For six months, a single painful nodule on the right lower leg of an 85-year-old man in eastern China multiplied, and some of the bumps merged into plaques. By the time dermatologists examined him, the lesions were covered in thick, dark gray crusts that looked like oyster shells.
A biopsy pointed away from infection, cancer, and psoriasis. The diagnosis was verrucous porokeratosis, a rare variant of a group of disorders in which skin cells mature abnormally. His doctors at Zhuji People's Hospital in Zhejiang Province described the case in Clinical, Cosmetic and Investigational Dermatology on Sept. 14, writing that, to the best of their knowledge, it is the first reported case of rupioid porokeratosis on the lower leg.
A Shape Usually Linked to Psoriasis and Syphilis
Dermatologists use the word rupioid for thick, cone-shaped scales that cling tightly to the skin and resemble limpet or oyster shells. According to the authors, that appearance is typically associated with conditions such as rupioid psoriasis and secondary syphilis.
The man's blood tests for syphilis and HIV were negative. Under the microscope, a sample that included the raised edge of one nodule showed cornoid lamellae, thin vertical columns of abnormally retained cells cutting through the outer skin. That feature is the hallmark of porokeratosis, and the pathologists found it consistently across multiple sections. Because the clinical and tissue findings pointed away from infection, the team did not order microbiology cultures.
The doctors also worked through a long list of look-alikes. Based on what the tissue did and did not show, they excluded hypertrophic lichen planus, tuberculosis of the skin, reactive perforating collagenosis (a skin disorder that can occur in people with diabetes), deep fungal infection, prurigo nodularis, keratoacanthoma, and squamous cell carcinoma.
Verrucous porokeratosis itself is uncommon. It classically forms thick, wart-like plaques, and a 2023 review in the journal Metabolites notes that the variant is also known as genitogluteal porokeratosis. The case authors wrote that earlier reports have emphasized the palms and soles or the genital and buttock region, that involvement of the lower leg is exceptionally rare, and that none of the few earlier reports of porokeratosis at that site described this shell-like crust.
Diabetes, a Cancer History and Open Questions
The man had long-standing type 2 diabetes managed with oral medication. About a year before he came to the clinic, he had surgery to remove a rectal adenocarcinoma and received no additional treatment afterward.
Atypical forms of porokeratosis have been linked to a weakened immune system. The authors raised the possibility that his cancer history or diabetes-related circulation problems played a role in the unusual appearance, but they stressed that these ideas are speculative and cannot be confirmed in one patient.
No genetic testing was done, which the authors note is typical, because such testing is mainly a research tool rather than part of routine diagnosis. Research has tied porokeratosis to changes in genes of the mevalonate pathway, which cells use to make cholesterol and related molecules. A 2015 study in eLife of 134 patients found a mutation in at least one of four such genes, MVK, PMVK, MVD and FDPS, in 98% of familial cases and 73% of cases with no family history.
Six Months of Treatment, Several Drugs at Once
There is no standard treatment for porokeratosis. Options include topical and oral medications, light therapy, lasers and surgery. Evidence for oral retinoids in the verrucous form has been mixed, with one earlier report showing partial improvement and another showing none.
The team prescribed low-dose oral isotretinoin, 10 milligrams daily, to help normalize how skin cells mature. They added a topical steroid-antibiotic ointment and, because of his diabetes, medications aimed at improving small-vessel circulation, plus an oral antihistamine to ease itching.
After six months, the crusting, thickness, and redness had markedly decreased. His pain and itching were gone, the lesions were fewer and smaller, and monthly blood tests showed no significant liver, kidney, or cholesterol problems.
The authors were candid about the limitation. Because several drugs were started together, the improvement cannot be credited to isotretinoin or any single component.
Why a Stubborn Crusted Spot Deserves a Biopsy
Porokeratosis is more than a cosmetic problem. A Swedish nationwide registry study that identified 2,277 people diagnosed with the condition between 2001 and 2020 found higher skin cancer rates than in matched controls, with hazard ratios of 4.3 for squamous cell carcinoma, 2.42 for basal cell carcinoma, and 1.83 for melanoma.
A 2024 review in the journal Medicina describes porokeratosis as a precancerous condition, with squamous cell carcinoma the most commonly reported cancer. These findings show links, not proof that porokeratosis itself causes cancer, and most people with the condition will not develop skin cancer.
The Chinese case does not change treatment guidance. Its lesson, the authors wrote, is to consider porokeratosis for atypical thickened plaques, even in unusual places, and to keep a low threshold for biopsy. Anyone with a crusted, scaly, or wart-like skin lesion that keeps growing or will not heal should have it checked by a dermatologist.
Key Questions Answered
What is porokeratosis?
It is a group of skin disorders in which skin cells mature abnormally, forming scaly or thickened patches. It is diagnosed by finding cornoid lamellae on biopsy.
What made this case unusual?
The lesions on the man's lower leg had thick, oyster-shell-like crusts. The authors say rupioid porokeratosis had not been reported on the lower leg before, to the best of their knowledge.
Could the crusts have been cancer or infection?
Doctors considered those possibilities. Biopsy findings and negative syphilis and HIV tests pointed instead to verrucous porokeratosis.
Did isotretinoin cure it?
His lesions improved markedly over six months, but he took several drugs at once, so the benefit cannot be credited to isotretinoin alone.
Is porokeratosis linked to skin cancer?
A large Swedish registry study found higher skin cancer rates in people with porokeratosis, especially squamous cell carcinoma. The studies show an association, not proof of cause.
Published by Medicaldaily.com
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