Sun exposed skin disease.

A wide variety of dermatoses may arise in exposed areas and are at the same time induced or exacerbated by irradiation from the sun. The spectrum may range from acute sunburn to chronic effects of sun damage, including elastosis and ultraviolet-induced skin cancer. Inflammatory ultraviolet-induced dermatoses have a confusing nomenclature and classification that often leads to difficulties in the differential diagnosis. Modern nosology differentiates primary from secondary photodermatoses. Primary photodermatoses are believed to be mainly irradiation-induced and immunologically mediated. If the pathophysiology is not clearly defined, they are also called idiopathic. In cases of a known photosensitizer, local and systemic phototoxic or photoallergic reactions can be differentiated. Secondary photodermatoses have an established pathophysiology; for example, an enzyme defect such as occurs in the porphyrias or xeroderma pigmentosum, which leads to the abnormal sun sensitivity. Finally, preexisting dermatoses may be exacerbated by irradiation from the sun, as in systemic lupus erythematosus or Darier disease.