Actinic Prurigo (AP) is a rare chronic, idiopathic skin condition, which is characterised by abnormal cutaneous responses to ultraviolet radiation (i.e. photosensitivity). AP is thought to be mediated by an abnormal immune response in the background of genetic predisposition. AP is a rare condition and is usually seen in certain populations of the Americas. Skin lesions can appear hours to days after sun exposure and rarely, non-exposed skin can be affected. AP is also known as Hutchinson prurigo. Although AP is not associated with mortality, it can cause significant morbidity in afflicted individuals.
Typically, AP first appears in the sunnier months and patients often report exacerbations in symptoms during summer and spring. Rarely, however, the symptoms are worse during winter and autumn, and immunological tolerance during summer is thought to be responsible for this phenomenon. AP tends to occur equally in both the sexes in children and adolescents. In adults, however, females are twice as frequently affected as males. A positive family history of AP or PLE is present in one-fifth of individuals with APThe prevalence of AP in the general population is not known, but is thought to be less than 5% in the above-mentioned populations. In Europe and the Asia-Pacific, where a pathogenetically similar but clinically distinct disease known as polymorphic light eruption (PLE) is more common, rare cases of AP have been reported.AP usually begins in childhood. In some individuals, it may resolve before adulthood. In others, however, it is chronic and tends to recur persistently. Rarely, the disease may arise in adults and the clinical course in these cases is usually chronic.Actinic prurigo can affect the skin of all races, although it is more commonly observed in Latin-American Mestizo and Native American populations. Sporadic cases have been reported in the United Kingdom, the United States, Europe, Australia and Japan.
Sun-exposure, i.e. exposure to ultraviolet A and B radiation, is the predominant cause of actinic prurigo, yet the reason for the abnormal response to sun exposure is not fully understood. AP is believed to be delayed-type hypersensitivity reaction to antigens exposed or changed following exposure of the skin to ultraviolet radiation. The exact nature of these antigens is not known. It is also believed that genetic predilection may play a role in the pathogenesis of AP, with up to 90% of patients with AP showing various human leukocyte antigen (HLA) variants. In particular, HLA type DRB1*0401 and HLA type DRB1*0407 have been associated with AP.
Cellular involvement in actinic prurigo
Langerhans cells (LC) are believed to play a role in the pathogenesis of AP. LC are antigen presenting cells that are involved in the induction of cell-mediated immune responses to antigens located in the skin. LCs in patients with AP show a reduced susceptibility to ultraviolet radiation. UV-resistant LCs in patients with AP activate immune responses via putative antigens. Because of the presence of UV-resistant LCs, the putative antigens may be delivered to lymphocytes in large amounts or in a chronic fashion, leading to inflammation seen in AP. Thus, a combination of altered expression of adhesion molecules and UV-resistant LCs may be involved in the pathogenesis of AP. Further studies are required to elucidate the identity of these putative antigens and to gain a better understanding of the disease process.Expression of cell adhesion and activation molecules has been shown to be increased in AP. This suggests that lymphocytes in skin lesions of AP are, therefore, activated and may play a role in the damage observed in this condition. An increased adhesiveness of lymphocytes, as a result of increased expression of adhesion molecules, enables them to migrate through the endothelium and extracellular matrix. Similarly, the activation molecules can act on their respective receptors to induce lymphocyte proliferation and activation. Ultraviolet radiation is thought to activate adhesion molecules, such as ICAM-1, via activated lymphocyte infiltrates in keratinocytes, leading to inflammation that is observed in AP.Skin lesions in AP have been shown to be infiltrated by CD4+ T cells. An abnormal immune sensitization occurs against epidermal antigens in AP. AP can be considered as an autoimmune disease, because lymphocyte activation occurs in response to patient’s own ultraviolet-irradiated keratinocytes.
The symptoms of actinic prurigo include:
- Extremely itchy (pruritic) skin rash;
- Red and inflamed bumps (papules);
- Thickened patches (plaques); and/or
- Lumps (nodules)
The symptoms of AP are often described as similar to those seen in atopic dermatitis (eczema).
In addition to the symptoms listed above, skin ulcerations, crusting and scaling can occur.
Although areas exposed to the sun, such as the cheeks, nose and hands, are more commonly affected, non-exposed areas can rarely occur in severe AP. Pseudo-alopecia of the eyebrows can occur if the face is constantly scratched. In severe cases, permanent mild scarring and hypopigmented lesions may develop.
In about 65% of the patients affected with AP, the lips are affected. Inflammation of the lip (cheilitis) and pruritis are commonly observed. Other features such as oedema, crusting, ulceration and scales may be present. Interestingly, in 10% of the patients with AP, the lips are the only sites that are affected.
