A definition of: Pustule
Apustule is a circumscribed elevation formed of pus, i.e., yellow-white fluid composed of neutrophils and debris of neutrophils. In some instances of infectious cause, neutrophils are joined by microorganisms such as bacteria. Pustules, like papules, lend themselves to being classified as follicular or non-follicular. Most pustules are follicular and consist of collections of neutrophils housed within a dilated infundibulum. The cause of the suppurative folliculitis may be a bacterium, e.g., folliculitis decalvans, dermatophyte, e.g., Majocchi’s granuloma, spirochete, e.g., follicular syphilids, or virus, e.g., herpesvirus folliculitis. The most common of all suppurative folliculitides is seemingly non-infectious acne vulgaris. Virtually all non-follicular pustules are intra-epidermal, and they may be positioned within the spinous, granular, or cornified layers, or all of those sites concurrently, depending on when during the course of migration of neutrophils from capillaries in dermal papillae to cornified layer of the epidermis biopsy interrupts the process. Examples of pustular dermatitides devoid of proclivity for follicles are those of pustular psoriasis, in which there are no infectious agents, and those of dermatophytosis (exclusive of Majocchi’s folliculitis) and candidiasis, in which there are. Pustules may be both follicular and non-follicular concurrently, as in toxic erythema of the newborn.
Several clues enable the follicular nature of pustules to be identified clinically: (1) relative equidistance of them from one another on hair-bearing skin, e.g., gram negative folliculitis (“hot tub” folliculitis) because the follicles themselves are nearly equidistant from one another and (2) a hair emerging from them, e.g., staphylococcal folliculitis, because follicles house a hair shaft. Pustules may arise de novo, e.g., toxic erythema of the newborn, develop from vesicles consequent to secondary bacterial infection, i.e., impetiginization, e.g., of allergic contact dermatitis, or come into being as a result, at least in part, of necrosis of an epidermis that forms a roof of a vesicle, e.g., in gonococcemia. When the morphologic features just referred to are taken into account, it is possible to conceive of pustules in the fashion that follows, examples of which are not presented in an exhaustive list because they are meant to be representative of a concept, namely, that elevations, in this instance pustules, can be understood in terms of histopathologic findings.
A Classification of Pustules
1. Acne vulgaris (sterile except for the presence of normal microbiological flora). Suppuration within infundibula and often outside of them, i.e., perifollicular abscesses, granulomatous perifolliculitis, and sometimes fibroplasia
2. Rosacea. Suppuration within and around infundibula, and, in time, granulomatous perifolliculitis
a. Impetigo. Suppuration plus bacteria within infundibula, and a perifollicular infiltrate of neutrophils and histiocytes followed sometimes by fibroplasia
a. Majocchi’s granuloma
3. Spirochetal. Hyphae and spores of dermatophytes within cornified elements of follicles, suppuration within infundibula, and usually suppurative granulomatous perifolliculitis that may be a prelude to fibrosis
a. Syphilis, acneiform. Suppuration within infundibula in conjunction with a superficial and deep perivascular and often lichenoid infiltrate of histiocytes and plasma cells mostly. Spirochetes in number (as evidenced by specialized silver stains or an immunoperoxidase stain specific for T. Pallidum) within foci of infundibular suppuration
1. Pustular psoriasis. Subcorneal, spongiform, and/or Monro’s pustules within the epidermis, mounds of parakeratosis with neutrophils at their summits, and a mixed-cell infiltrate in the upper part of the dermis
a. Dermatophytosis. Subcorneal and/or spongiform pustules within epidermal epithelium, neutrophils often in mounds of parakeratosis, and hyphae in zones of orthokeratosis
b. Candidiasis. Subcorneal and/or spongiform pustules within epidermal epithelium, and spores and pseudohyphae in the cornified layer
III. BOTH FOLLICULAR AND NON-FOLLICULAR
A. Toxic erythema of the newborn. Collections of neutrophils and eosinophils within infundibula and in the epidermis
a. Impetiginization of spongiotic vesicles
1. Allergic contact dermatitis, nummular dermatitis, and dyshidrotic dermatitis. Spongiotic vesiculation replete with neutrophils beneath scale-crusts that also contain neutrophils
B. Secondary to epidermal necrosis in vesicles
1. Herpesvirus infections. Multinucleate, ballooned, necrotic, acantholytic keratinocytes plus neutrophils within an intra-epidermal vesicle
2. Gonococcemia. Spongiotic and ballooning vesiculation replete with neutrophils beneath necrotic epithelium above vasculitis in the form of thrombi within venules, neutrophils and lymphocytes in perivascular distribution, and neutrophils in the interstitium
In sum, pustules are elevations that result from accumulation of pus within adnexal or non-adnexal epithelium and, rarely, both together. Clinical features of pustules can be explained by histopathologic findings. In the next issue of the journal, another type of elevation, namely, scale, will be explained in terms of changes discernible by microscopy.
An Atlas of pustules
Drawings by Larry Parsons, M.D.
Allergic contact dermatitis, impetiginized
Toxic erythema of the newborn
From the Institute for Dermatopathology, Jefferson Medical College, Philadelphia, Pennsylvania. Larry Parsons, M.D. is a visiting fellow at the Institute. Reviewed by Mario DiLeonardo, M.D.