Hydroa vacciniforme is a rare disorder belonging to the group of photodermatoses and is most often diagnosed in the first decade of life. The recurrent appearance of vesicles and crusts after exposure to the sun and subsequent scarring is the hallmark of this cutaneous disease. The diagnosis rests on clinical criteria and findings on histopathology.
Hydroa vacciniforme (HV) is a rare, but potentially debilitating disorder arising exclusively after exposure of the skin to the ultraviolet rays of the sun (termed photodermatoses)   . Until recently, the etiology was considered unknown (and thus, HV was classified into the group of idiopathic photodermatoses) , but the strong association between Epstein-Barr virus (EBV) described across numerous reports indicates the clear role of this virus in the pathogenesis of hydroa vacciniforme    . The prevalence is estimated to be 0.34 per 100,000 individuals, suggesting its rare occurrence in clinical practice . The clinical presentation is distinguished by the development of erythematous lesions, vesicles, papules, and necrotic (sometimes hemorrhagic) crusts in the pediatric population within a few hours after sun exposure   . Hence, the most common areas where the typical lesions appear are the face (ears, nose, and cheeks) and the upper extremities (the lower arm and the hands) . The lesions are followed by scarring and depression of the skin at the site of the lesion, as well as hyper and hypopigmentation .The course of the disease is chronic, as many patients develop recurrent episodes for a prolonged period of time, although a spontaneous resolution of lesions is usually seen by late teenage years and adolescence  . Atypical forms of hydroa vacciniforme have been described in the literature, encompassing similar lesions that develop in non-exposed areas and ocular symptoms (such as conjunctivitis)  .
Despite the benign and self-limiting course of the disease, hydroa vacciniforme can severely impair the quality of life, especially in the absence of an early diagnosis. Since numerous photodermatoses exist, a thorough clinical and laboratory workup should be performed in all patients with sun exposure-induced skin lesions. A detailed patient history is useful in determining whether certain preexisting conditions (eg. systemic lupus erythematosus, allergies, dermatomyositis, pellagra, etc.) may be the underlying cause , whereas the course and progression of symptoms (and confirmation of their recurrence after sun exposure) are vital information for raising clinical suspicion. A meticulous physical examination of the skin is equally important, as it can further identify the distribution of the lesions and their characteristics. Laboratory studies usually yield no pathological findings in the case of hydroa vacciniforme , and because porphyrias (both hepatic and erythropoietic) have a strikingly similar clinical presentation to hydroa vacciniforme, urine and blood levels of porphyrins must be evaluated . Some reports have assessed the presence of EBV through detection of its genetic material by polymerase chain reaction (PCR) testing  . To make a definite diagnosis, a sample of the lesion through a biopsy should be obtained, showing necrosis of the epidermis and intraepidermal vesiculation .