Primary Lesions: Alphabets of Diagnosis

Introduction

The primary lesions are the fundamental morphological units of dermatology. Accurate identification distinguishes between vast categories of disease (e.g., a vesicle implies a totally different differential to a papule).

This chapter defines the strict semiological criteria for each lesion type.

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2.1 Non-Palpable Lesions

Macule (<1cm) and Patch (>1cm)

Definition: A change in color without any change in skin texture, thickness, or elevation.

• Test: If you close your eyes and run your finger over it, you cannot feel it.
• Colors & Meaning:
  • Red (Erythema): Vasodilation (blanches with pressure). Ex: Viral exanthem.
  • Red (Purpura): Extravasation of blood (does NOT blanch). Ex: Vasculitis.
  • Brown: Melanin in epidermis/dermis. Ex: Lentigo.
  • White: Hypopigmentation (loss of melanin) or depigmentation (absence). Ex: Vitiligo.
  • Blue: Deep pigment (Tyndall effect). Ex: Mongolian spot.

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2.2 Palpable Solid Lesions

Papule (<1cm)

Definition: Solid, elevated lesion.

• Surface Differentiation:
  • Acuminate (Pointed): Follicular origin. Ex: Keratosis Pilaris.
  • Dome-shaped: Dermal origin. Ex: Intradermal Nevus.
  • Flat-topped: "Table mount". Ex: Lichen Planus.
  • Umbilicated: Central dell. Ex: Molluscum Contagiosum.
  • Verrucous: Rough, finger-like projections. Ex: Verruca vulgaris.

Plaque (>1cm)

Definition: A solid, raised lesion where the diameter is greater than the height ("Plateau-like").

• Formation: Often formed by the coalescence of papules.
• Examples: Psoriasis, Eczema, Mycosis Fungoides.
• Differential: Differentiate from a "Patch" (which is flat). A plaque has palpable substance.

Nodule (>1cm)

Definition: A solid lesion with a significant deep component (dermal or subcutaneous).

• Differentiation: It feels like a "marble" or "knot" under the skin, whereas a plaque feels like a thickening of the skin.
• Examples: Erythema Nodosum, Lipoma, Cyst.

Wheal (Hive)

Definition: A firm, edematous plaque resulting from infiltration of the dermis with fluid.

• Key Features: Transient (individual lesions last <24h). Compressible. Often surrounded by a red "flare".

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2.3 Fluid-Filled Lesions

Vesicle (<1cm)

Definition: Circumscribed elevation containing clear fluid (serum).

• Arrangement:
  • Grouped: Herpes Simplex.
  • Linear: Contact Dermatitis (Poison Ivy).
  • Scattered: Varicella.

Bulla (>1cm)

Definition: Large fluid-filled blister.

• Tense vs Flaccid:
  • Tense: Roof is thick (full thickness epidermis). Hard to rupture. Ex: Bullous Pemphigoid.
  • Flaccid: Roof is thin (stratum corneum/partial epidermis). Ruptures easily. Ex: Pemphigus Vulgaris.

Pustule (Any Size)

Definition: Circumscribed elevation containing purulent fluid (neutrophils).

• Color: White, Yellow, or Green-tinged.
• Follicular: Centered on a hair (Folliculitis).
• Non-Follicular: On smooth skin (Pustular Psoriasis).

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2.4 Surface Changes (Secondary)

Scale

Definition: Flaking of the Stratum Corneum.

• Types:
  • Silvery/Micaceous: Psoriasis.
  • Greasy/Yellow: Seborrheic Dermatitis.
  • Fine/Powdery: Tinea Versicolor.
  • Collarette: Ring of scale peeling outwards. Pityriasis Rosea.

Crust

Definition: Dried exudate (serum, blood, or pus).

• Honey-colored (Meliceric): Impetigo (Staph/Strep).
• Hemorrhagic: Vasculitis or Excoriation.

Erosion vs Ulcer

• Erosion: Loss of Epidermis ONLY. Heals without scarring.
• Ulcer: Loss of Epidermis + Dermis. Heals with scarring.

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2.5 Diagnostic Table: Palpability vs Content