluteal region is widely recognized as a characteristic feature of porokeratosis ptychotropica rather than a mandatory criterion for VP.^5,6 Histopathologically, the presence of cornoid lamellae with loss of the underlying granular layer, combined with the characteristic verrucous and rupioid clinical morphology, supports the diagnosis of a VP variant. This distinction is important because the diagnostic value of our case lies in its unique morphological features and unusual location, rather than adherence to a specific subtype distribution. The term “rupioid” refers to thick, conical, limpet-like, adherent scales with a distinctive heaped-up and peripherally adherent appearance resembling oyster shells. In our case, the crusts showed raised edges with central adherence, unlike the flat, lamellar scaling typical of hyperkeratotic VP. While rupioid morphology is typically associated with conditions such as rupioid psoriasis and secondary syphilis, the histopathological features in our case, including the presence of cornoid lamellae and the absence of psoriasiform epidermal hyperplasia or plasma cell infiltrates, allowed differentiation from these entities. Hypertrophic lichen planus (HLP) was considered but excluded by the absence of typical violaceous papules. Although a band-like lymphocytic infiltrate was present in our case, this finding can occasionally be observed in porokeratosis and is therefore not definitive. Histopathologically, HLP typically exhibits wedge-shaped hypergranulosis and saw-tooth acanthosis, whereas our case demonstrated characteristic cornoid lamellae with underlying loss of the granular layer and overall diminished granular layer, features that are not seen in HLP.⁷ Tuberculosis verrucosa cutis (TBVC) was ruled out due to the absence of tuberculous granulomas, caseation necrosis, or acid-fast bacilli.⁸ Reactive perforating collagenosis (RPC), although clinically plausible given the patient’s diabetes, was discounted by the lack of transepidermal elimination of collagen bundles.⁹ We also considered other differential diagnoses beyond the classical mimickers of rupioid lesions. Lichenoid keratosis, which can present as a solitary hyperkeratotic plaque, was excluded by the absence of prominent basal cell vacuolization and the characteristic band-like lymphocytic infiltrate with predominantly basal layer involvement, which are typical features of this entity. Furthermore, the presence of cornoid lamellae is not a feature of lichenoid keratosis, making this diagnosis unlikely.¹⁰ HIV-associated epidermodysplasia verruciformis was also considered, given the rupioid and verrucous appearance, but was ruled out by the patient’s negative HIV status, the localized clinical presentation, and the absence of large vacuolated keratinocytes in the upper epidermis.¹¹ Prurigo nodularis was excluded by the absence of compact orthokeratosis and vertical collagen streaking in the papillary dermis, features that are typical of this entity, whereas cornoid lamellae are not.¹² Hypertrophic squamous cell carcinoma and keratoacanthoma were excluded by the absence of atypical keratinocytes, mitotic figures, or invasive growth patterns.¹³ Deep fungal infections and chromoblastomycosis were ruled out by the absence of fungal elements on histopathological examination. Hypertrophic eczema, psoriasis, and hypertrophic lichen simplex chronicus were considered unlikely due to the absence of spongiosis, psoriasiform epidermal hyperplasia with suprapapillary thinning, or compact orthohyperkeratosis with prominent granular layer.¹⁴ The presence of cornoid lamellae remains the defining feature that distinguishes our case from these entities. We also carefully considered porokeratoma in the differential diagnosis. This rare benign neoplasm typically presents as a solitary keratotic nodule with a prominent cornoid lamella, usually without significant epidermal dysplasia or dermal inflammation.¹⁵ In contrast, our case showed marked verrucous and rupioid epidermal hyperplasia, extensive hyperkeratosis, dyskeratotic cells within the spinous layer, and a superficial band-like lymphocytic infiltrate with focal neutrophilic aggregates. These findings, together with the clinical course of multiple coalescing nodules progressing over six months, are more consistent with a porokeratosis variant than with porokeratoma.

Regarding the diagnostic approach to porokeratosis, several modalities are available. Clinical examination remains the first step, with characteristic presentations varying by subtype. Histopathological examination of a representative lesional biopsy remains the gold standard for diagnosis, in which the identification of cornoid lamellae plays a central role. The diagnosis in this case was based on three criteria. First, the clinical morphology showed well-defined verrucous and rupioid plaques with peripheral hyperkeratosis, consistent with a porokeratosis variant. Second, histopathological examination of multiple sections consistently revealed typical cornoid lamellae at the peripheral raised ridge of the lesion (Figure 1B and C), characterized by a thin vertical column of parakeratotic cells with abrupt loss of the underlying granular layer and focal dyskeratosis of adjacent keratinocytes. Third, other porokeratosis subtypes were excluded by clinical distribution, as the patient had no disseminated lesions, no photodistribution and no genitogluteal involvement. The lower leg is an unusual location for this condition, but it does not preclude the diagnosis. Alternative diagnoses were ruled out by histopathology as discussed above. Taken together, these findings support the diagnosis of a rupioid variant of porokeratosis. Although genetic testing was not performed in this case, testing for mevalonate pathway genes, including MVK, PMVK, MVD, and FDPS, has recently been increasingly used in research to characterize familial and sporadic cases and to explore genotype-phenotype correlations.^16,17 However, it is important to note that these genetic investigations are predominantly research oriented and are not currently mandated as part of the standard diagnostic workup in routine clinical practice. In the vast majority of reported porokeratosis cases, the diagnosis has been established based on characteristic clinical morphology and histopathological confirmation of cornoid lamellae, without the need for genetic testing. This is particularly true for solitary or localized lesions, which often represent sporadic cases with post-zygotic somatic mutations that may be difficult to detect due to mosaicism and tissue sampling limitations. Thus, while we acknowledge the value of genetic testing in advancing our understanding of porokeratosis, we maintain that the diagnosis in our case is sufficiently supported by the integration of characteristic clinical morphology, histopathological confirmation of cornoid lamellae, and exclusion of other clinical mimickers. The novelty of our case lies not in its genotype but in its unprecedented phenotypic presentation, namely the striking rupioid morphology on the lower leg, which expands the clinicopathological spectrum of porokeratosis regardless of the underlying genetic alteration.

