Distribution: Geographical Map of Diagnosis

Introduction

The Distribution of a rash (where it is) is often more diagnostic than the Morphology (what it looks like). Diseases have "preferred habitats" determined by anatomy (hair follicles, sweat glands), physics (sunlight, gravity), or nervous system innervation.

This chapter maps dermatoses to their anatomical territories.

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10.1 Generalized Distributions

Universal vs Generalized

• Generalized: Widespread but has areas of normal skin intact.
  • Examples: Viral exanthem, Drug eruptions.
• Universal: 100% of skin surface involved (including hair/nails).
  • Examples: Alopecia Universalis.
• Erythroderma: Redness >90% of BSA.

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10.2 Photosensitive Distribution ("V-Sign")

Lesions strictly limited to sun-exposed areas.

• Key Areas Involved: Face (sparing philtrum/upper lip fold - "Nose Shadow"), V of Neck, Dorsum of Hands/Forearms.
• Key Areas Spared: Submental (under chin), Retroauricular (behind ears), Watch strap area.
• Differential Matrix:
  • Lupus Erythematosus: Butterfly rash.
  • Dermatomyositis: Heliotrope, Shawl sign.
  • Pellagra: Casal's necklace.
  • Phototoxic Drug: Severe sunburn pattern.
  • Polymorphous Light Eruption (PMLE): Papules/vesicles in Spring.

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10.3 Acral Distribution (Extremities)

Involving the hands, feet, ears, and nose.

• Palmoplantar: Palms and Soles.
  • Syphilis: Copper-red papules + Biett's collarette.
  • RMSF: Petechiae.
  • Hand-Foot-Mouth: Elliptical gray vesicles.
  • Erythema Multiforme: Target lesions.
• Distal Fingers/Toes:
  • Chilblains (Pernio): Cold-induced violaceous papules.
  • Vasculitis: Palpable purpura.

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10.4 Flexural vs Extensor (Atopic vs Psoriatic Rule)

Extensor Surfaces

Elbows, Knees, Lumbar Back, Scalp.

• Pattern: Skin is thicker and subject to trauma (Koebner).
• Top Dx: Psoriasis, Dermatitis Herpetiformis, Erythema Elevatum Diutinum.

Flexural Surfaces (Intertriginous)

Antecubital fossa, Popliteal fossa, Axillae, Groin, Inframammary.

• Pattern: Skin is occluded, warm, and moist.
• Top Dx: Atopic Dermatitis (Child/Adult), Inverse Psoriasis, Tinea Cruris, Candidiasis, Erythrasma.
  • Differentiation in Groin:
    • Tinea: Spares the scrotum. Active border.
    • Candida: Beefy red, satellite pustules. Involves scrotum.
    • Inverse Psoriasis: Shiny red, no scale. Well defined.

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10.5 Seborrheic Distribution

Areas rich in sebaceous glands (T-zone of face, Scalp, Pre-sternal, Interscapular).

• Top Dx: Seborrheic Dermatitis, Acne Vulgaris, Pityriasis Versicolor, Pemphigus Foliaceus.

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10.6 Zosteriform (Dermatomal) Distribution

Unilateral, strictly distinguishing the movement.

• Pattern: Follows a sensory nerve dermatome. DOes NOT cross the midline.
• Major Dx: Herpes Zoster (Shingles).
• Pearl: If it crosses the midline significantly, it is likely NOT Zoster (consider Herpes Simplex or Contact Dermatitis).

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10.7 Blaschkolinear Distribution

Follows lines of Blaschko (embryonic cell migration paths).

• Appearance: V-shaped on back, S-shaped on abdomen, Linear on limbs.
• Significance: Indicates Mosaicism (two cell lines in one person).
• Dx: Incontinentia Pigmenti, Epidermal Nevus, Lichen Striatus.

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10.8 Lymphangitic Distribution

Linear red streak extending proximally from a wound towards a lymph node.

• Dx: Acute Lymphangitis (Strep), Sporotrichosis (Sporothrix schenckii - "Rose Gardener's" nodular lymphangitis).

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Diagnostic Algorithm: By Location