Parapsoriasis is a rare and diverse group of skin disorders characterized by chronic, scaly patches. While its exact definition is unclear due to the condition’s varied clinical appearance and nonspecific findings under a microscope, it is known to primarily affect middle-aged adults, especially men. 

Etiology and Pathophysiology

The exact cause of parapsoriasis is not fully understood, but both environmental and genetic factors are thought to play a role. It is believed that small plaque parapsoriasis (SPP) and large plaque parapsoriasis (LPP) may represent different stages in the development of cutaneous T-cell lymphoma, which ranges from chronic skin inflammation (dermatitis) to lymphoma. While some cases of LPP show abnormal T-cells, it has not yet been proven that this increases the risk of cancer.

In parapsoriasis, the immune system’s T-cells become dysregulated, leading to abnormal skin growth and the presence of unusual lymphocytes (a type of white blood cell) in the skin. In LPP, there is significant lymphocyte buildup in the deeper layers of the skin, along with changes in the skin’s basal layer and a mix of skin color changes, known as poikiloderma. Unlike mycosis fungoides, LPP usually does not show Pautrier microabscesses, which makes diagnosis more difficult.

Clinical Presentation

• Small Plaque Parapsoriasis (SPP): SPP typically appears as small, pink to yellow-brown patches or spots, usually between 2 and 5 cm in size. These patches are often round or oval and have mild scaling. They are most commonly found on the trunk, sides, and upper limbs, and are generally not painful or itchy. Under a microscope, the skin shows mild swelling, uneven growth of skin layers, and small lymphocytes (a type of white blood cell). A variation of SPP, called digitate dermatosis, presents as longer patches with a thin, paper-like surface, usually along the skin folds on the flanks.
• Large Plaque Parapsoriasis (LPP): LPP presents as larger, irregularly shaped patches that are usually dusky red or brown, with fine scaling. These patches are commonly found on areas of the body not exposed to the sun, such as the buttocks, thighs, lower back, and underarms or breasts. These lesions may lead to thinning of the skin. The skin changes are more noticeable and include thickened skin, irregular pigmentation, and significant infiltration of lymphocytes. The absence of Pautrier microabscesses (which are seen in a related condition, mycosis fungoides) can suggest LPP, though this isn’t always a conclusive diagnosis.

Diagnosis

To diagnose parapsoriasis, both a clinical evaluation and a skin biopsy are needed. A punch biopsy is often preferred because it allows the doctor to examine the full thickness of the skin and gather the necessary samples for diagnosis. Since parapsoriasis lesions can look different, doctors may recommend taking biopsies from several areas of the skin. In cases of large plaque parapsoriasis (LPP), repeated biopsies may be needed over time to check for any changes or progression to cancer. Dermatologists also consider other possible conditions, such as eczema, psoriasis, or other skin disorders involving abnormal growth of lymphocytes, when assessing suspicious skin lesions.

Treatment Approaches

The treatment of parapsoriasis aims to manage symptoms, control skin inflammation, and prevent progression, especially in cases of LPP, where there is a risk of developing cancer. The approach varies depending on whether the condition is SPP or LPP, and based on the severity of the condition.

• Treatment for SPP: The first-line treatment for SPP often involves using moderate to high-potency topical corticosteroids, such as clobetasol propionate, for 8 to 12 weeks. This treatment helps reduce inflammation and control the lesions. If there is no significant improvement (at least 50%), phototherapy may be considered. This typically involves ultraviolet B (UVB) therapy 2-3 times a week and is especially useful when lesions are widespread or corticosteroids do not fully control the disease.
• Treatment for LPP: LPP usually requires a more aggressive treatment due to the potential for malignancy. High-potency or super-high-potency topical corticosteroids like clobetasol are used for 12 weeks to reduce inflammation and flatten the lesions. If the lesions are extensive or do not respond to topical treatments, phototherapy with UVB or psoralen combined with ultraviolet A (PUVA) therapy is introduced early in the treatment, usually 2-3 times per week. In some cases, systemic therapies like oral retinoids or immunosuppressants may be considered, especially for severe cases or those at high risk of progressing to cutaneous T-cell lymphoma (CTCL).

Follow-up and Prognosis

For SPP,  follow-up visits are typically needed once a year to monitor for any changes or recurrence. However, patients with LPP require more frequent follow-up, usually every 6 months, because of the risk that it could progress to a more serious form, such as mycosis fungoides. Regular check-ups with a dermatologist are important to catch any early signs of malignancy, as early detection can lead to more effective treatment.

The outlook for SPP is generally good, as it is a benign, chronic condition. In contrast, LPP carries a higher risk of developing into CTCL, with about 10% of people with LPP progressing to mycosis fungoides over time. However, with early and appropriate treatment, the risk of this transformation can be reduced and the condition can be managed more effectively.

Conclusion

Parapsoriasis is a complex and rare skin disorder that presents in two forms, small plaque parapsoriasis (SPP) and large plaque parapsoriasis (LPP). While SPP is a benign, chronic condition, LPP has a higher risk of progressing to cutaneous T-cell lymphoma. Treatment strategies are based on the subtype and severity of the disease, with topical corticosteroids and phototherapy being the cornerstone of management. Close monitoring, particularly for LPP, is essential to detect and treat potential malignant transformation early. Ongoing research is needed to further elucidate the pathophysiology and optimize treatment options for this challenging condition.

References

1. Gniadecki, R., & Vestergaard, C. (2018). Parapsoriasis: A review of the clinical presentation, diagnosis, and treatment. Journal of the American Academy of Dermatology, 78(1), 33-39. https://doi.org/10.1016/j.jaad.2017.06.037
2. Stern, R. S. (2020). Parapsoriasis: An overview of diagnosis, clinical course, and management. Dermatologic Clinics, 38(3), 391-398. https://doi.org/10.1016/j.det.2020.03.005