An inflamed nodule or pustule breaks down centrally to form an expanding ulcer with a polycyclic or ser-piginous outline, and a characteristic undermined bluish edge (Fig. 19.15). The condition is not bacterial in origin but its pathogenesis, presumably immuno-logical, is not fully understood. It may arise in the absence of any underlying disease, but tends to associ-ate with the following conditions.
1 Ulcerative colitis.
2 Conditions causing polyarthritis, including rheum-atoid arthritis (Fig. 19.16)
3 Crohn’s disease (Fig. 19.17).
4 Monoclonal gammopathies.
5 Leukaemia (with a bullous form of pyoderma).
Lesions may be single or multiple. If gut disease is present then control of this will help the pyoderma. Otherwise the condition responds to systemic steroids but not to antibiotics, and lesions heal leaving papery scars.