INTRODUCTION — Pustules are circumscribed collections of white blood cells and serous fluid. It is important to recognize the morphologic pattern of pustules because it may imply a different spectrum of differential diagnosis as well as treatment.

While the differential diagnosis of pustules is broad (table 1), most diagnoses will fit one of the diseases below. Several defining features can aid in narrowing down the possibilities in an efficient manner:

●The patient's age and general health

●The distribution of lesions

●The duration of the lesions

Simple procedures can confirm the diagnosis when questions persist.

DISTRIBUTION

Face

Acne vulgaris — Acne vulgaris is the most common skin disorder affecting adolescents and young adults [1]. Patients typically have pustules and/or inflamed papules on the face and, less frequently, on the back and chest (picture 1). Acne vulgaris is discussed in detail separately. (See "Pathogenesis, clinical manifestations, and diagnosis of acne vulgaris" and "Acne vulgaris: Overview of management".)

Acneiform eruptions — Acneiform eruptions may be induced by exposure to drugs (table 2), cosmetics containing comedogenic ingredients, industrial chemicals (eg, cutting oils, coal tar, chlorinated hydrocarbons), or environmental factors (eg, elevated temperatures, ionizing radiations). (See "Pathogenesis, clinical manifestations, and diagnosis of acne vulgaris", section on 'Acneiform eruptions' and "Acneiform eruption secondary to epidermal growth factor receptor (EGFR) and MEK inhibitors".)

Acne keloidalis nuchae — Acne keloidalis nuchae is a condition involving chronic inflammation and scarring of the hair follicles of the posterior neck that is seen more frequently in dark-skinned patients [2]. Follicular papules, pustules, and hypertrophic scars may result (picture 2). The clinical manifestations, diagnosis, and treatment of acne keloidalis nuchae are discussed in detail separately. (See "Acne keloidalis nuchae: Pathogenesis, clinical manifestations, and diagnosis".)

Rosacea — Rosacea is a common chronic disorder that may present with pustular eruptions, particularly in moderate and severe cases [3]. Pustular rosacea is found frequently on the central face and neck and, occasionally, in atypical locations, such as the retroauricular area or scalp [4]. Look for telangiectasia, erythema, papules, nodules, thickening of the soft tissue, and sebaceous prominence of the central face (picture 3A-C). Exacerbating factors for rosacea that have been anecdotally reported include alcohol, spicy food, hot beverages, temperature extremes, and psychologic stress. (See "Rosacea: Pathogenesis, clinical features, and diagnosis" and "Management of rosacea".)

Perioral dermatitis — Perioral dermatitis (ie, periorificial dermatitis) presents as small papules, vesicles, and/or tiny pustules with erythema and scaling around the mouth, nose, or periorbital region (picture 4A-C) [5]. When in a perioral distribution, the eruption classically spares the skin immediately surrounding the vermilion border of the lips. A burning sensation or pruritus may be present [6]. (See "Perioral (periorificial) dermatitis".)

Bacterial folliculitis and impetigo — Bacterial folliculitis may occur anywhere on the body, including the face. It is in most cases caused by Staphylococcus aureus. Folliculitis is typically characterized by isolated pustules with a hair piercing the central aspect (picture 5). (See "Infectious folliculitis", section on 'Bacterial folliculitis'.)

Gram-negative folliculitis may occur on the face as a sudden pustular flare of acne previously controlled by chronic oral antibiotics [7]. It can be caused by Klebsiella, Enterobacter, and Proteus species. (See "Pathogenesis, clinical manifestations, and diagnosis of acne vulgaris", section on 'Gram-negative folliculitis'.)

Impetigo occurs most commonly on the face and can present with bullae, honey-colored crusts, erythema, edema, and exudate (picture 6) [8]. (See "Impetigo".)

Fungal folliculitis — Tinea barbae is a fungal infection of the beard region that can present with a significant pustular component (picture 7A-B). There is usually scaling and eventually a circular configuration. Early or mild cases may not have pustules and can have an appearance similar to tinea corporis. (See "Infectious folliculitis", section on 'Fungal folliculitis'.)

