# Pustular Psoriasis and Acute Generalized
## 5 Differential Diagnosis  
The differential diagnosis of AGEP presents a challenging clinical problem. Many of the histopathologic and clinical features of AGEP overlap with other pustular dermatologic conditions like PP, Sneddon-Wilkinson disease, IgA pemphbigus, and bullous tinea. It is particularly important to distinguish AGEP from PP since these two conditions vary in their clinical course and treatment but share significant overlapping features [32]. A comparative study by [PERSON] et al. examined the histopathologic and clinical features of 22 patients with AGEP and 11 patients with PP. Histopathological findings that supported a diagnosis of AGEP included eosinophilic spongiosis, vacuolar interface dermatitis, and dermal eosinophil [32]. The presence of at least 10 dermal CD161\({}^{+}\) cells per punch biopsy specimen strongly favored a diagnosis of PP [32]. Clinically, a history of psoriasis and evidence of scaling plaques favored PP as a diagnosis, while mucosal involvement, although uncommon, was a feature unique to AGEP [32]. Of note, within the EuroSCAR study, 7% of patients had a personal history of psoriasis, indicating that patient history should not solely guide the differentiation between AGEP and PP [18].  
Subcorneal pustular dermatosis (SPD) or Sneddon-Wilkinson disease is a rare but chronic pustular eruption that typically presents abruptly with flacied pustules overlying normal-appearing skin [33]. SPD presents similarly to PP, with some arguing it is a variant of PP, particularly the annular variant with pustules spreading centrifugally [33]. In contrast to the pinpoint pustules seen in PP and AGEP, the hypopyon pustules of SPD contain clear fluid overlying purulent material at the base [33]. SPD cannot be reliably distinguished histologically from PP, but a lack of psoriasisiform hyperplasia, mitotic figures, spongiform pustules, and telangiectasias favors SPD [33]. In addition, SPD lacks the presence of necrotic keratinocytes, vacuolar interface, and vasculitis that are variably identified in AGEP [33].  
IgA pemphbigus may only be differentiated from the classic form of SPD using immunofluorescence studies showing intercellular IgA deposition. In the SPD variant of IgA pemphbigus, human desmocolin 1 is the autoanti targeted by IgA autoantibodies [23]. Histopathologic examination typically shows subcorneal neutrophatic pustules, minimal acantholysis, and a mixed inflammatory infiltrate in the dermis [34].  
Figure 4: Acute generalized exanthematous pustulosis. Spongiform pustules and papillary dermal edema are consistent features. Orthokeratosis reflects the acuity of this process.  
Bullous tinea is an uncommon dermatophyte infection that typically presents with inflammatory subcorneal or intraperitoneal vesicles and bullae on the soles [24]. Uncommonly, the clinical presentation of a bullous dermatophyte reaction can appear similar to a localized patch of AGEP or PP given the pustules on an erythemat base. The primary diagnostic test is a potassium hydroxide (KOH) preparation of the affected area, although fungal culture or skin biopsy may be considered if segmented hyphae are not detected, and clinical suspicion remains high.  
In summary, differentiating PP and AGEP can present a significant diagnostic challenge. Both PP and AGEP present with widespread sterile pustules and may be accompanied by fever, malaise, and leukocytosis. Currently, there is no decisive feature that clearly distinguishes PP from AGEP, but a few clinical and histopathologic features can aid the clinician with discerning between these two entities. A history of psoriasis, the presence of scaling plaques, and the presence of dermal CD161\({}^{+}\) cells favor a diagnosis of pustular psoriasis [32]. However, a diagnosis of AGEP is more likely if the histopathologic features include eosinophil spongiosis, vacuolar interface dermatitis, and dermal eosinophil [32].