Patch testing and histopathology in Thai patients with hyperpigmentation due to Erythema dyschromicum perstans, Lichen planus pigmentosus, and pigmented contact dermatitis.

BACKGROUND Erythema dyschromicum perstans (EDP)/Ashy dermatosis (AD), Lichen planus pigmentosus (LPP) and Pigmented contact dermatitis (PCD) are common skin diseases featuring abnormal pigmentation which have overlapping clinical features. OBJECTIVE To search for differences in the natural history, clinical features, histopathology and relevant contact allergens in patients those were clinically diagnosed as AD, LPP and PCD in our outpatient clinic. METHOD 43 patients were enrolled into the study. Patients' demographic details, histological findings, DIF staining, provisional and histology diagnosis were recorded. Closed patch tests with standard fragrance and cosmetic series allergens were performed in all patients. RESULT 36 of the patients were female and all of them had dark skin complexions (Fitzpatrick's skin type IV-V), as normally found in AD and LPP. The most common histological finding was pigmentary alteration followed by lichenoid infiltration. DIF staining was positive in 6 out of 21 cases, the most common pattern being IgM colloid bodies. Patients with a provisional diagnosis of AD and LPP had positive patch tests in 40 and 36.36% of cases, respectively. CONCLUSION We have found some similarities and differences between these 3 clinically and histologically overlapping pigmentary disorder. Clinical history, histopathology and DIF are necessary together for making the diagnosis. Patch testing should be conducted in all cases that present with AD or LPP.