CASE #1: Pityriasis alba
Pityriasis alba (PA) is best thought of as an eruption usually related to eczema that results in hypopigmentation, particularly in children.1-3 Not all cases are associated with atopic dermatitis. PA is more apparent in dark-skinned patients; moreover, this condition’s prevalence appears to be higher among racial groups with darker skin. Some studies have related the incidence of PA and atopy, amount of sun exposure, lack of sunscreen use, and frequency of bathing. Studies suggest that its prevalence in children is 2%-5%. Other names for PA include erythema streptogenes, pityriasis streptogenes, and impetigo furfuracea.
PA usually manifests on the face and can be a single lesion or several lesions. Body lesions, particularly on the arms, sometimes accompany facial lesions. PA can be either extensive or generalized, which is more common in adults.4 PA also has an uncommon clinical pattern that manifests in a generalized distribution in psoriatic locations (e.g., the elbows and sacrum).5 Pigmenting PA seems to be a variant of classic disease, showing a strong association with dermatophyte infection, especially tinea capitis.6 PA may be related to lichenoid melanodermatitis and can overlap with such other conditions of hypopigmentation as leprosy or the pigmentation alteration form of molluscum.
The basis for PA is not fully understood but it is thought to be related to underlying skin melanin and UV-light exposure.7 The observation that darker-skinned individuals have a higher incidence of PA is underscored by the fact that most cases of atopic dermatitis are not accompanied by PA. Potential causes of this condition include xerosis, frequent bathing, hot baths, sun exposure, wind, and soap. A retrospective study found that patients with PA were exposed to more sunlight than healthy patients and did not use sunscreen regularly. Staphylococcus aureus is not an important etiopathogenic factor in PA.
A number of entities mimic PA. In Western countries, the most important diagnostic mimickers include vitiligo, nevus depigmentosus, pigmentary alteration (e.g., after tinea or molluscum), and hypopigmented seborrheic dermatitis. In India, indeterminate leprosy is an important entity to rule out. The most concerning entity on the differential diagnosis is hypopigmented mycosis fungoides, as it is a neoplastic process. With that being said, the vast majority of hypopigmented mycosis fungoides cases in the United States do not process to a systemic disease. Progressive macular hypomelanosis (PMH) is another entity on the differential diagnosis, although some think that PA and PMH are the same entity.8 PMH manifests with ill-defined nummular, nonscaly hypopigmented spots on the trunk, often confluent in and around the midline and sometimes extending to the proximal extremities and neck/head region. Both PA and PMH manifest with no itch, pain, or preceding inflammation. Pityriasis versicolor alba is a hypopigmented or depigmented variant of pityriasis versicolor, characterized by maculous, partly pityriasiform, scaly depigmented lesions occurring particularly in seborrheic areas.9 Some medications can cause an eruption that resembles PA. Finally, low serum levels of copper might have an association with PA.
The histology of PA is not extremely distinctive. There is pigment in the epidermis of lesional skin, but no significant difference in melanocyte count was found between areas affected by PA and normal skin. In skin affected by PA, ultrastructurally, degenerative changes in melanocytes and a reduced population of melanosomes within keratinocytes manifest. There is no detectable defect in melanosomal transfer to keratinocytes. Hyperkeratosis and parakeratosis are variable, as are intercellular edema and intracytoplasmic lipid droplets.
The best treatment for PA has yet to be defined. Literature supports the use of a calineurin inhibitor, such as tacrolimus or pimecrolimus, as these medications have antinflammatory properties and do not result in hypopigmentation. Topical corticosteroids can be used as well, but these can depigment the skin and must be used carefully. Extensive cases of PA can be treated with narrow-band UVB laser or oral methoxsalen UVA photochemotherapy.10 Side effects can arise from even mild topical treatments. Molluscum contagiosum infection associated with pimecrolimus use in pityriasis alba has been noted.11
In this patient, the clinicians decided to observe, as the rash was not serious. The boy was subsequently lost to follow-up.