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There are multiple causes of diffuse hyperpigmentation of the skin (DHP) and determination of the exact etiology can sometimes be difficult. A systematic classification based on both clinical and pathological findings that should aid the clinician who is confronted with this situation is presented in this article. The skin diseases that cause DHP are classified based on the pigment characteristics (melanin, non-melanin), pigment distribution (epidermal, epidermal-dermal, dermal) and density of melanocytes (normal, increased). The intent of this review is to provide a "histopathological tool" that increases diagnostic accuracy in the evaluation of a patient with DHP.

The dyschromatoses are a group of disorders characterized by the presence of both hyperpigmented and hypopigmented macules, many of which are small in size and irregular in shape. There are two major forms-dyschromatosis symmetrica hereditaria (DSH) and dyschromatosis universalis hereditaria (DUH), both of which are seen most commonly in Japan. DSH was first described by Toyama in 1929 and is characterized by a symmetrical distribution of hyperpigmented and hypopigmented macules on the extremities, especially over the dorsa of the hands and feet. In 1933, Ichikawa and Hiraga were the first to describe DUH as well-demarcated brown macules admixed with various-sized hypopigmented macules in a generalized as opposed to acral distribution. DUH was noted to appear within the first month of life. Some clinicians have suggested that DSH might be a subtype of DUH; however, we have to wait for the cloning of the causal genes of these diseases before coming to any definite conclusions. The differential diagnosis of dyschromatosis includes xeroderma pigmentosum, dyschromic amyloidosis, and exposure to chemicals such as diphenylcyclopropenone and monobenzyl ether of hydroquinone.