A Case of Palmoplantar Porokeratosis With Hypokeratosis: A New Subtype of Porokeratosis?

To the Editor: Porokeratosis (PK) is a primary disorder of the epidermis that is characterized by annular plaques with an atrophic center and hyperkeratotic edges.1 According to the clinical manifestations, 6 clinical types of PK are currently known: PK of Mibelli, actinic PK, disseminated superficial actinic PK, linear PK, punctate PK, and genitogluteal PK. Here, we describe a case of PK in an elderly woman who presented with hypokeratosis, which was indicative of circumscribed palmar or plantar hypokeratosis (CPH). CPH was first described and named by Pérez et al2 in 2002, and several cases were later reported. However, to the best of our knowledge, only one other case of PK concomitant with CPH has been reported before.3 Therefore, the findings of this case would be valuable for the differential diagnosis of similar cases in the future. A 74-year-old woman presented with lesions on the thenar area of the right palm that had first appeared 9 years ago and had gradually increased in size due to repeated scratching and irritation, and it was occasionally accompanied by itching. However, she had not sought treatment at the time. She did not have a history of chronic diseases, such as diabetes and hypertension. A cutaneous examination revealed a well-defined, reddish, annular plaque about 2 × 3 cm in size, with hyperkeratotic edges (Fig. 1). Initial histopathological examination of a paraffin-embedded section showed an abrupt decrease in the thickness of the stratum corneum that resembled a vertical knife cut and had a reduced granular layer below. Dyskeratotic cells were observed at the junction of the 2 layers. The spinous layer was thickened and extended into the dermis. The superficial dermis contained a sparse, perivascular, predominantly lymphocytic infiltrate (Fig. 2). Further examination of a deeper section revealed the presence of an incomplete cornoid lamella above the aforementioned dyskeratotic cells that was located in the area where the stratum corneum was reduced. On the basis of the histopathological findings, a definite diagnosis of PK was made.FIGURE 1.: Clinical presentation. A pale red elliptical plaque on thenar eminence of the right hand, characterized by a well-defined border, mildly elevated margins, and central depression.FIGURE 2.: Pathological features. An abrupt decrease in the thickness of the stratum corneum, resembling a vertical knife cut, with reduced granular layer below. Dyskeratotic cells were observed at the junction. The spinous layer was thickened, extending downward into the dermis. A few inflammatory cells infiltrated around superficial dermal blood vessels (A, H&E, ×20). An abrupt decrease in the thickness of the stratum corneum, with absence of the granular layer. Dyskeratotic cells were observed at the junction (B, H&E, ×40). The deeper pathological examination demonstrated an abrupt thinning of the stratum corneum and a reduction in the underlying granular layer at the site of the skin lesion. At the junction and within the area of decreased cornification, 2 incomplete cornoid lamellae were observed inserting into the epidermis, accompanied by a cluster of dyskeratotic cells below (C, H&E, ×40). The deeper pathological examination revealed the insertion of cornoid lamellae into the epidermis, with underlying dyskeratotic cells, absence of the granular layer, and vacuolar degeneration (D, H&E, ×100).While the patient presented with the characteristic histopathological findings of PK: cone-shaped lamellar parakeratosis columns extends throughout the stratum corneum, the underlying granular layer is either absent or markedly reduced, and vacuolar degeneration. Furthermore, a distinct feature of this case was reduced keratinization. It should be noted that in the initial pathological examination, typical cornoid lamellae were not observed, and only abrupt thinning of the stratum corneum and a reduction in the underlying granular layer were observed. This presentation closely resembled that of CPH. However, the clustering of dyskeratotic cells, the presence of epidermal hyperplasia beneath the reduced keratinization layer, the thickening of the spinous layer, and more pronounced infiltration of inflammatory cells in the superficial dermis were less consistent with the features of CPH. In addition, the lesion clinically presented as a central depression on a patch, rather than a simple concave shape. Based on these findings, we performed a deeper sectioning of the original paraffin block and ultimately confirming the diagnosis. Based on Groysman et al's4 classification of CPH, it may be appropriate to diagnose pseudo-CPH concurrently, which refers to secondary keratinization abnormalities resulting from trauma and certain skin diseases, such as traumatic poroma, dermatofibroma, common warts, and squamous cell carcinoma. In the one other case of PK occurring with hypokeratosis reported previously by Kim et al,3 the clinical and histopathological manifestations were consistent with the patient's manifestations. However, in their case, the lesions were smaller, and the hyperplasia was not as pronounced. Given the rarity of these cases, the connection between PK and hypokeratosis is unclear. Some viewpoints suggest that CPH is associated with localized developmental abnormalities of the epidermis and clonal proliferation of genetically abnormal keratinocytes.5 This mechanism shares similarities with the pathogenesis of PK, indicating a potential common pathway or link between the 2 conditions. Accordingly, Blanco-Barrios et al6 also proposed a possible association between PK and CPH. This type of isolated palmoplantar PK does not correspond to any of the existing 6 clinical types, and the coexistence of localized hypokeratosis in the histopathological findings may represent a distinct variant. Further observation and research are needed to explore the current classification of PK, as well as the pathogenic mechanisms of and the relationship between PK and hypokeratosis in patients with this presentation. This case provides valuable insights into the histopathological features of CPH and demonstrates that its features can be similar to those of palmoplantar PK. Therefore, when clustered dyskeratotic cells, parakeratosis, and inflammatory cell infiltration are observed, it is advisable to section multiple areas and search for evidence of PK.