Summary: Skin Lesions Morphology Histopathology And Investigations

Table of Contents
1. Classification of Skin Lesions
2. Levels of Fluid Collection and Blisters
3. Special Lesions and Purpura
4. Secondary Lesions
5. Shapes and Arrangement of Lesions
6. Basic Histopathology of Skin
7. Important Tests in Dermatology
View more

Classification of Skin Lesions

• Primary lesions: initial lesions. Key types:
  • Macule: flat, well-defined change in color, usually <1 cm.
  • Patch: larger macule >1 cm.
  • Papule: small, solid elevation <1 cm.
  • Plaque: elevated area >1 cm, often from fused papules.
  • Nodule: solid, palpable lesion >1 cm, deeper in skin.
  • Tumour: mass >2 cm.
  • Vesicle: clear fluid-filled blister <1 cm.
  • Bulla: fluid-filled blister >1 cm.
  • Pustule: pus-filled lesion (size variable).
  • Wheal: transient edematous elevation from plasma leakage.

Levels of Fluid Collection and Blisters

• Fluid can collect at epidermal, subcorneal, dermal-epidermal junction (DEJ), granular layer or suprabasal levels.
• Blister types: very flaccid, flaccid, and dense (tense) - differing in fragility and clinical visibility.
• Clinical outcomes after blistering:
  • Crusts/scales (burst blisters), erosions, tense blisters (heal without scarring), scarring with milia, or normal healing.
  • Pigmentation change after healing: hyperpigmentation or hypopigmentation may occur.
• Abscess: localized pus deep in dermis/subcutis.
• Pustule: pus within epidermis or follicle; may show hypopyon sign (horizontal pus level).

Special Lesions and Purpura

• Telangiectasia: permanent dilation of superficial blood vessels.
• Poikiloderma: combination of atrophy, telangiectasia and pigmentary change.
• Burrow: thread-like tunnel in stratum corneum made by mites.
• Comedone (open/black): follicular plug with oxidized content.
• Purpura: extravasation of red blood cells into skin or mucosa.
  • Types: non-palpable (e.g., due to low platelets) and palpable (small-vessel vasculitis).
  • Size terms: petechiae (small pinpoint) and ecchymoses (larger bruise-like).

Secondary Lesions

• Scale: visible shedding of epidermal cells. Various morphologies exist (silvery, greasy, powdery, collarette, leaf-like, fish-like).
• Crust: dried serum, pus or blood mixed with cells/debris (various colours).
• Excoriations: linear erosions from scratching.
• Erosions / Ulcers / Fissures: loss of epidermis (ulcers may have rolled/raised edges; fissures are linear splits).
• Scar: connective tissue replacing dermis after deeper injury.
• Lichenification: thickened, hyperpigmented skin with exaggerated markings from chronic rubbing.
• Atrophy: tissue loss from epidermis or dermis.
• Induration: increased firmness to touch.
• Mnemonic to remember common secondary changes:
  • L - Lichenification
  • E - Erosion
  • A - Atrophy
  • F - Fissure
  • S - Scale
  • U - Ulcer
  • C - Crust
  • K - Kind-of-hard to feel (induration)
  • S - Scar

Shapes and Arrangement of Lesions

• Annular: ring-shaped with active raised edge and clear center.
• Discoid / Nummular: coin-shaped lesions without central clearing.
• Lesions may arise on normal skin after trauma or follow patterns:
  • Iso: new lesions resemble the original.
  • Reverse Koebner's phenomenon: trauma causes local lesion reduction; remote reverse form affects distant lesions after single trauma.
• Other arrangements:
  • Linear nodules with discharging sinuses: spread along lymphatics producing nodules that may discharge.
  • Target lesions: concentric zones (three zones typical; two-zone atypical targets in severe forms).
  • Dermatomal: follow spinal/cranial nerve distribution (trunk horizontal, limbs longitudinal).
  • Blaschkoid: follow embryonic migration lines of skin cells, not nerves/vessels/lymphatics.

Basic Histopathology of Skin

• Acanthosis: thickening of spinous layer.
• Colloid (Civatte) body: degenerated keratinocyte appearing as eosinophilic ovoid body in lower epidermis/dermis.
• Dyskeratosis: abnormal keratinization of epidermal cells.
• Corps rond: round epidermal cells (term for grain-like cells).
• Epidermotropism: migration of atypical lymphoid cells into epidermis.
• Hypergranulosis: thickened granular layer.
• Hyperkeratosis: thickened stratum corneum.
• Parakeratosis: nucleated cells in stratum corneum.
• Spongiform pustule of Kogoj, Munroe's microabscess, Papillary tip microabscess, Pautrier's microabscess: neutrophil or lymphocyte collections at characteristic epidermal/dermal sites.
• Spongiosis: intercellular edema within epidermis.
• Acantholysis: loss of intercellular connections between keratinocytes.

Important Tests in Dermatology

• Wood's Light Examination: ultraviolet light ~365 nm used with a specific filter to assess fluorescence of skin and lesions; helps highlight pigment changes and fluorescent substances.
• Tzanck Smear: smear of blister fluid stained (Giemsa/Wright/Pap) to show characteristic cells or inclusion bodies; may reveal uninucleate or multinucleate acantholytic cells and viral or parasitic inclusions; negative in certain blistering diseases.
• Skin Prick Test: introduces small amounts of suspected allergens into epidermis with a prick; measure wheal and flare after ~15 minutes to identify immediate hypersensitivity.
• KOH Mount: potassium hydroxide preparation clears keratin to visualize fungal elements, mites or characteristic fungal bodies in skin, hair and nails.