Herpes simplex — Herpes simplex eruptions can present as grouped pustules or vesicles on an erythematous base typically located on the vermilion border of the lips (picture 8). They may present similarly in the genital or sacral areas, particularly with recurrent disease. Patients frequently have a history of a prodrome prior to the onset of lesions. Precipitating factors include fever, wind or sunburn, trauma, or stress. (See "Epidemiology, clinical manifestations, and diagnosis of herpes simplex virus type 1 infection".)

Herpes zoster — Herpes zoster may present early on as isolated pustules or vesicles on the face in association with significant discomfort. Look for grouped pustules and/or vesicles on an erythematous base that occur in a dermatomal distribution (picture 9A-B). Whether zoster occurs on the face, trunk, or extremities, it is usually associated with symptoms of pain, aching, and burning. (See "Epidemiology, clinical manifestations, and diagnosis of herpes zoster".)

Pityrosporum folliculitis — Pityrosporum or Malassezia folliculitis is a fungal acneiform disorder that can present as fine pustules, often pruritic, on the face (picture 10). It tends to be unresponsive to traditional acne therapies and worsens with humidity and heat. A KOH examination will reveal copious budding yeast and pseudohyphae. (See "Infectious folliculitis", section on 'Fungal folliculitis'.)

Trunk and extremities

Acne vulgaris — Patients with acne vulgaris often have pustules and/or inflamed papules on the back and upper chest (picture 11). Acne vulgaris is discussed in detail separately. (See "Pathogenesis, clinical manifestations, and diagnosis of acne vulgaris" and "Acne vulgaris: Overview of management".)

Acneiform eruptions — Acneiform eruptions may be induced by exposure to drugs (table 2), cosmetics containing comedogenic ingredients, industrial chemicals (eg, cutting oils, coal tar, chlorinated hydrocarbons), or environmental factors (eg, elevated temperatures, ionizing radiations). (See "Pathogenesis, clinical manifestations, and diagnosis of acne vulgaris", section on 'Acneiform eruptions' and "Acneiform eruption secondary to epidermal growth factor receptor (EGFR) and MEK inhibitors".)

Keratosis pilaris — Keratosis pilaris, while often asymptomatic, may produce pruritic and pustular lesions of the lateral face, trunk, upper and lower arms, thighs, and buttocks. It is caused by the plugging of the follicle by keratin that has failed to exfoliate, leading to a sterile, papular or pustular eruption (picture 12A-B). Prepubertal children can have significant involvement of the lateral cheeks. (See "Keratosis pilaris".)

Miliaria — Miliaria is a transient skin disorder caused by accumulation of sweat beneath eccrine sweat ducts obstructed by keratin. Depending on the level of duct obstruction, miliaria is divided into miliaria crystallina, the most superficial form (picture 13A-B); miliaria rubra (picture 14A-B); and miliaria profunda (picture 15). The clinical manifestations, diagnosis, and treatment of miliaria are discussed in detail elsewhere. (See "Miliaria".)

Bacterial folliculitis — S. aureus folliculitis can involve the upper trunk, buttocks, and legs (picture 16A-B). Gram-negative folliculitis is often seen as a generalized, pustular eruption that ranges from mildly symptomatic to being associated with considerable pain and pruritus. Truncal involvement is typically seen in healthy patients after use of recreational hot tubs, with Pseudomonas aeruginosa identified as the pathogen on culture (picture 17) [9]. (See "Pseudomonas aeruginosa skin and soft tissue infections", section on 'Hot tub-associated eruptions'.)

Dyshidrotic eczema — Hand and foot eczema typically presents with vesicles, bullae, erythema, scale, and intense itching (picture 18A-B). Isolated pustules also may occur but rarely are the predominant primary lesions. A KOH preparation is needed to rule out underlying fungal infection. (See "Acute palmoplantar eczema (dyshidrotic eczema)".)

