Common Dermatoses: Differential Diagnosis

Introduction

Common things are common. However, they are often confused with each other. This chapter focuses exclusively on the differentiation between the "Big Five" red scaly rashes: Psoriasis, Eczema (Atopic), Seborrheic Dermatitis, Tinea, and Pityriasis Rosea.

11.1 "Papulosquamous" Differential

When you see a red, scaly plaque, use this matrix to distinguish the etiology.

Feature	Psoriasis	Atopic Dermatitis (Eczema)	Seborrheic Dermatitis	Tinea Corporis	Pityriasis Rosea
Primary Lesion	Well-defined Plaque	Ill-defined Patch/Plaque	Greasy Macule/Patch	Annular Plaque	Oval Macule
Scale	Silvery, Micaceous, Thick	Fine, diffuse	Yellow, Greasy, Bran-like	Peripheral only	Collarette (inward)
Edge	Sharp ("cliff-drop")	Fades out	Indistinct	Active, Raised	Distinct
Site	Extensor, Sacrum, Scalp	Flexural, Face, Neck	Sternal, Glabella, NLF	Exposed skin, Asymm	Trunk ("Christmas Tree")
Nails	Pitting, Oil spots	Polishing (shiny)	Normal	Onychomycosis	Normal
Symptoms	Pain/Itch	Intense Itch	Mild Itch	Mild Itch	Asymptomatic/Mild
Sign	Auspitz +	Lichenification	-	KOH Positive	Herald Patch

11.2 Acne vs Rosacea vs Perioral Dermatitis

Three common facial rashes that mimic each other.

Acne Vulgaris

Defining Feature: Comedones (Blackheads/Whiteheads).
Deep Lesions: Nodules/Cysts common.
Site: T-zone, extends to chest/back.
Scarring: Yes (Ice-pick, boxcar).

Rosacea

Defining Feature: Erythema + Telangiectasia.
Lesions: Papules/Pustules (inflammatory) but NO Comedones.
Site: Central face convexities (Cheeks, Nose, Chin, Forehead). Spares peri-ocular skin.
Triggers: Flushing (Alcohol, Heat, Spice).

Perioral Dermatitis

Defining Feature: Monomorphic micropapules/pustules.
Site: Around mouth, nose (Perialar), eyes.
KEY SIGN: Vermilion Border Sparing (a thin rim of clear skin typically surrounds the lips).
Trigger: Often steroid-induced.

11.3 Tinea vs Mimics

Tinea Corporis (Ringworm)

Morphology: Annular. Center is clear(er). Border is active (red/scaly/raised).
Confirmation: Scrape the edge (active border) for KOH.

Granuloma Annulare (Mimic)

Morphology: Annular. Center is clear. Border is smooth and dermal (composed of papules), NOT scaly.
Differentiation: No scale = Not fungal.

Erythema Migrans (Lyme)

Morphology: "Bull's Eye". Center red, clear ring, outer red ring. Rapidly expanding (>5cm).
Differentiation: Macular (flat), non-scaly. History of tick exposure.

11.4 Scabies: "Great Imitator"

Scabies mimics eczema, but eczema treatment (steroids) makes Scabies worse ("Scabies Incognito").

Diagnostic Triad:

Burrows: Wavy, gray, 5mm lines. (Web spaces, wrists).
Nocturnal Pruritus: Itch worsens significantly at night.
Family History: Others are itchy.

The "Circle of Hebra": Scabies involves the warm areas: Axillae, Belt line, Buttocks, Genitalia (nodules), Web spaces. It SPARES the mid-back and head (in adults).

11.5 Diagnostic Algorithm: Red Face

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