Gangrenous pyoderma: a problem of differential diagnosis in atypical localization.

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relevance
Gangrenous pyoderma (GP) is a rare immune-mediated skin disease that belongs to the group of neutrophilic dermatoses. The classic ulcerative form is characterized by an acute onset, painful ulcers with undermined purplish-blue edges, and a pronounced pathergy phenomenon. Despite the fact that the lower extremities are the most typical location of the pathological process, the disease can affect any area of the skin, including intertriginous zones [1, 2]. However, isolated involvement of the axillary regions is extremely rare in clinical practice, which creates significant difficulties in timely diagnosis verification.
The axillary localization of GP is a diagnostic challenge due to its clinical uniqueness and similarity to other inflammatory diseases of the apocrine glands. First of all, it is necessary to conduct a differential diagnosis with purulent hidradenitis (PH), which is characterized by a chronic course, the formation of fistulous tracts, and scarring changes in the axillary armpits [3, 4]. The clinical similarity is so great that in some cases these nosologies are considered within the framework of syndromal forms, for example, PASH syndrome (GP, PH, acne) [5]. The erroneous interpretation of axillary GP as an infectious process or exacerbation of hidradenitis can lead to the prescription of ineffective antibacterial therapy and, more critically, to surgical intervention. Given the high risk of pathergy, any skin trauma in the active phase of GP can provoke a sharp progression of the ulcerative defect and worsen the patient's overall condition [6, 7].
The pathogenetic basis of both diseases is the activation of certain autoinflammatory genes, in particular, the genes MEFV, NLRP3, NLRP12, LPIUN2, NOD2, and others, which leads to dysregulation of innate immunity in GP with the formation of an inflammasome and excessive production of proinflammatory cytokines, such as IL-1β and IL-17 [8, 9, 10]. Nevertheless, the management of patients is fundamentally different: while surgical sanitation of the foci is possible in hidradenitis, in gangrenous pyoderma, systemic immunosuppressive therapy is given priority[11]. The absence of specific diagnostic markers requires a thorough analysis of clinical and anamnestic data, histological findings, and the exclusion of an infectious etiology.
Local status:
In the axillary regions, multiple deep ulcerative defects of irregular shape with a diameter of up to 7 cm are visualized, with undermined edges and sharp borders. The bottom of the ulcers is covered with a black, purulent-necrotic scab with openings that simulate fistulous tracts with abundant yellow-greenish purulent discharge with bloody streaks. Palpation of the lesions is extremely painful.
Laboratory results:
In the general blood test, there is leukocytosis (9.75 × 10⁹/L) and an increase in ESR to 25 mm/h. In the biochemical blood test, there is an increase in the level of C-reactive protein to 7.7 mg/L (normal value is 5 mg/L), as well as a slight increase in ALT (80 U/L) and GGT (61 U/L). The culture of the discharge revealed the growth of *Escherichia coli*.
Histological examination of the biopsy:
A fragment of adipose tissue with a section of hyalinized fibrous tissue (probably fascia). The adipose tissue shows focal and confluent necrosis with moderate neutrophilic leukocyte infiltration and accumulations of nuclear detritus. There are no signs of a specific inflammatory process or a neoplastic process within the examined area.

discussionscussion
ented clinical observation illustrates a classic diagnostic dilemma that arises in the case of atypical localization of neutrophilic dermatoses. The nine-month period from the onset of the disease to the verification of the diagnosis indicates a lack of awareness among primary care physicians regarding intertriginous HP. An analysis of diagnostic errors reveals several key factors that make timely diagnosis of axillary HP challenging. Firstly, the rare occurrence of HP in this unusual location, its peculiar course with the formation of a perforated necrotic crust resembling a fistula, and the abundance of purulent discharge inevitably lead to a differential diagnosis of infectious diseases (bacterial pyoderma, mycoses) or HP. Secondly, the absence of pathognomonic histological features also complicates the diagnostic process and may lead to overdiagnosis of autoimmune bullous diseases, such as vegetating pemphigus, which resulted in the administration of high-dose glucocorticosteroid therapy without the use of immunosuppressive drugs in the early stages of the patient's management. The presence of fistulous tracts in the axillary regions, although understandable due to the depth of the ulcerative process in GD, requires long-term monitoring for the development of signs of purulent hidradenitis or PASH syndrome. Given the refractoriness of the disease in the early stages and the high need for immunosuppression, it is advisable to consider the possibility of prescribing genetically engineered biological drugs (inhibitors of TNF-α or IL-17) in case of possible relapse, as their effectiveness has been proven in resistant forms of neutrophilic dermatoses.

conclusion
In summary, isolated axillary involvement in GD is a challenging diagnostic task that requires the exclusion of a wide range of differential diagnoses, including infectious, autoimmune, and inflammatory skin diseases. Key markers that suggest GD at the pre-histological stage include painful lesions, undermined ulcer margins, lack of response to standard antibiotic therapy, and the pathergy phenomenon. Despite the nonspecific nature of the findings, histological examination plays a crucial role in ruling out alternative diagnoses such as pemphigus, deep mycoses, and vasculitis.

Fig. 1. Clinical presentation of gangrenous pyoderma of the left axillary region in patient L.:

a — appearance of the ulcerative defect under a necrotic crust after treatment with aniline dyes (brilliant green solution) before therapy;

b — deep ulcerative–necrotic defect with undermined violaceous-bluish edges and irregular borders during treatment;

c — ulcerative defect with isolated areas of necrosis and bright-red granulation tissue at the base. A zone of perifocal erythema is observed around the ulcer.

Figure 2. Positive dynamics of the skin process shown by a reduction in the area of the ulcerative defect, decreased severity of perifocal inflammation, and signs of scarring.

About the authors

Olga Yurievna Olisova

I.M. Sechenov First Moscow State Medical University (Sechenov University)

Email: olisovaolga@mail.ru
ORCID iD: 0000-0003-2482-1754
SPIN-code: 2500-7989

MD, Dr. Sci. (Med.), Professor, Corresponding Member of the Russian Academy of Sciences

Russian Federation, 8 Trubetskaya St., building 2, Moscow, 119048, Russia

Olga Valentinovna Grabovskaya

I.M. Sechenov First Moscow State Medical University (Sechenov University)

Email: olgadoctor2013@yandex.ru
ORCID iD: 0000-0002-5259-7481
SPIN-code: 1843-1090

MD, Cand. Sci. (Med.), Professor

Russian Federation, 8 Trubetskaya St., building 2, Moscow, 119048, Russia

Anna Evgenievna Bobkova

I.M. Sechenov First Moscow State Medical University (Sechenov University)

Email: anya_bobkova98@mail.ru
ORCID iD: 0000-0003-3611-0917
SPIN-code: 5345-5746

Graduate student

Russian Federation, 8 Trubetskaya St., building 2, Moscow, 119048, Russia

Valentina Vadimovna Glushak

I.M. Sechenov First Moscow State Medical University (Sechenov University)

Author for correspondence.
Email: valentina.glushak@inbox.ru
ORCID iD: 0000-0002-3007-5498

Мedical resident

8 Trubetskaya St., building 2, Moscow, 119048, Russia

References

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