What Are These Erythematous Plaques In The Axillae?

A 46-year-old man presented to the dermatology clinic for complaints of a rash that had been present in the armpits for approximately 1 month. The rash was associated with erythema and pruritus. The patient had tried halobetasol (Ultravate) cream several times for 2 weeks prior to his visit without obvious signs of improvement. On examination, there were several well-demarcated pink patches in the right and left axillae (Figures 1 and 2). Potassium hydroxide examination of the axillary scale scraping were negative for fungal elements.

Figure 1 and 2.

What is Your Diagnosis?

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Diagnosis: Inverse Psoriasis 

Inverse psoriasis is a variant of psoriasis that affects only the intertriginous areas such as the inguinal fold, gluteal cleft, and axillae (Figures 1 and 2). This is unlike typical psoriasis which sees common involvement of scalp, trunk, and extensor surfaces.¹ Thus, inverse psoriasis has been classified as a variant of the more common plaque-type psoriasis due to solely the location of the lesions.²

Psoriasis affects approximately 2% of the US population with increase prevalence among the White population.³ Studies have shown that the prevalence of inverse psoriasis varies among different populations. In Chinese populations, prevalence can reach up to 7%⁴ in contrast Europeans have had rates up to 36%.² Some believe this discordance in rates is due to the fact that there is no precise definition of inverse psoriasis.⁵

While inverse psoriasis and common psoriasis may distinguish themselves by location on examination, their immunopathogenesis is identical. In each condition, there is an autoimmune reaction that involves CD4⁺ T helper cells that react against self-antigens in the skin. This causes an inflammatory response leading to increased levels of IL-17 and interferon gamma. These mediators react with mast cells, neutrophils, dermal dendritic cells, and macrophages that ultimately lead to histologic and clinical changes.⁶  

Figure 1. Physical examination revealed several well-demarcated pink patches with no scale in the right axillae.

Figure 2. Physical examination revealed several well-demarcated pink patches with no scale in the left axillae. 

Psoriasis is an immune-mediated disease that can affect both the skin and joints. While many forms of the disease exist, plaque-type psoriasis is the predominant form (90%). In this type, the lesions present as well-demarcated erythematous plaques covered by silvery scale. 

Plaque-type psoriasis and inverse psoriasis can commonly coexist on patient presentation and there are a few characteristics of each that can help differentiate them. Plaque-type psoriasis often occurs on extensor skin surfaces, while inverse psoriasis is localized to the axillae, groin, intergluteal cleft, and many other locations on the skin where 2 skin surfaces come together.^1,2 The area of most common involvement of inverse psoriasis is the inguinal area which is followed by the axillae.⁷ A retrospective study of 48 patients showed that the area of most common involvement was the groin (95.8%).⁴ 

The warm and moist environment of areas such as the groin and axillae cause the lesions to appear to have less scale than the plaque-type psoriasis. Lesions will often be sharply demarcated and erythematous, while also appearing moist and shiny.¹ This is due to the large number of sebaceous, apocrine, and eccrine glands in those areas. This environment can also lead to increased maceration within the lesions causing a Koebner phenomenon as well as infection.^1,2    

While plaque-type psoriasis and the inverse type can present together, it has also been reported that inverse psoriasis presents more commonly in patients with nail involvement than in patients without nail involvement.⁸ Studies have also shown that patients with palmar psoriasis have increased likelihood of having inverse psoriasis compared with plaque-type psoriasis. Fransson and colleagues⁹ found inverse psoriasis 5.3 times more likely than plaque-type in 107 patients with palmar psoriasis.⁹   

There is no histologic difference between inverse psoriasis and common psoriasis.¹ In early psoriasis, histology may show edema and dilated vessels in the papillary dermis, intracorneal deposits of neutrophils (Munro microabcesses), epidermal spongiosis, and depletion of granular layer. Later in the disease, the rete ridges elongate and widen.¹⁰ Some of these changes on histology can be classified as a psoriasiform reaction pattern, a pattern shared by other disease processes as well.¹ The pathognomonic finding of psoriasis includes the Munro microabcesses and the Kogoj micropustules (a neutrophilic pustule located in the stratum spinosum). Although psoriasis is usually a clinical diagnosis, a biopsy may be useful to rule out other disease processes and prove helpful in the final diagnosis.¹  

Differential Diagnosis 

The differential diagnosis for inverse psoriasis (Table) include bacterial and fungal (eg, tinea corporis) infections, especially in the absence of an overlying scale. Other entities that should be considered in the differential of an erythematous rash in the intertriginous areas of the body are intertrigo with or without a secondary Candida infection, extramammary Paget disease, glucagonoma syndrome, Langerhans cell histiocytosis, Hailey-Hailey disease (benign familial pemphigus), flexural Darier disease (keratosis follicularis), and subcorneal pustular dermatosis (Sneddon-Wilkinson disease).¹ HIV-associated skin disorders may also be on the differential for inverse psoriasis.¹¹ 

Treatment for inverse psoriasis should be individualized. However, first-line treatment for flexural psoriasis is topical corticosteroids.⁷ Short-term, intermittent use of moderate to potent topical corticosteroids followed by long-term, low-potency topical corticosteroids is the first-line recommendation.¹² Topical corticosteroids should be used with caution in order to avoid adverse events, like atrophy of the skin, that are at a higher risk of occurrence in the intertriginous areas.^1,2 

If the side effect profile with corticosteroids are of concern, alternative primary treatment modalities may include vitamin D₃ analogues (calcipotriol) and calcineurin inhibitors (tacrolimus).^2,7 If first-line options fail or if the patient experiences unwanted side effects, coal-tar preparations are another possibility with decades of experience-based evidence behind their use.^7,12 

Aside from topical therapies, phototherapy using UV-A and UV-B radiation, as well as psoralen–UV-A can be used in the treatment of psoariasis.^1,7 Systemic therapeutic options also exist, with some evidence supporting the use of methotrexate, cyclosporin, or dapsone (100 mg daily).^1,7,12 Biologic therapies, such as adalimumab (Humira) and etanercept (Enbrel), also have documented success in the treatment of inverse psoriasis; therefore, they are valid treatment options depending on the severity and individual circumstances.^12,13 

Upon further examination of the patient, the elbows and scalp revealed slivery scaly plaques along with some nail pitting.

Because of the clinical appearance of the lesion, histological diagnosis was deemed unnecessary. The patient initially failed to improve while on halobetasol cream and the decision to prescribe 0.1% tacrolimus twice daily was made. 

At the patient’s third visit, he experienced mild improvement, but wished to seek other treatment. The patient was then prescribed clobetasol cream twice daily for the body and extremities, and desonide lotion twice daily for the face and scrotum. After further educating and counseling the patient on other therapeutic modalities, a decision to initiate etanercept was considered pending how the patient responded to the latest treatment plan. The patient was told to follow-up in 3 months. 

Psoriasis presents no immediate harm to the patient, however, it can be a chronic and often debilitating disease. The diagnosis of inverse psoriasis should remain high on the differential when encountering a patient with an erythematous rash in intertriginous areas.

However, it is important to consider other differentials, as the use of topical corticosteroids on a fungal infection could cause worsening of symptoms.¹ This case highlights the complexity of this disease process and the difficulty that comes in managing this chronic condition, especially when located in sensitive areas of the body.