Palmoplantar pustulosis — Palmoplantar pustulosis (PPP) is a chronic, pustular skin disorder of unknown etiology that usually occurs in adults. PPP presents as recurrent crops of pustules on the palms and/or soles (picture 19), sometimes associated with nail dystrophy and psoriasis-like skin lesions. (See "Palmoplantar pustulosis: Epidemiology, clinical features, and diagnosis".)

Hidradenitis suppurativa — Hidradenitis suppurativa occurs in areas of hormonally influenced apocrine sweat glands, including the axillae, mammary, and inguinal regions, frequently in patients who are obese (picture 20). Pustules may be evident in early lesions. Follicular rupture and involvement of the apocrine gland occurs deeply, resulting over time in extensive scarring and sinus tract formation in many patients (picture 21). Comedonal lesions are usually present as well and help to define the diagnosis, with chronic, recurrent disease as the norm. (See "Hidradenitis suppurativa: Pathogenesis, clinical features, and diagnosis" and "Hidradenitis suppurativa: Management".)

Fungal infections

Pityrosporum folliculitis — Pityrosporum or Malassezia folliculitis is a fungal acneiform disorder that manifests with asymptomatic or pruritic, pustular lesions typically involving the trunk and upper arms, especially in patients with a history of exposure to an extremely humid environment (picture 22A-B). The lesions represent an overgrowth of normal yeast flora. Extensive lesions may require treatment with oral antifungal drugs. More limited involvement often responds to topical therapy (eg, ketoconazole, ciclopirox cream). (See "Infectious folliculitis", section on 'Fungal folliculitis'.)

Candida infection — Candida infections tend to be associated with beefy red areas with scaling, predominantly in areas of moisture, such as the inframammary folds (picture 23), neck folds, inguinal folds (picture 24), and axillae (picture 25). Satellite pustules may occur, particularly beyond the erythematous plaques (picture 26). A KOH preparation of the roof of the pustule will demonstrate the characteristic organisms. (See "Intertrigo".)

Dermatophyte infection — Dermatophyte (superficial fungal) infections, such as tinea pedis or corporis, typically occur with papules, plaques, and peripheral scale (picture 27). Pustules may also be a key primary lesion when follicular units are involved, particularly on the legs, scalp, or forearms. A KOH preparation of the roof of the pustules will demonstrate hyphae (picture 28) [10]. (See "Dermatophyte (tinea) infections".)

Scabies — Scabietic infestations can produce isolated papules, vesicles, and pustules that are intensely pruritic, particularly located in the interdigital web spaces, volar wrists, axillae, breasts, umbilical, and groin areas (picture 29A-B). Family members are often symptomatic. Scrapings from the base of isolated, nonexcoriated pustules, vesicles, or papules may reveal mites, eggs, or fecal material. (See "Scabies: Epidemiology, clinical features, and diagnosis".)

Fire ant bites — Fire ant bites are painful and may cause isolated or grouped pustules (picture 30), especially in patients with a history of outdoor exposure. Significant erythema and edema can occur in patients who have a hypersensitivity reaction to the insect venom. (See "Stings of imported fire ants: Clinical manifestations, diagnosis, and treatment".)

PUSTULES IN PATIENTS WITH FEVER OR OTHER SYSTEMIC SYMPTOMS

Eosinophilic folliculitis — Eosinophilic folliculitis is a pustular skin eruption predominantly located on the scalp, face, neck, and upper chest that occurs in immunosuppressed patients, particularly in those with advanced human immunodeficiency virus infection (picture 31A-B). (See "HIV-associated eosinophilic folliculitis".)

Varicella — Varicella lesions may present with both vesicles and pustules in a generalized fashion. Look for lesions that occur in varying stages in febrile patients (picture 32). Oral lesions also can occur (picture 33A-B). Secondary staphylococcal infection may result in a pustular eruption of not only a viral but also a bacterial origin. (See "Clinical features of varicella-zoster virus infection: Chickenpox".)

Acute generalized exanthematous pustulosis — Acute generalized exanthematous pustulosis is a rare drug eruption most often caused by antibiotics (picture 34). Patients experience the rapid onset of a widespread pustular eruption approximately 24 hours after ingesting the drug. (See "Acute generalized exanthematous pustulosis (AGEP)".)

