**10. Differential diagnosis**

As typical forms of SS exhibit a very characteristic clinicopathological picture they rarely cause problems with differential diagnosis. The disease that clinically more resembles SS is

(HIV infection and other immune disorders), b) it is less associated with malignancy (although it is necessary to investigate this condition), and c) it is particularly susceptible to

It has been described as located subtypes of SS cases with clinical lesions limited to a particular body´s area. The neutrophilic dermatosis of the dorsal hands shows characteristics as much SS as pyoderma gangrenosum exclusively located in this area. There is a controversy about if this entity is a subtype of SS or it is an independient disease (Walling et al, 2006; Laguna et al, 2007; Takahama & Kanbe, 2010). The same consideration is

It is very characteristic and one of the diagnostic criteria of SS: a diffuse infiltrate of neutrophils located in the upper half of the dermis accompanied by intense edema. This edema causes the clinical appearance of pseudovesicular or bullous plaques. Leukocytoclasia is frequently present and may be prominent, but obvious vasculitis (neutrophils and fibrin deposits into blood vessel walls) must be absent in order to set the diagnosis. Ocasionally swollen endothelial cells, scattered eosinophils (more typical of druginduced SS), and epidermal exocytosis of neutrophils (even with formation of subcorneal pustules) can be observed. In older lesions the neutrophilic infiltrate is substituted by linfohistiocytic infiltrate (Jordaan, 1989). Requena et al (Requena et al, 2005; Requena, 2007) have described the called *histiocytoid Sweet syndrome*, characterized by a dermal infiltrate constituted by immature neutrophilic granulocytes that have an appearance indistinguishable from histiocytoid cells on optic microscopy with routine stains. The majority of this histiocytoid SS is associated with hematological malignancies, although it has recently been reported an histiocytoid SS induced by trimethoprim-sulfamethoxazole therapy with bone marrow granulocytic maturation arrest (Wu et al, 2008) and two patients with inflammatory bowel disease (Requena et al, 2005; Spencer et al, 2008). Immunohistochemical analysis is necessary when histyocites are present in SS in order to distinguish histiocytoid SS (immature neutrophils) from true histiocytes that can be present in the typical neutrophilic infiltrate, sometimes in a moderate or predominant amount

Typical forms of SS are easily diagnosed by means of criteria of Su and Liu published in 1986 (Su & Liu, 1986) (table 4). Von den Driesch provided a more evolved modification of these criteria in 1994 (von den Driesch, 1994) (table 5), but it has had less acceptation. As we previously indicated, there are patients with "atypical" SS, transitional forms of SS and other ND, as well as cases in which it is only possible to set a generic diagnosis of ND.

As typical forms of SS exhibit a very characteristic clinicopathological picture they rarely cause problems with differential diagnosis. The disease that clinically more resembles SS is

discussed about the located form in facial region (Whittle et al, 1968).

(specially in older lesions) (Corazza et al, 2008).

recurrences (Mohr et al, 2010).

**7.5 Located forms** 

**8. Histopathology** 

**9. Diagnosis** 

**10. Differential diagnosis** 

the erythema multiforme. Other clinical differential diagnosis are drug eruptions and Behçet disease. All of these entities can be ruled out by means of a skin biopsy. Other ND (atypical pyoderma gangrenosum, bowel bypass syndrome, neutrophilic eccrine hidradenitis), vasculitis (specially erythema elevatum diutinum), and erythema nodosum may occasionally set problems with differential diagnosis both from the histopathological and clinical points of view.
