Sweet's syndrome, also known as Acute Febrile Neutrophilic Dermatosis, is a rare inflammatory condition. It is considered to be the prototype disease of neutrophilic dermatoses, and presents with acute onset dermal neutrophilic lesions, leukocytosis, and pyrexia. Several variants have been described both clinically and histopathologically. Classifications include classic Sweet's syndrome, malignancy associated, and drug induced. The cellular and molecular mechanisms involved in Sweet's syndrome have been difficult to elucidate due to the large variety of conditions leading to a common clinical presentation. The exact pathogenesis of Sweet's syndrome is unclear; however, new discoveries have shed light on the role of inflammatory signaling, disease induction, and relationship with malignancy. These findings include an improved understanding of inflammasome activation, malignant transformation into dermal infiltrating neutrophils, and genetic contributions. Continued investigations into effective treatments and targeted therapy will benefit patients and improve our molecular understanding of inflammatory diseases, including Sweet's syndrome.

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Stevens-Johnson syndrome/toxic epidermal necrolysis (SJS/TEN) is a rare, severe mucocutaneous reaction with few large cohorts reported. This multicenter retrospective study included patients with SJS/TEN seen by inpatient consultative dermatologists at 18 academic medical centers in the United States. A total of 377 adult patients with SJS/TEN between January 1, 2000 and June 1, 2015 were entered, including 260 of 377 (69%) from 2010 onward. The most frequent cause of SJS/TEN was medication reaction in 338 of 377 (89.7%), most often to trimethoprim/sulfamethoxazole (89/338; 26.3%). Most patients were managed in an intensive care (100/368; 27.2%) or burn unit (151/368; 41.0%). Most received pharmacologic therapy (266/376; 70.7%) versus supportive care alone (110/376; 29.3%)-typically corticosteroids (113/266; 42.5%), intravenous immunoglobulin (94/266; 35.3%), or both therapies (54/266; 20.3%). Based on day 1 SCORTEN predicted mortality, approximately 78 in-hospital deaths were expected (77.7/368; 21%), but the observed mortality of 54 patients (54/368; 14.7%) was significantly lower (standardized mortality ratio = 0.70; 95% confidence interval = 0.58-0.79). Stratified by therapy received, the standardized mortality ratio was lowest among those receiving both steroids and intravenous immunoglobulin (standardized mortality ratio = 0.52; 95% confidence interval 0.21-0.79). This large cohort provides contemporary information regarding US patients with SJS/TEN. Mortality, although substantial, was significantly lower than predicted. Although the precise role of pharmacotherapy remains unclear, co-administration of corticosteroids and intravenous immunoglobulin, among other therapies, may warrant further study.

Neutrophil-mediated skin diseases, originally named neutrophilic dermatoses (NDs), are a group of conditions due to an altered neutrophil recruitment and activation, characterized by polymorphic cutaneous manifestations with possible internal organ involvement. Although a number of diseases are included in this setting, the two prototypic forms are pyoderma gangrenosum (PG) and Sweet's syndrome (SS) which usually present with skin ulcers and plaque-type lesions, respectively. They have central features significantly overlapping with autoinflammatory conditions which manifest as repeated episodes of tissue inflammation. However, in contrast to appropriate inflammatory responses to insults or to autoimmune disease, there is an absence of identifiable pathogens, autoantibodies, or autoreactive lymphocytes. The recognition of monogenic autoinflammatory diseases which can present with NDs has led to study several genes involved in autoinflammation in NDs. Based on discovering of a number of mutations involving different autoinflammatory genes, neutrophil-mediated skin diseases are nowadays regarded as a spectrum of polygenic autoinflammatory conditions. Although disease mechanisms have not yet been completely elucidated, NDs are recognized as diseases involving dysfunctional cellular signaling mediated by pathways mainly related to inflammasome and IL-1 with the contributory role of IL-17 and other effector molecules. The precise elucidation of the above-mentioned pathologic mechanisms may pave the way to tailored treatments for patients with different neutrophil-mediated skin diseases.

Pyoderma gangrenosum is a rare inflammatory skin disease classified within the group of neutrophilic dermatoses and clinically characterized by painful, rapidly evolving cutaneous ulcers with undermined, irregular, erythematous-violaceous edges. Pyoderma gangrenosum pathogenesis is complex and involves a profound dysregulation of components of both innate and adaptive immunity in genetically predisposed individuals, with the follicular unit increasingly recognized as the putative initial target. T helper 17/T helper 1-skewed inflammation and exaggerated inflammasome activation lead to a dysregulated neutrophil-dominant milieu with high levels of tumor necrosis factor-α, interleukin (IL)-1β, IL-1α, IL-8, IL-12, IL-15, IL-17, IL-23, and IL-36. Low-evidence studies and a lack of validated diagnostic and response criteria have hindered the discovery and validation of new effective treatments for pyoderma gangrenosum. We review established and emerging treatments for pyoderma gangrenosum. A therapeutic algorithm based on available evidence is also provided. For emerging treatments, we review target molecules and their role in the pathogenesis of pyoderma gangrenosum.