Disseminated gonococcemia — Disseminated gonococcemia can present with lesions that initially begin as papules and vesicles, ultimately causing no more than 10 pustules that occur along with a necrotic base (picture 35). In addition to the rash, patients may have a fever, migratory polyarthritis, or tenosynovitis. A Gram stain obtained after unroofing the pustule reveals the causative organism. (See "Disseminated gonococcal infection".)

Secondary syphilis — Rash is the most characteristic finding of secondary syphilis. The rash is classically a symmetric papular eruption involving the entire trunk and extremities including the palms and soles. Individual lesions are discrete red or reddish-brown and measure 0.5 to 2 cm in diameter (picture 36). They are often scaly but may be smooth and rarely pustular. (See "Syphilis: Epidemiology, pathophysiology, and clinical manifestations in patients without HIV".)

Pustular psoriasis — Pustular psoriasis is typically localized to the palms and soles but may rarely occur in an acute, generalized form (von Zumbusch-type). Acute, generalized pustular psoriasis is characterized by the abrupt development of widespread, painful, erythematous patches that rapidly become studded with hundreds of pinhead-sized, sterile pustules (picture 37). Systemic symptoms include fever, malaise, and arthralgias. (See "Pustular psoriasis: Pathogenesis, clinical manifestations, and diagnosis".)

Pyoderma gangrenosum — Pyoderma gangrenosum is an inflammatory skin disease often associated with underlying systemic disorders such as inflammatory bowel disease, arthritis, and lymphoproliferative disorders. The eruption may begin as an isolated pustule or scattered lesions on the trunk or extremities (picture 38). There is surrounding edema and purplish induration with rapid progression into a large ulcer, which heals ultimately with cribriform scars (picture 39). The diagnosis is typically made after all infectious etiologies have been ruled out. Histologic examination is helpful but not diagnostic in characterizing this disease. (See "Pyoderma gangrenosum: Pathogenesis, clinical features, and diagnosis".)

Sweet syndrome — Sweet syndrome (acute febrile neutrophilic dermatosis) is an uncommon inflammatory disorder characterized by the abrupt appearance of painful, edematous, and erythematous papules, plaques, or nodules on the skin, accompanied by fever and leukocytosis. In some patients, it can present with vesicular or bullous lesions (picture 40). Sweet syndrome can be idiopathic or associated with infections, inflammatory bowel disease, medications, or malignancy. (See "Sweet syndrome (acute febrile neutrophilic dermatosis): Pathogenesis, clinical manifestations, and diagnosis".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topic (see "Patient education: Rosacea (The Basics)")

SUMMARY

●Pustules are elevated skin lesions containing collections of purulent fluid. A variety of skin conditions may present with pustules, including both infectious and noninfectious disorders. (See 'Introduction' above.)

●Knowledge of the history and distribution of skin lesions, as well as patient demographics and comorbidities, is useful for narrowing the differential diagnosis of a pustular eruption. In cases in which infection is suspected, microbial cultures can assist in the identification of a causative organism. A potassium hydroxide preparation is a quick procedure that is useful for the diagnosis of superficial fungal infections. (See 'Introduction' above and 'Distribution' above and "Office-based dermatologic diagnostic procedures", section on 'Potassium hydroxide preparation'.)

●Acute generalized exanthematous pustulosis is a drug-induced eruption in which widespread pustules rapidly appear. Examples of other conditions that may present with generalized pustules include varicella and pustular psoriasis. (See "Acute generalized exanthematous pustulosis (AGEP)" and "Clinical features of varicella-zoster virus infection: Chickenpox" and "Pustular psoriasis: Pathogenesis, clinical manifestations, and diagnosis" and "Dermatoses of pregnancy", section on 'Pustular psoriasis of pregnancy'.)

●Pyoderma gangrenosum is an inflammatory disorder of the skin that clinically may resemble an infectious process. The diagnosis of pyoderma gangrenosum is made only after infectious processes have been excluded. (See "Pyoderma gangrenosum: Pathogenesis, clinical features, and diagnosis".)