The development of porokeratosis, particularly its disseminated or atypical forms, has been linked to immunosuppression.^18,19 It is also theoretically have contributed to the unusual phenotype in our case, although these remain speculative. Although the patient did not receive chemotherapy or other systemic immunosuppressive therapy, it is theoretically possible that a history of malignancy contributed to aberrant keratinocyte proliferation, possibly through chronic immune dysregulation or altered local cytokine milieu that may facilitate clonal expansion of atypical keratinocytes.²⁰ However, this remains speculative and cannot be confirmed in a single case. Similarly, the patient’s long-standing diabetes mellitus might have contributed to local microcirculatory disturbances and altered immune responses, but direct evidence supporting a causal role in the development of the rupioid morphology is lacking. These hypotheses cannot be confirmed in a single case and should be interpreted with caution.²¹ An incisional biopsy possible that this localized lesion represents a mosaic form of porokeratosis, where post-zygotic mutations in genes involved in keratinization (such as MVK, PMVK, etc.) led to this peculiar phenotype.^6,22 However, no genetic testing was performed in this patient, so this hypothesis remains entirely speculative and cannot be substantiated in this case.

There is no standardized treatment for porokeratosis. Topical and systemic medications, laser therapy, phototherapy, and surgical intervention are the main current treatment strategies.⁶ In this case, the treatment strategy distinguished between therapy directed at the porokeratosis itself and supportive measures for associated comorbidities. We chose oral isotretinoin (10 mg once daily) as a systemic agent to normalize keratinization. Although the evidence for systemic retinoids in porokeratosis is limited, several reports have documented their use in certain subtypes. Isotretinoin has been used in linear porokeratosis with good cosmetic effect, although complete resolution was not achieved, and has also shown favorable responses in porokeratosis palmaris, plantaris et disseminata.^20,23 In verrucous porokeratosis, its efficacy has been variable, with one report showing partial improvement (combined with topical 5-fluorouracil) and another demonstrating no response.²⁰ Given the refractory nature of our patient’s lesion and the lack of established treatment guidelines, isotretinoin was prescribed with careful monitoring. Topical compound flumetasone and polymyxin B ointments were used as adjunctive therapy for anti-inflammatory effects and secondary infection prevention. Given the patient’s diabetes, we incorporated adjunctive agents targeting microcirculation, including beraprost sodium (40 μg twice daily) to improve cutaneous blood flow in the setting of diabetes-associated microangiopathy, diosmin (0.45 g twice daily) for its venoactive and anti-inflammatory properties, and oral epinastine hydrochloride (10 mg nightly) to alleviate potential pruritus and inflammation. The combination regimen was well tolerated and associated with clinical improvement, as demonstrated by post-treatment photographs showing significant reductions in lesion thickness, crusting, and erythema (Figure 1D). At six-month follow-up, the clinical improvement was sustained, with complete resolution of pain and pruritus and marked decreases in both the number and size of the lesions. No recurrence was observed, and no significant abnormalities in liver or renal function or lipid profiles were detected on monthly monitoring. However, because multiple agents were administered simultaneously, improvement cannot be confidently attributed to isotretinoin or to any individual component. The simultaneous use of multiple treatments represents a limitation of this case report. This observation suggests that a combined approach targeting both keratinocyte dysplasia and diabetes-associated microangiopathy may be worth considering in similar cases, although further studies are needed to establish causality.

Conclusions

This case underscores the importance of considering porokeratosis in the differential diagnosis of atypical hyperkeratotic plaques, even when they occur at non-classic sites or exhibit unusual morphologies such as rupioid features. A low threshold for biopsy is crucial in such cases, as the histopathological identification of cornoid lamellae remains the characteristic diagnostic feature. However, clinicopathological correlation is equally essential, since cornoid lamellae can occasionally be encountered as a secondary phenomenon in other dermatoses.

Funding Statement

This work was supported by grants from the Zhejiang Province Medical and Health Science and Technology Plan Project (No. 2023XY199).

Data Sharing Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Ethics Approval

Institutional approval for the publication of anonymized case details was granted by the Ethics Committee of Zhuji People’s Hospital of Zhejiang Province. This study was performed in accordance with the Declaration of Helsinki.

Consent Statement

A written informed consent form has been obtained from the patient, consenting to the publication of anonymous patient information in this article.

Author Contributions

All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

Disclosure

The authors declare no conflicts of interest in this work.