In about 45% of AP patients conjunctivae of the eyes are affected. Hyperaemia, brown pigmentation, photophobia, epiphora (excess production of tears), and formation of pseudopterygium (where the conjunctiva adheres to the cornea) are potential symptoms associated with the involvement of conjunctivae.
Observational diagnosis of actinic prurigo
Nevertheless, blood tests may be performed to rule out systemic diseases with involvement of the skin. The presence of HLA type DRB1*0401, or DRB1*0407 in genetic testing is suggestive of AP. Histological studies may be helpful. As a skin disease that does not involve internal organs, no blood tests are available to diagnose AP.Histologically, mild acanthosis, exocytosis, and spongiosis of the epidermis are observed. Lymphocyte infiltration and lymphoid follicles may be present. Eosinophils are often present. With the conjunctival histology, the epithelium appears thinned and atrophied. The basal cells are vacuolised with lymphocytic infiltration in the sub-mucosal follicles. Eosinophils are conjunctival pigmentation are common findings. The presence of lymphoid follicles in the mucosal and conjunctival laminae is the most characteristic pathological finding in AP.AP is diagnosed based on clinical assessment and the pathological study of the mucosae of the lips, conjunctivae, or the skin.
Phototesting and actinic prurigo
Other laboratory tests are used to rule out other systemic diseases with a photosensitivity component, rather than to diagnose AP. The presence of anti-nuclear antibodies and extractable nuclear antibodies should be undertaken to rule out lupus. The highly contagious Scabies should always be ruled out prior to diagnosing AP. When the skin on the nose is not affected, the condition is more likely to be photosensitive atopic dermatitis than AP. Polymorphic light eruption and prurigo nodularis are other morphologically similar diseases that should be excluded during the diagnosis of AP.Phototesting may help with the diagnosis, but is non-specific and does not rule out other photosensitive disorders. The minimal erythema dose, the minimum dose of narrow-band ultraviolet (UVB) radiation that is required to produce redness 24 hours after exposure, is reduced in patients with AP. It should be noted that a negative cutaneous phototesting does not exclude the diagnosis of AP.
Actinic prurigo is seldom cured and so preventative measures are important in managing actinic prurigo. The disease may resolve before adulthood in some individuals. In others, however, it is persistent and seasonal outbreaks during summer and spring may occur. Rarely, AP may arise in adulthood and persist throughout life. Treatment is aimed at prevention and controlling symptoms. Avoiding UV exposure and appropriate sun protection measures may be beneficial.
Prophylactic phototherapy may be helpful in some cases. Other treatment options include:
- Emollients to relieve itching.
- Topical steroids as anti-inflammatories. Oral steroids may be necessary in more severe disease. Hypersensitivity and infections are possible side-effects of steroid use.
- Anti-malarial drugs, such as chloroquine, may be effective in some cases. These drugs have anti-inflammatory and photoprotective effects. Also, topical calcineurin inhibitors, such as tacrolimus, may be used to treat relatively mild AP
- Oral thalidomide, an immune suppressant, is usually effective in treating more resistant AP in all age groups. Adverse effects include drowsiness, headache and weight gain. Nerve conduction studies should be performed regularly to assess for peripheral neuropathy, another side-effect of thalidomine treatment. Thalidomide is teratogenic and pregnancy must be avoided.
- Oral immunosuppressants, such as azathioprine or cyclosporine.
- Prophylactic phototherapy with UVB or PVB in spring, when the symptoms usually worsen, may be effective in some patients.
Actinic prurigo is an idiopathic disease – i.e. the precise cause(s) of AP are unknown. As sun exposure is the primary trigger of the disease, prudent steps must be taken to reduce or avoid exposure to the sun. The use of sunglasses, wearing appropriate clothing, and the appropriate use of sunscreen may be beneficial. In case of excoriations, sunscreen should be used cautiously. Patients should be taught about the appropriate application of sunscreens. Patients should also be made aware that even minor exposure to ultraviolet radiation may result in outbreaks or exacerbations of symptoms. Annual prophylactic phototherapy may be of benefit in some patients.
Actinic prurigo usually arises in childhood, and persists into adulthood. In some individuals, AP can improve or resolve before adulthood. Rarely, it may arise in adulthood, and persist chronically. Although not a fatal disease, AP can lead to significant morbidity, with exacerbations during the sunnier months. Secondary infections and impetigo are potential complications. Irritant contact dermatitis can manifest with inappropriate use of sunscreens.
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