Pyoderma gangrenosum occurs worldwide. Accurate epidemiological data on PG are missing. The peak of incidence occurs between the ages of 20 and 50 years with women being more often affected than men [11]. Cases in infants and adolescents account for only 4 % of PG. PG in elderly people has occasionally been reported [12]. The general incidence has been estimated to be between 3 and 10 per million per year [10].

The etiology of PG is unknown and its pathogenesis is poorly understood. Currently an immune-mediated process is thought to play an important role. Both humoral and cell-mediated abnormalities have been reported with PG, but none of these findings have been demonstrated consistently and it is not clear whether they are of primary importance or represent an epiphenomenon [2].

Pathergy phenomenon is reported to occur in 20–30 % of patients with PG. This refers to heavy neutrophilic infiltrates or pustular reaction that develops at the site of nonspecific trauma [13]. Pathergy phenomenon is elicited by injecting 0.1 ml of normal saline obliquely to the depth of 5 mm with a 20–22-gauge needle [13]. An erythematous papule of more than 2 mm at the prick site develops within 48 h.

Humoral defects described in PG include autoantibodies against skin and bowel, a dermo-necrotic factor and a serum factor [14, 15]. Some studies have suggested the mechanism to be consistent with Arthus and Shwartzman reaction [14–17], in which circulating immune complexes are deposited in blood vessels leading to activation of complement pathways.

Cross-reactivity between cutaneous and bacterial antigens, especially Escherichia coli, has been suspected. A recent support for this possibility was documented in a study [18]. Appearance of PG-like lesions in these patients represents an abnormal reaction to bacteria rather than a form of pathergy and suggests a local failure to terminate IL8 production resulting in marked neutrophilic infiltration. However, the role of bacterial antigens warrants further study. Cell-mediated defects described in PG include cutaneous anergy to candida, streptokinase and purified protein derivative, as well as altered production of macrophage inhibition factor by lymphocytes [19]. Oligoclonal T-cell response due to antigenic stimulus and trafficking between the skin and other organs occurs in PG [20]. Decreased neutrophil chemotaxis and impaired monocyte phagocytosis have been reported in association with PG [21]. The leukocyte abnormalities may contribute to the pathergy phenomenon [11, 22, 23]. Elevated levels of IL8 in the blood have been documented in PG [24], and high IL8 levels have been demonstrated in fibroblasts from PG lesions [25]. Cytokines like IL1 or TNF-α stimulate production of IL8.

Although ulcerative colitis is more commonly associated with PG than Crohn’s disease, the latter became increasingly recognized as having association with PG; both classic and peristomal PG have been reported with ulcerative colitis, Crohn’s disease, and collagenous colitis. Smoking is a significant factor favoring the development of extraintestinal manifestations of ulcerative colitis including PG and erythema nodosum [26].

PG is not associated with vasculitis, but cases with positive tests for c-ANCA or p-ANCA have been reported specially drug-induced PG (thiouracil). Recently a patient with PG and c-ANCA specific for h-lamp-2 has been reported [27]. Several families with inherited PG have been described [28, 29]. There is predisposition to PG in patients with mutation in caspase recruitment domain 15 leading to autosomal dominant autoinflammatory syndrome of pyogenic arthritis, pyoderma gangrenosum, and acne; the mutations affect CD2-binding protein 1 [30, 31].

Pyoderma gangrenosum can have a variety of clinical presentations. The lesions can be classified morphologically as (a) classic ulcerative, (b) bullous, (c) pustular, (d) vegetative (superficial granulomatous), and (e) pyostomatitis vegetans. These clinical variants of PG differ in their clinical presentation, location, and associated diseases.

Patients with PG complain of severe pain that is out of proportion to the clinical appearance of lesions. Pathergy occurs in 20–30 % of patients; it is defined as the development of lesions at sites of cutaneous trauma (needle stick, venesection, insect bite, scalds, varicella, or surgical procedures, especially breast surgery, may act as trigger).

Cutaneous lesions most frequently develop on lower extremities especially on pretibial areas but can occur anywhere including mucous membranes. Sterile neutrophilic abscesses of internal organs (lungs, bones, joints, central nervous system, cardiovascular system, eye, intra-abdominal viscera) can occur with or precede cutaneous PG.

Skin lesions begin as tender papulopustule with surrounding erythematous or violaceous induration, or erythematous nodule or bulla on violaceous base. Necrosis occurs forming a central ulcer; a fully developed ulcer has purulent base with irregular undermined and overhanging, gunmetal color border which extends centrifugally. Re-epithelialization from margins occurs and lesions heal with atrophic cribriform pigmented scarring. Ulcers may be single or multiple and are rapidly progressive, but some may be less inflammatory and expand slowly.

PG-associated hematological disorders or drug-induced PG presents with acute onset hemorrhagic or purulent bullous lesions with widespread distribution and rapid progression. Fever and signs of toxicity are present. PG associated with inflammatory disorders is usually a chronic slowly enlarging ulcer with increased granulation tissue at base, and it sometimes regresses spontaneously. PG in children is clinically similar to adults, but the lesions occur more frequently on head and anogenital areas.

The histopathological findings of PG are variable and nonspecific but are useful in excluding other possible etiologies. The findings depend on clinical variant, type of lesion, site of lesion, site within the lesion, stage of evolution of lesion, and the treatment taken by the patient [2]. Site of biopsy is important because biopsy taken from the center of established ulcerative, bullous, or pustular PG lesions usually shows marked neutrophilic infiltration with abscess formation in mid and deep dermis extending to the panniculus, whereas those taken from peripheral areas (ulcer edge or inflammatory zone of erythema) show mixed or predominantly lymphocytic inflammatory infiltrate.

Typical findings are central necrosis and ulceration of the epidermis and dermis, surrounded by acute inflammatory cell infiltrate with peripheral mixed or chronic inflammatory cells.

Each clinical variant has additional more specific features which are as follows:

In most patients with typical PG, chronic ulcers have inflammation at the edge of ulcer bed. Presence of vasculitis in PG is debatable. True vasculitides and infective causes should be excluded if vasculitis is evident. Lymphocytes may be seen to infiltrate vessel wall with intramural or intravascular fibrin deposition (lymphocytic vasculitis). Culture and staining of biopsy tissue for bacteria, fungi, and mycobacteria should be done if granuloma is present in absence of inflammatory bowel disease.

Clinical presentation of PG may be diverse, and there is neither a diagnostic laboratory test nor pathognomic histopathological findings; therefore, it is a diagnosis of exclusion. Most important considerations are exclusion of infection, vascular disease (stasis, occlusion, and vasculitis), and malignancy. Thorough history should be taken to rule out systemic involvement due to associated disorders, malignancy-related symptoms, and exposure to drugs (iodides, bromides, hydroxyurea), and physical examination should be done.

Skin biopsy and tissue culture should be done to rule out other causes. The best site for skin biopsy (incisional) is from the edge of the lesion (reducing potentially misleading features that may occur in any chronic ulcer) [2]. The patient should be investigated to rule out associated disorders (arthritis, inflammatory bowel disease, and hematological malignancies). Serological evaluation may also be performed (antinuclear antibodies, antiphospholipid antibodies, serum protein electrophoresis, ANCA).

There is no confirmatory diagnostic test for pyoderma gangrenosum. The following diagnostic criteria have been proposed for cutaneous lesions of classic ulcerative PG [36]. Diagnosis requires both of the major criteria and at least two minor criteria (Table 18.2).