[Cutaneous manifestations of internal diseases: purpura, livedo, pyoderma gangrenosum]

Dermatology and internal medicine have in common that many systemic diseases manifest with skin symptoms that are easily accessible for both diagnostic and investigative procedures. Even if modern practice of medicine tends towards organ specific particularity of specialists, dermatology traditionally strives for interdisciplinary communication. Cutaneous manifestations of internal diseases present in various forms. In a syndrome, a number of symptoms present in an identical pattern, and pattern recognition is diagnostic. Vice versa, a distinct skin sign can be a clue to various internal disorders, that have to be considered in the differential diagnosis. Purpura and livedo due to either inflammatory (immune complex mediated or associated with ANCA) or vasoocclusive microthrombotic vascular disease are presented as examples. Finally, subtile variations in key symptoms of the skin, such as of pyoderma gangrenosum, may give a clue to a specific underlying disease, such as rheumatoid arthritis, inflammatory bowel disease, monoclonal gammopathy, or hematologic disease, as exemplified by the variants of pyoderma gangrenosum.