Pyoderma gangrenosum is a rare, inflammatory skin disease where painful pustules or nodules become ulcers that progressively grow. Pyoderma gangrenosum is not infectious. Treatments may include corticosteroids, ciclosporin, infliximab, or canakinumab. The disease was identified in 1930. It affects approximately 1 person in 100,000 in the population. Though it can affect people of any age, it mostly affects people in their 40s and 50s. Types There are two main types of pyoderma gangrenosum: the 'typical' ulcerative form, which occurs in the legs an 'atypical' form that is more superficial and occurs in the hands and other parts of the body Other variations are: Peristomal pyoderma gangrenosum comprises 15% of all cases of pyoderma Bullous pyoderma gangrenosum Pustular pyoderma gangrenosum Vegetative pyoderma gangrenosum Presentation Associations The following are conditions commonly associated with pyoderma gangrenosum: Inflammatory bowel disease: Ulcerative colitis Crohn's disease Arthritides: Rheumatoid arthritis Seronegative arthritis Hematological disease: Myelocytic leukemia Hairy cell leukemia Myelofibrosis Myeloid metaplasia Monoclonal gammopathy Autoinflammatory disease: Pyogenic sterile arthritis, and acne syndrome (PAPA syndrome) Causes Though the cause is not well understood, the disease is thought to be due to immune system dysfunction, and particularly improper functioning of neutrophils. In support of an immune cause, a variety of immune mediators such as interleukin (IL)-8, IL-1β, IL-6, interferon (IFN)-γ, granulocyte colony-stimulating factor, tumor necrosis factor alpha, matrix metalloproteinase (MMP)-9, MMP10, and elafin have all been reported to be elevated in patients with pyoderma gangrenosum. Also in support of an immune cause is the finding that at least half of all pyoderma gangrenosum patients suffer from immune-mediated diseases. For instance, ulcerative colitis, rheumatoid arthritis, and multiple myeloma (MM) have all been associated with pyoderma gangrenosum. It can also be part of a syndromes such as PAPA syndrome. One hallmark of pyoderma gangrenosum is pathergy, which is the appearance of new lesions at sites of trauma, including surgical wounds. Diagnosis Diagnosis of PG is challenging owing to its variable presentation, clinical overlap with other conditions, association with several systemic diseases, and absence of defining histopathologic or laboratory findings. Misdiagnosis and delayed diagnosis are common. It has been shown that up to 39% of patients who initially received a diagnosis of PG have an alternative diagnosis. In light of this, validated diagnostic criteria have recently been developed for ulcerative pyoderma gangrenosum. Diagnostic criteria In addition to a biopsy demonstrating a neutrophilic infiltrate, patients must have at least 4 minor criteria to