Introduction 

Pyoderma gangrenosum is a rare but serious painful ulcerating skin disease (neutrophilic dermatosis) of unknown cause, characterised by tiny reddish papules, nodules or pustules which eventually break to form an ulceration that occurs most often on the lower limbs.¹ It can develop on skin that has been recently injured or operated on. It begins as a red bump or blister which might resemble a pimple or an insect bite. The bump or blister opens as a painful ulcer, which expands rapidly. The ulcer has an undermined border, which is dusky or purple. These ulcers grow together to form a larger ulcer.³ It is a hyperactive inflammatory response to injury, swelling or neoplastic process, and so is classified as a systemic autoinflammatory disease.⁴

Associated systemic conditions are found in up to 50% cases The most common associated conditions include inflammatory bowel disease in up to 30% of cases, seronegative arthritis and rheumatoid arthritis in up to 10% of cases, malignancies such as haematological or monoclonal gammopathies, i.e., immunoglobulin A gammopathy, in 5% of cases and other malignancies in up to 5%  of cases and certain other rarely associated conditions are inflammation and chronic infection.⁴

Note: 5p's of Pyoderma gangrenosum: Painful, Purple, Progressive, Pretibial, Pathergy.²

Pathergy 

It is a general overreaction of the body to minor skin injuries caused by needle pricks. This reaction shows up as small raised bumps or pus-filled spots. Several things can influence it, like how many times the skin is pricked, the type of disease, a person’s gender, existing health conditions, and the kind of needle used. The reaction doesn’t stay the same throughout the illness; it can change over time and tends to become less sensitive. How common this reaction is also depends on where people are from; for example, it's more common in places like Turkey, the Middle East, and China, but less common in Northern Europe.

Histopathological examination revealed a mixed inflammatory infiltrate in the dermis, primarily composed of lymphocytes, neutrophils, and occasional eosinophils, with a tendency to localise around blood vessels. In a controlled temporal analysis, an early infiltration of polymorphonuclear cells and the formation of intraepidermal pustules were observed within 4 hours of skin challenge, intensifying by 24 hours with additional mononuclear cell involvement. By 48 hours, the inflammatory response either stabilised or declined, without changes in mast cell density or evidence of vasculitis. Notably, such pustular formations were absent in both healthy individuals and those with recurrent aphthous stomatitis, despite minor inflammatory infiltration. Consistent findings across studies support the presence of superficial perivascular inflammation, intraepidermal pustules, and the absence of vasculitic changes or immune complex deposition.⁸

Pathergy testing 

There isn’t a single agreed-upon way to perform the pathergy test yet, but the most common method involves pricking the skin with a needle. Some versions of the test involve injecting substances like saltwater, uric acid crystals, or specific proteins. There are two types of pathergy tests: one done on the skin (called the Skin Pathergy Test or SPT) and one inside the mouth (Oral Pathergy Test or OPT). The skin test is usually more accurate.

To get the best results, the skin test is usually done by making two small pricks on the forearm using a slightly thick needle at a 45-degree angle. Using thicker, duller needles tends to produce more positive results, while thinner needles often produce fewer. Even though reusable needles might work better, disposable ones are preferred to avoid the risk of infection. Cleaning the skin before testing might reduce the chances of getting a positive result, so it’s often skipped.

Findings 

The pathergy test is usually checked 24 to 48 hours after the skin has been pricked with a needle. A normal result shows just some redness without any swelling. If the test is positive, a small bump or pus-filled spot appears on the skin, and the bigger or more severe the reaction, the stronger the positive result.

The oral version of the test is easier to interpret; if there's any sore or pus-filled spot in the mouth, it's considered positive. Doctors often use a system called the Dilsen method to rate how strong the skin reaction is, based on the size of the bump and the type of needle used. Dull, blunt needles tend to cause more noticeable reactions. The results are usually graded on a scale from 1+ to 4+, with higher numbers meaning stronger reactions. However, it's still not clear how much these reactions reflect how active Behcet’s disease really is.⁹

Conditions with positive pathergy phenomenon

The pathergy phenomenon, characterised by an exaggerated skin injury response, is observed in various medical conditions. It is most notably associated with Behcet's disease, where it serves as a diagnostic criterion. Pyoderma gangrenosum (PG) also exhibits this reaction in approximately 25% of cases, prompting clinicians to avoid surgical interventions like debridement or grafting due to the risk of exacerbating lesions. Additionally, individuals undergoing interferon alpha treatment for chronic myeloid leukaemia may show pathergy. Other inflammatory or immune-mediated disorders linked to this phenomenon include Sweet’s syndrome, eosinophilic pustular folliculitis, and inflammatory bowel disease. Interestingly, a pathogenic response can occasionally be seen in healthy individuals and, albeit rarely, in patients with spondyloarthropathies.¹⁰

Role of trauma in lesion formation

Surgical Intervention in pyoderma gangrenosum is usually not considered due to the risk of pathergy, a condition where an injury can worsen the disease. In several patients, surgery might be considered where the active disease is being controlled for reconstruction after severe loss of tissue.

Immunosuppression can slow or stop the progress of pyoderma gangrenosum, but not the loss of tissue; therefore, a surgical procedure amid the course of therapy is required.⁵

Extensive large ulceration- If the sores are large and aren't healing, skin grafting can be performed. In this procedure, the surgeon attaches a piece of skin from the graft site over the open sores.³

Treatment 

Non-responsive to medical management- Initially, when the corticosteroids and immunosuppressive drugs are unresponsive to treatment, surgical procedures like split-thickness skin graft and vacuum-assisted closure are important treatment options for slow-healing lesions.⁷ When the topical and systemic treatment of cyclosporine and corticosteroids fails, allogeneic cultured dermal substitutes can be useful tools for the treatment of obstinate ulcer.⁶

Drug-based therapy can stop the progression of pyoderma gangrenosum, but tissue damage can be a continuing problem. Reconstructive surgical treatments like microvascular free flaps, autologous and allogenic skin transplantation, and debridement act as an essential part of therapy and can be  reliable for selected patients.⁵

Summary 

The pathergy phenomenon is when the body overreacts to small skin injuries, like needle pricks, by developing bumps or pus-filled spots. It’s influenced by factors like the type of needle used, the number of pricks, the person’s health, and where they live, being more common in places like Turkey and China. The reaction changes over time and is mostly seen in conditions like Behcet’s disease and pyoderma gangrenosum. A pathergy test, often done by pricking the skin, helps doctors spot this reaction. If the skin swells or forms pus within 24–48 hours, the test is considered positive. It’s not only found in certain diseases but can occasionally appear in healthy people, too. In severe cases, especially when medication doesn’t work, surgery, like skin grafts or tissue repair, may be needed to manage stubborn wounds.