Skin Nodules (Diffuse)

• Nodule: circumscribed skin elevation; Depth up to 2 cm: deeper skin layers affected (dermis → subQ fat)

• Pustule: skin elevation with visible collection of leukocytes (”pus”); Depth less than 1 cm: superficial skin layers affected (epidermis → dermis)

The Premise 🧠

• If you think of Skin Nodules as representing deeper diseases of the skin affecting the dermis & subQ fat, an anatomical breakdown of the DDx makes sense (see above)!

• 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!

• Infiltrative diseases land in the dermis due to its rich vascular supply, dense connective tissue (allowing for accumulation & collection of infiltrative cells/molecules), & active immune surveillance. (If you think of the dermis as being strategically located between the epidermis & subQ tissue, it makes sense this is a key place for WBCs to accumulate & fight off foreign cells.)

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 (pustule → 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

• Panniculitis: inflammation of the subcutaneous fat

• Leukemia cutis: cutaneous infiltration of cancer cells (Occurs in 10-15% of AML patients, Less common in chronic myeloproliferative neoplasms)

⚫

Skin Pustules (Diffuse)

• Framework adapted from @rabihmgeha’s