Of the 34 patients studied, 15 were males and 19 were females. Their ages ranged from 12 to 55 years (mean=35 years). The majority of the lesions were located on the trunk. In 26% of the cases, the secondary eruptions were preceded by a herald patch. In 76 % (26/34) of the cases, a definite diagnosis of PR was given based on history and clinical findings without any differential diagnosis. A minority of cases (8/34) had a primary clinical diagnosis of PR but other differential diagnoses were considered such as eruptive psoriasis, parapsoriasis, viral exanthem, and tinea infection. The average duration from the onset of the eruption to resolution was 31 days. None of the eruptions exceeded a duration of 8 weeks. All of the cases responded well to conservative management with oral antihistamines and emollients.

The histopathological findings of pityriasis rosea are summarized in Table 2. The table also includes data of 26 cases of seborrheic dermatitis and 28 cases of allergic contact dermatitis for comparison.

The majority of the sections showed a perivascular pattern in the dermis with multi-focal spongiosis (82.35%) in the epidermis. The infiltrates were located superficially in the papillary dermis with some extension into the mid-dermis and the interstitium. The "mounds of parakeratosis" were very characteristic in a majority of sections examined (88.24%) and differed from the confluent and peri-infundibular parakeratosis commonly observed in allergic contact dermatitis and seborrheic dermatitis, respectively. Lymphocytes in the epidermis were present often (82.35%). "Simulators of Pautrier's microabscesses" were present in 50% of the cases of PR but not in seborrheic dermatitis and in only few cases of allergic contact dermatitis (17.86%). Other prominent epidermal findings included a mild to moderate acanthosis (73.53%) and a normal granular layer (85.89%). Very few necrotic keratinocytes (20.59%) and red blood cells (20.41%) were seen in the epidermis with no considerable difference from those seen in seborrheic dermatitis and allergic contact dermatitis. There was mild dermal edema with few extravasated red blood cells and very few pigment-laden macrophages. The perivascular infiltrate consisted mostly of lymphocytes with very few histiocytes. No periadnexal extension was observed in the sections examined.

Photomicrographs of histopathologic findings are shown in Figs. 1- 5.

CD1a stain demonstrated multifocal positivity in areas of spongiosis (100%) with increased density beneath the parakeratotic mounds. Dermal staining was likewise observed in 32/34 (94%) of cases and comprised 10-30% of the perivascular infiltrate with very sparse interstitial distribution. Fifty to 75% of the cells which constitute the spongiotic microvesicles stained positively for CD1a and were observed in 12/34 (35%) of cases.

Forty to 60% of the perivascular infiltrate stained for the T-cell marker CD3, where there is variable density ratio between CD8-positive lymphocytes (20-80%) and CD4 lymphocytes (20-80%). However, the lymphocytes within the epidermis were observed to stain positively for CD8 in 75% of cases. CD20 stained less than 10 lymphocytes/hpf in the perivascular infiltrate. CD68 stained 10-50% of the cells within and surrounding the spongiotic microvesicle and positive staining was detected in less than 5% of the perivascular and interstitial infiltrates. Less than 5 cells/hpf in the perivascular infiltrate stain for the plasma cell marker CD138.

There was scattered Ki67 expression in the basal layer of the epidermis with increased density of staining in the suprabasal layers in areas of spongiosis, beneath parakeratotic mounds, and just below the spongiotic microvesicles in 16/34 (47%) of the cases with staining of occasional mitotic figures. Scattered bcl2 expression was observed in 22/34 (65%) of cases, in the basal layer and lower half of the epidermis and it was prominent within the focal areas of spongiosis, staining some cells within the spongiotic microvesicle. There was diffuse (29%) and multifocal (70.5%) cytokeratin 16 expression in the upper half of the Malphigian layer with more pronounced expression beneath parakeratotic mounds.

Immunohistochemical results are pictured in figs. 6- 12.