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Signature ✍️
Pustules vs. Nodules ⚫
- Pustule: skin elevation with visible collection of leukocytes (”pus”); Depth less than 1 cm: superficial skin layers affected (epidermis → dermis)
- Nodule: circumscribed skin elevation; Depth up to 2 cm: deeper skin layers affected (dermis → subQ fat)
Definitions ⚫
- Neutrophilic dermatoses: spectrum of disorders united by dense, sterile neutrophilic infiltrate, as well as pathergy (dermal injury → pustule → ulcer). The infiltrate begins in the dermis (nodule) & can then extend into the epidermis (pustular “eruption” → ulcer)
- Sweet syndrome: acute febrile neutrophilic dermatosis (“juicy,” edematous papules & plaques)
- Pyoderma gangrenosum: rolled borders with violaceous “gun-metal grey” border → net-like cribiform scarring
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DDx 🏳️🌈
Non-infectious ⚫
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- SAPHO syndrome
- Synovitis: anterior chest wall (e.g. sternocostal, sternoclavicular joints) involvement is nearly pathomnemonic, peripheral joints involved in < 30%
- Acne: typically severe in the form of hidradenitis suppurativa, nodulocystic acne w/ scarring
- Pustulosis: palmoplantar pustular disease is highly suggestive (neutrophilic dermatosis spectrum)
- Osteitis: bone pain +/- swelling early in disease → MRI w/ hyperostosis later in disease
- Other Autoinflammatory Diseases (Fever = Less Prominent)
- PAPA syndrome
- PASH syndrome
- DIRA syndrome
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- Sweet Syndrome 2/2 __
- Endogenous trigger
- Malignancy (AML & MDS = most common)
- Autoimmune disorders (IBD, Behcet's, SLE, RA, VEXAS, …)
- Exogenous trigger
- Drug-related (ICIs, TKIs, ATRA, G[M]-CSF, TMP-SMX)
- Post-infectious/vaccination (e.g. pneumococcus)
- Pyoderma Gangrenosum 2/2 __
- IBD (most common), RA
- Paraneoplastic
- Behcet's & others
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- Presentation
- Inflammation: high fevers, leukocytosis, LAD, atypical lymphocytosis
- Mild (more common): acute-onset (hours-days) eruption (usually beginning on flexural surfaces) of pinhead-sized pustules on background of erythroderma (i.e. edematous erythema)
- Severe (less common): the pustules coalesce, become bullous, & ulcerate (mucosal involvement is rare but possible, & a mimicker of SJS/TEN) → desquamation
- Endpoint DDx
- ~90%: drug reaction (antibiotics, anticonvulsants, calcium channel blockers)
- ~10%: immunologic reaction to infection (SARS CoV 2, PB19, CMV, Mycoplasma)
- Pathology
- Dense neutrophilic infiltrate of variable layers (epidermis-predominant)
- Necrotic keratinocytes, Intra-/Subcorneal spongiform pustules
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- Psoriasiform papules & plaques are “studded” with pustules
- Frequently affects the palms & soles (DIP involvement = clue)
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- Presentation
- Chronic, recurrent, sterile “folliculitis” affecting the seborrheic areas (face, chest, upper back); “folliculitis” means the pustules are centered on hair follicles (indicating predilection for the sebaceous glands)
- Endpoint DDx
- Classic (idiopathic): in young healthy adults (E. Asian most reported)
- Immunosuppression-related: AIDS-defining, hematologic malignancy, BMT/SOT
- Drug-related: rare
- Pathology
- Dense eosinophilic infiltrate of sebaceous glands
🧠 Mnemonic: “SNAPE”
Infectious ⚫
- Viral: VZV, HSV, Monkeypox
- Bacterial: secondary syphilis, disseminated gonococcemia
- Fungal: benign (dermatophytes), opportunistic (cryptococcosis, histoplasmosis, blastomycosis, talaromycosis)
- Parasitic: scabies
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Principles❗️
The Premise 🧠
- When you consider that Skin Pustules are fluid (WBC)-filled collections visible to the naked eye, it makes sense that this group of diseases are superficial diseases of the skin affecting the epidermis & dermis
- Because the dermis is the middle-most skin layer & affected in both Pustular/Nodular disease, the Neutrophilic Dermatosis DDx (i.e. disease of the dermis) is present in both frameworks!
- Below is a slide showing epidermal pathology (subcorneal pustules, neutrophilic spongiosis) in late-stage, severe AGEP
- Sebaceous Glands are located within the dermis, & this is the basis for EPF pathology showing eosinophilic infiltrate within the dermal layer
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References 📚
- 6/12/2025 NEJM Image Challenge
