Actinic prurigo (AP) is a rare form of an idiopathic photodermatosis that primarily affects sun-exposed areas of the skin. The affected regions of the skin typically include the face, neck, and dorsal surface of the upper extremities. Sun protected areas of the skin such as the buttocks have also been described. Actinic prurigo typically manifests in the spring as a symmetric intensely pruritic papulonodular dermatitis and can persist into the winter months. In severe cases, excoriations, cheilitis, conjunctival disease, and scarring may develop. Actinic prurigo is typically described in prepubescent females but can occur at any age or gender. The disease has a strong genetic component and is more commonly seen in American Indians of North, Central, and South America. Diagnosis is mainly clinical. Disease management begins with sun protection and sunlight avoidance. Treatment involves topical antihistamines and steroids, photochemotherapy (PUVA), and systemic therapies for severe cases. Without treatment, this disease course remains chronic and can persist into adulthood.
The etiology remains unclear, but there is a strong genetic component suggesting an autoimmune basis for actinic prurigo. The human leukocyte antigen DR4 allele variant is present in 90% of cases, especially the DR4 subtype DRB1*0407 which is seen in 60% of cases.
Actinic prurigo is a rare photodermatosis in the United States. However, actinic prurigo is more common in the American Indian populations of North, Central, and South America. The disease can affect both genders but is more commonly described in female patients. Actinic prurigo can manifest at any age, although the disease typically presents in prepubescent individuals with the mean onset before age 10. Some studies suggest an increased prevalence of actinic prurigo in certain geographic areas with dry, warm climates at an altitude of at least 1000 meters above sea level.
The pathophysiology of actinic prurigo remains largely unclear, but evidence suggests the disease process is driven by a delayed type-IV hypersensitive response to ultraviolet A and B (UVA and UVB) radiation in genetically predisposed individuals. Both TH1 and TH2 lymphocytic processes have been implicated in the disease process. Multiple studies note the presence of eosinophils and mast cells in the dermal layers of involved tissue which suggests a type-IVb (TH2)-driven response specifically. TH2 lymphocytes secrete IL-4, IL-5, and IL-13 which promote IgE and IgG4 production by B cells. These immunoglobulins stimulate eosinophils and mast cells. Individuals with moderate to severe actinic prurigo have been found to have markedly elevated IgE levels, further supporting a type-IVb (TH2) hypersensitive reaction.
Biopsy alone is not sufficient to make a diagnosis of actinic prurigo, but histopathology can be helpful in aiding diagnosis and excluding other disease processes with similar presentations. Skin biopsies will show hyperkeratosis, spongiosis, and acanthosis in the epidermis with lymphocytic perivascular infiltration in the dermis. A lip biopsy will show the presence of lymphoid germinal centers in the lamina propria, which can help distinguish actinic prurigo from the polymorphous light eruption. Studies have also described an infiltration of eosinophils and mast cells in the underlying mucosa of affected lesions which correlate with a delayed type-IVb hypersensitive response.
Patients with actinic prurigo will typically present in early spring as sunlight becomes more prevalent and individuals spend more time exposing their skin to sunlight outdoors. Patients will present with intensely itchy, erythematous papulonodular lesions on predominantly sun-exposed areas of the skin. The face, especially over the zygomatic arches, nasal bridge, lower lip, and conjunctiva, is typically affected; although, lesions may develop anywhere on the neck, trunk, extremities, and even non-sun exposed areas such as the buttocks. Since the lesions are intensely pruritic, excoriations, skin thickening, scarring, and hyper- or hypopigmentation may occur.
Actinic cheilitis has been described as a hallmark finding in patients with actinic prurigo. Individuals may present with actinic cheilitis as a solitary finding or in concurrence with more widespread papulonodular lesions.
Actinic prurigo is typically diagnosed clinically with a detailed history and physical exam. Patients will report the characteristic symptoms of severely pruritic papulonodular erythematous skin lesions that began in the spring or summer months. Patients may or may not report a direct correlation with exposure to sunlight and disease onset. Diagnosis can be aided by skin photo testing, histologic evaluation, and genetic screening for the HLA DR4 allele variant. Laboratory testing and immunofluorescence studies are often performed to rule out lupus erythematosus and other photosensitive dermatoses.
Minor cases of actinic prurigo can be treated with sun-avoidance alone. Proper sun protection includes avoiding sunlight by staying indoors or in shaded areas, wearing protective clothing, sunglasses, and wide brim hats, and using a broad-spectrum sunscreen. Topical steroids and non-sedating antihistamines will provide relief for acute episodes. The more severe and persistent disease requires treatment with systemic therapies such as antimalarials, tetracyclines, and systemic steroids.
Thalidomide has been described as the hallmark therapy for severe refractory cases of actinic prurigo. Treatment with thalidomide has a major limiting side effect profile which includes peripheral neuropathy and teratogenicity. Treatment with thalidomide is contraindicated during pregnancy or in individuals trying to become pregnant. Females of childbearing age must use contraception during therapy with thalidomide. Screening for peripheral neuropathy is typically initiated pre-treatment and continued throughout the treatment process.
Alternative immunosuppressive regimens with agents like cyclosporine A have been described as successful as well. Photochemotherapy with psoralen and ultraviolet A (PUVA) has also been shown to be successful in managing symptoms and treating skin changes caused by actinic prurigo.
Originally, actinic prurigo was thought to be a hereditary type of polymorphous light eruption (PMLE), a more common idiopathic photodermatoses. Genetic testing and the unique clinical presentation of actinic prurigo now supports two separate disease entities. The HLA-DR4 allele, specifically the DRB1*0407 subtype is strongly associated with actinic prurigo and not PMLE. Actinic prurigo also has an earlier age of onset and frequently presents with cheilitis and sometimes conjunctivitis, which is never seen in patients with PMLE. When suspecting actinic prurigo, laboratory tests should also be performed to rule out systemic diseases such as lupus erythematosus or porphyria.
Actinic prurigo is a chronic disease process and can reoccur with repeated sun exposure. Treatment with immunosuppressive agents like thalidomide and cyclosporine A have been shown to be effective at long-term suppression of symptoms. Some adolescents may have spontaneous resolution of symptoms and may not have disease progression into adulthood.
Actinic prurigo does not resolve with time and is marked by frequent relapses. Hence the role of the pharmacist and nurse are critical. All patients must be told to avoid sun exposure. Even going out during the day can cause a relapse of the condition within minutes. Thus, the patient should be educated by the clinician, nurse or pharmacist about wearing long dark sleeved garments, a hat, and sunglasses. No part of the skin should be exposed to the sun and participation in outdoor sports is not recommended. The use of sunscreens is not recommended as they contain chemicals that can exacerbate the skin lesions. Only mineral sunscreens can be used. To avoid exacerbations, the sunscreen must also be applied properly. Before going out, the patient must be aware of the UV index and weather. This skin condition is very labile and even minor exposure to the sun can lead to an outbreak. (Level III)
For most patients with actinic prurigo, the outcomes are poor. The condition is associated with frequent relapses, especially during the warmer months of the year. The condition rarely spontaneously resolves and is chronic. The overall quality of life is poor. (Level III)