A 24-year-old Black woman at 20 weeks’ gestation presents with a pruritic rash that began 5 weeks prior. Examination shows multiple flat, oval, erythematous patches with a collarette of fine scale diffusely distributed on the chest, abdomen, back, and buttocks as well as the bilateral arms and legs. The rash spares the palms, soles, and face. She has no chronic medical conditions or drug allergies and is taking daily prenatal vitamins. The patient denies a history of skin disorders or skin cancer. Periodic acid–Schiff (PAS) stain of a 4mm punch biopsy performed on the mid back is negative.
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Pityriasis rosea (PR) is a common self-limited disease that is not associated with long-term sequelae.1 The peak age of onset is between 10 and 35 years of age, and the disease is found worldwide and affects all races.1 Pityriasis rosea is hypothesized to have an infectious etiology based on evidence of viral exanthem associated with reactivation of Human herpesvirus 6 (HHV-6) and/or HHV-7; both of which are Roseolovirus and closely related betaherpesvirinae.2
Pityriasis rosea manifests as an acute exanthematous eruption, initially presenting with a single 3- to 5-cm oval plaque on the trunk with a collarette scaling inside the periphery, known as a herald patch.1 Exanthem occurs 1 to 2 weeks after the initial plaque and is a generalized eruption of similar appearing but smaller lesions. The rash develops most prominently on the trunk and proximal extremities and its pattern is described as resembling a Christmas tree given its diagonally oriented distribution along the lines of skin cleavage.2 The eruption may be preceded by various prodromal symptoms such as malaise, nausea, headache, gastrointestinal, and/or upper respiratory symptoms.3
The diagnosis of PR is made clinically and is rather straightforward in classic-appearing cases. The various atypical presentations, however, can pose a diagnostic challenge. Routine blood tests are typically normal and nonspecific. The histologic features are likewise nonspecific. When a biopsy is needed to make a definitive diagnosis, it may show patchy or diffuse parakeratosis, absence of granular layer, slight acanthosis, focal spongiosis, and microscopic vesicles. Occasional dyskeratotic cells with an eosinophilic, homogenous appearance may be noted. In the dermis, there is typically edema and perivascular infiltration of mononuclear cells 1,2
Conditions considered in the differential diagnosis of PR include various other papulosquamous disorders such as nummular eczema, guttate psoriasis, lichen planus, pityriasis lichenoides, tinea corporis, parapsoriasis, and seborrheic dermatitis.1 Dermatophyte infection is often difficult to distinguish from the herald patch of PR, making mycologic investigations such as potassium hydroxide (KOH) examination necessary to differentiate between the 2 disorders. Pruritic urticarial papules and plaques of pregnancy (PUPPP, also known as polymorphic eruption of pregnancy) is another disease to consider if the patient is pregnant. Typical lesions of PUPPP are 1- to 2-mm erythematous urticarial papules surrounded by a narrow pale halo. The eruption begins on the abdomen, classically within the striae gravidarum, and demonstrates periumbilical sparing.4 Pruritus will generally parallel the eruption and rapidly spread to the thighs, buttocks, breasts, and arms.4
Since PR is self-limited, it does not require treatment in many cases.1 Supportive treatment such as oral antihistamines and/or topical corticosteroids may help relieve pruritus. For severe cases, acyclovir may hasten recovery and lessen symptoms.2 Ultraviolet B phototherapy or natural sunlight exposure in the first week of eruption is also reported to have some benefits.5 A short course of systemic glucocorticoids is another option for severe cases.2
Dominique Jacobs, BS, is a fourth-year medical student and aspiring dermatologist attending the Philadelphia College of Osteopathic Medicine. Katrina Hansen, DO, is the incoming dermatology chief resident at Beaumont Farmington Hills in Michigan. Her interests include medical and surgical dermatology.
1. Clark M, Gudjonsson JE. Pityriasis rosea. In: Kang S, Amagai M, Bruckner AL, et al. eds. Fitzpatrick’s Dermatology, 9th ed. McGraw-Hill; 2019. Accessed May 19, 2021.
2. Blauvelt A. Pityriasis rosea. In: Goldsmith LA, Katz SI, Gilchrest BA, Paller AS, Leffell DJ, Wolff K. eds. Fitzpatrick’s Dermatology in General Medicine, 8th ed . McGraw-Hill; 2012. Accessed May 19, 2021.
3. Drago F, Ciccarese G, Broccolo F, Cozzani E, Parodi A. Pityriasis rosea in children: clinical features and laboratory investigations. Dermatology. 2015;231(1):9-14. doi:10.1159/000381285
4. Wiznia LE, Pomeranz M. Skin changes and diseases in pregnancy. In: Kang S, Amagai M, Bruckner AL, et al. eds. Fitzpatrick’s Dermatology, 9th ed. McGraw-Hill; 2019. Accessed May 19, 2021.