PITYRIASIS RUBRA PILARIS
Pityriasis rubra pilaris (PRP) is an idiopathic rash that has many cutaneous manifestations. It is an uncommon entity that often manifests with nearerythroderma. There are several clinical variations of the condition, and it has a characteristic histological pattern, although this pattern is not always seen on microscopic examination.
Clinical Findings: PRP has a unique bimodal age of distribution, with an early onset of disease in the first 5 years of life and adult onset in the sixth decade. There is no gender or racial predilection. PRP tends to run a chronic course. It starts insidiously with small follicular, keratotic, pink to red papules. These papules have been described as “nutmeg grater” papules. The papules coalesce into larger patches and plaques. Eventually, large surface areas are involved, with a near-erythro-derma. Characteristic islands of sparing occur within the erythrodermic background. These islands of completely normal skin are usually small, a few centimeters in diameter, but they can be much larger. The islands typically have an angulated shape, and they are rarely perfectly round or oval. The palms and soles become thickened and yellowish. This is highly characteristic of PRP and is called “carnauba wax–like” palms and soles. Fissuring is very common within the keratoderma and can be a source of pain and a site for secondary infection.
PRP has historically been separated into five subtypes: classic adult, classic juvenile, atypical adult, atypical juvenile, and a circumscribed or localized form. The classic adult and classic juvenile forms were described earlier. They typically run a chronic clinical course, with most cases spontaneously resolving a few years after onset. Paraneoplastic variants of PRP have been described. Onset of the malignancy precedes the rash of PRP, and the patients seem to improve with treatment of the underlying tumor. This is a very rare clinical scenario. Patients with human immunodeficiency virus infection seem to be at a higher risk for developing PRP.
The differential diagnosis of classic forms of PRP includes psoriasis, drug rash, and cutaneous T-cell lymphoma. Skin biopsy and clinical pathological correlation help the clinician make a firm diagnosis.
Pathogenesis: The etiology is undetermined. Theories on the formation of PRP have centered on abnormal metabolism of vitamin A or an abnormal immune response to a foreign antigen. These theories have not been thoroughly studied or proven. The report of a familial form of PRP may shed some light on the etiology.
Histology: The pathognomonic histological finding in PRP is the appearance of alternating layers of parakeratosis and orthokeratosis, both in a vertical and a horizontal direction, lending the appearance of a chess board. This pattern is not always present, and sometimes it can be seen only with close inspection.
Treatment: Therapy for PRP is difficult. Many agents have been used with varying degrees of success. Topical corticosteroid wet wraps, oral retinoids, and ultraviolet therapy have long been used as first-line agents. The retinoids are considered first-line therapy, and both isotretinoin and acitretin have been used. Other immunosuppressants have been used, including methotrexate, azathioprine, and the newer anti tumor necrosis factor inhibitors. No randomized, placebocontrolled trials have been performed to date.