PO:22:032 | Beyond the joints: a rare case of subcorneal pustular dermatosis in Rheumatoid Arthritis

Sara Casella^1|2, Carlo Smirne^1|2, Roberta Zaira Pedrazzoli³, Mattia Perazzi^1|3, Edoardo Cammarata⁴, Francesca Zottarelli⁴, Paola Savoia⁵, Renzo Boldorini⁶, Mario Pirisi^1|2, Pier Paolo Sainaghi^1|3 | ¹Dipartimento di Medicina Traslazionale, Università del Piemonte Orientale Novara, Italy; ²Unità di Medicina Interna, Azienda Ospedaliera Maggiore della Carità Novara, Italy; ³Unità di Medicina Interna e Reumatologia, Azienda Ospedaliera Maggiore della Carità Novara, Italy; ⁴Unità di Dermatologia, Azienda Ospedaliera Maggiore della Carità Novara, Italy; ⁵Dipartimento di Scienze della salute, Università del Piemonte Orientale Novara, Italy; ⁶Unità di Anatomia Patologica, Azienda Ospedaliera Maggiore della Carità Novara, Italy

A 68-years-old woman with rheumatoid arthritis (RA) presented to our Rheumatology Clinic due to the exacerbation of a vesiculopustular rash over the past 3 months. The eruption followed a Herpes Zoster virus infection, treated with antivirals. The patient denied taking new medications and reported a family history of psoriasis. The lesions, ranging from 1 to 7 millimeters and exhibiting the hypopyon sign, involved the elbows, left axilla and left hip. Systemic symptoms were not present. In the suspicion of a bullous disease, namely subcorneal pustular dermatosis (SPD), a skin biopsy was performed. Later, the patient's clinical condition deteriorated, marked by the emergence of systemic manifestations. Thus, she was admitted to our department for diagnostic workup. The differential diagnosis included SPD, pustular psoriasis, and IgA pemphigus. Treatment with acitretin was started, due to its efficacy across all three conditions. Following an initial improvement, the patient worsened abruptly, presenting fever, dissemination of pustules, and secondary impetiginization. Consequently, steroids and doxycycline were initiated, with no significant response. Thus, second-line therapy with infliximab was started. Later, histopathological examination confirmed the suspicion of SPD. The patient then exhibited resolution of the eruption and defervescence. Thus, she was discharged with an outpatient follow-up plan and continued infliximab, which controlled both cutaneous manifestations and underlying RA. SPD, also known as Sneddon Wilkinson disease, is a rare neutrophilic dermatosis, mainly affecting middle-aged women. It is marked by flares of flaccid pustules, typically distributed on intertriginous zones and flexural regions, while sparing face and mucosal surfaces. SPD’s diagnosis can be challenging due to its overlap with other pustular disorders, including pustular psoriasis, IgA pemphigus, and acute generalized exanthematous pustulosis. Distinguishing features are its usual confinement to the skin, without systemic involvement, and gradual onset. Pustules’ distribution and the hypopyon sign are valuable diagnostic clues. However, diagnosis cannot rely solely on clinical features; histopathological evaluation is essential for diagnostic confirmation and shows a neutrophilic infiltrate in the subcorneal layer of the epidermis with normal dermis; definitive diagnosis relies on negative direct and indirect immunofluorescence. First-line treatment includes dapsone and, when contraindicated, acitretin. In refractory cases, anti-tumour necrosis factor a has shown promising results. Disease relapse is common, often necessitating long-term therapy. This case depicts an exceptionally rare condition frequently misdiagnosed due to its clinical similarity to other bullous diseases. Atypical features, such as systemic symptoms or abrupt onset, can complicate the diagnosis, as happened in our case. Also, lack of response to initial therapy challenged our initial diagnostic hypothesis. Nevertheless, histopathological confirmation validated our suspicion. This case also underscores the critical importance of early treatment, particularly in rapidly progressive forms.