Pityriasis rosea is a common idiopathic rash with a characteristic onset and distribution. It is a self-limited rash that spontaneously resolves within a few months. A few distinct clinical variants have been described. The main goal in treatment is to differentiate pityriasis rosea from other rashes that can have a similar clinical picture.
Clinical Findings: Pityriasis rosea is a common rash of young adults and children. It has no racial predilection. It is most often seen during the spring and fall months. Clustering of cases has been reported. A small but significant subset of patients have had a preceding upper respiratory tract infection. This has led some to search for a viral cause of the rash, although none have been found. The rash of pityriasis rosea can have a varying morphology, but it most commonly begins with a herald patch. The herald patch, or mother patch, is the first noticeable skin lesion. It typically precedes the entire outbreak of pityriasis rosea by a few days. The herald patch is a 2- to 4-cm, pink-red patch with fine adherent scale that commonly occurs on the trunk. After a few days, smaller, oval-shaped patches 0.5 to 1 cm in diameter begin appearing on the trunk and extremities. The rash follows the skin tension lines and has a peculiar “fir tree” pattern. This pattern mimics the down-sloping branches of a fir tree. The rash typically spares the face and glabrous skin.
Patients may complain of mild to moderate pruritus, but most are asymptomatic. The main differential diagnosis includes guttate psoriasis and, in cases that affect the palms and soles, secondary syphilis. Pityriasis rosea is a self-limited, spontaneously resolving rash. It typically does not last longer than 2 to 3 months. Guttate psoriasis usually begins after a streptococcal infection and does not exhibit a herald patch. The teardrop shaped patches of guttate psoriasis also do not follow the skin tension lines, and this fact can be used to differentiate the two. Tinea corporis is almost always in the differential diagnosis of any rash that has a patch type morphology and fine surface scale. Tinea corporis can be easily diagnosed with a microscopic evaluation of a small scraping of the skin. Widespread tinea is almost always associated with onychomycosis, and it is more commonly seen in patients who are taking chronic immunosuppressive agents or using topical steroids. These traits can be used to help differentiate the two conditions. The rash of secondary syphilis is the great mimicker. Any patient who has pityriasis rosea that affects the palms and or soles should be tested for syphilis.
A few unique variants of pityriasis rosea exist. One is papular pityriasis rosea. This form more commonly affects school-aged children with Fitzpatrick type IV, V, or VI skin. This version tends to be a bit more wide spread and more pruritic. Instead of small, oval-shaped patches, this variant consists of small (0.5 cm) papules that have a small amount of surface scale. It runs the same benign course, with self-resolution after a few weeks to months. On healing, postinflammatory hyper-pigmentation or hypopigmentation may result and may persist for several months.
Histology: A superficial and deep lymphocytic and histiocytic infiltrate is seen surrounding the vessels of the dermis. Varying amounts of extravasated red blood cells are appreciated within the upper dermis. The stratum corneum shows varying degrees of acanthosis and parakeratosis.
Pathogenesis: Many attempts to isolate a viral or a bacterial element in patients with pityriasis rosea have been met with frustration. To date, no infectious cause has been determined. The true nature and cause of pityriasis rosea remain elusive.
Treatment: No therapy is needed. Most cases are asymptomatic and mild. Pruritus can be treated with oral antihistamines and adjunctive topical steroids. The use of oral erythromycin, twice a day for 2 weeks, was shown to decrease the duration of the rash. Ultraviolet therapy is very helpful in treating the rash and pruritus. If there is any consideration for syphilis in the history or the physical examination, a rapid plasma regain (RPR) blood test should be performed.