AL

Skin Lesions: Primary and Secondary – Quick Reference

Primary skin lesions

  • Initial morphological changes of the skin; categorized as: macule/patch, papule/plaque, tuber, nodule, vesicle/bulla, pustule, wheal, ulcer, crust, tumor.

Macule / Patch

  • Circumscribed flat discoloration on the skin.

  • Color changes due to: hemoglobin, melanin, exogenous pigment; can be vascular origin or endogenous pigment.

  • Examples: freckles (ephelis), vitiligo, halo/Sutton nevus, exogenous pigment tattoo.

Infiltrative primary skin lesions

  • Papule/plaque/tuber/nodule represent solid infiltrative lesions.

Papule

  • Infiltrative, elevated solid lesion.

  • Epidermal papule: < 0.5 cm in diameter.

  • Plaque: > 0.5 cm in diameter.

  • Intradermal variant exists (papule within dermis).

  • Intradermal papule (usually does not go passed the papillary dermis)

Tuber

  • Cellular infiltration involving the entire depth of the dermis.

Nodule

  • Circumscribed, elevated lesion with cellular infiltration in the dermis and subcutis.

Examples linked to primary lesions

  • Lichen planus, psoriasis (papule/plaque type), erythema multiforme.

  • Peutz-Jeghers syndrome (STK11 gene mutation); ephelis (freckles); vitiligo; halo/Sutton nevus; tattoo;

  • Erythema (general redness); telangiectasia; erythema ab igne (erythematous reticular hyperpigmentation).

Exudative primary skin lesions

  • Urticaria / wheal: transient dermal edema.

  • Vesicle: fluid within the epidermis or just below the basal layer (intraepidermal or subepidermal).

  • Bulla: larger fluid-filled blister (larger than a vesicle).

  • Pustule: collection of pus in or beneath the epidermis.

  • Tumor: large solid mass of tissue in the skin.

Secondary lesions

  • Scales / desquamation

  • Erosion / Excoriation

  • Fissure / rhagades

  • Ulcer

  • Crust

  • Scar (cicatrix) / hypertrophic scar / keloid / atrophic scar

  • Atrophy

  • Lichenification

Scales / desquamation

  • Cornified epithelial cells or detached fragments of stratum corneum.

  • Types: pityriasis (fine scales, dandruff); psoriasiform (white, noncoherent scales); ichthyosiform (large scales); exfoliative (large sheet-like); collarette (scales around a lesion).

  • Color cues: silvery-white (psoriasis), oily/yellow (seborrheic dermatitis).

Erosion vs Excoriation

  • Erosion: loss of epidermis above the basement membrane; no scarring.

  • Excoriation: loss of papillary dermis; may scar.

Fissure

  • Painful, deep linear opening of the skin.

  • Often at skin folds (mouth corners, hands) due to stretching of brittle skin.

Ulcer

  • Skin defect with loss of epidermis and upper papillary dermis; may extend deeper into subcutis.

  • Always occurs with pathologically altered tissue; can form a scar.

Crust

  • Dried secretion (various colors: blood-red/brown, serum-yellow, purulent honey-yellow/green).

  • Associated with necrosis (ecthyma).

Scar / Cicatrix

  • Permanent fibrous tissue from repair.

  • Types: hypertrophic scar, keloid, atrophic scar.

Atrophy

  • Thinning of the dermis and epidermis; loss of skin appendages; regressive changes (age/actinic).

Lichenification

  • Increased epidermal thickness with distinct skin markings (cobblestone surface); from chronic rubbing or scratching.

Quick recall concepts

  • Primary vs Secondary: primary are initial lesion types; secondary arise from evolution of primary (e.g., scales from papules, crust from vesicles).

  • Depth and location differentiate lesions (epidermal vs intradermal vs subcutaneous; intraepidermal vs subepidermal vesicles/bulla).

  • Color and texture clues help infer etiology (pigment vs vascular changes; inflammatory vs infectious processes).