Differential Diagnosis of Erythematous Macules

Introduction

Erythema, defined as redness of the skin due to capillary dilation, is one of the most common yet challenging dermatological signs. The differential diagnosis is vast, ranging from trivial causes (sunburn) to life-threatening emergencies (necrotizing fasciitis). This chapter focuses on the systematic evaluation of erythematous macules and patches—flat areas of redness without elevation or palpable texture change.

The key clinical distinctions are:

Blanching vs. Non-Blanching: Does the redness disappear with pressure (diascopy)?
Permanence: Is the erythema fixed or paroxysmal (comes and goes)?
Configuration: Is it annular, reticulate (livedo), or diffuse?
Color Nuance: Pink vs. Violaceous vs. Orange-red.
Associated Symptoms: Pain, warmth, edema (inflammation), or systemic signs (fever).

17.1 Diascopy Test: Blanching vs. Non-Blanching

This fundamental bedside test separates vascular dilation from extravasated blood.

[!IMPORTANT] Technique: Press a glass slide (or dermatoscope's glass plate) firmly against the lesion and observe.

Blanching (Erythema): Redness disappears completely. Blood is within vessels.

Non-Blanching (Purpura): Redness persists. Blood is extravasated into dermis.

Finding	Implication	Examples
Complete Blanching	Inflammatory erythema	Erysipelas, Urticaria, Drug eruption
Partial Blanching	Mixed vasodilation + extravasation	Vasculitis (early), Stasis dermatitis
Non-Blanching	Purpura	Vasculitis (leukocytoclastic, IgA), Thrombocytopenia
Blanching with "Apple Jelly" residue	Granulomatous inflammation	Sarcoidosis, Lupus Vulgaris (Cutaneous TB)

17.2 Classification by Temporal Pattern

17.2.1 Paroxysmal (Transient) Erythema

Erythema that comes and goes, lasting minutes to hours.

Flushing:
- Neurogenic: Emotional flush (face/neck), Menopausal (hot flashes).
- Pharmacologic: Niacin, Calcium channel blockers, Alcohol (esp. with Disulfiram or in Asian faces).
- Carcinoid Syndrome: Paroxysmal flushing + diarrhea + bronchospasm. Due to serotonin secretion.
- Pheochromocytoma: Paroxysmal hypertension + pallor OR flushing.
Urticaria: Wheals (evanescent, migratory, pruritic). Individual lesions last <24 hours.
Autoinflammatory Diseases:
- Familial Mediterranean Fever (FMF): Erysipelas-like erythema on lower legs + serositis + fever.
- Adult-Onset Still's Disease: Evanescent salmon-pink rash on trunk during fever spikes.

17.2.2 Fixed (Persistent) Erythema

Erythema that remains in the same location.

Infectious:
- Erysipelas: Well-demarcated, raised, tender, warm plaque (usually lower leg or face). "Peau d'orange" texture.
- Cellulitis: Poorly demarcated, deeper erythema and edema.
- Erythema Migrans: Expanding annular erythema at site of tick bite (Lyme disease).
- Herpes Zoster (Early): Dermatomal erythema BEFORE vesicles appear.
Neoplastic:
- Mycosis Fungoides (Patch Stage): Fixed, well-demarcated erythematous patches, often on "bathing trunk" areas. May have subtle atrophy or scaling.
- Angiosarcoma: Purplish-red plaque or bruise-like lesion on the scalp of elderly patients. HIGH MORTALITY.
Inflammatory/Autoimmune:
- Lupus Erythematosus (Acute ACLE): Malar ("butterfly") rash, sparing nasolabial folds.
- Dermatomyositis: Heliotrope (violaceous) erythema of eyelids. Shawl sign.
- Fixed Drug Eruption: Round, well-demarcated erythematous plaque that recurs at the same site upon drug re-exposure.

17.3 Classification by Configuration

17.3.1 Annular Erythema

Ring-shaped lesions with central clearing (or normal skin in the center).

Condition	Key Features
Erythema Migrans (Lyme)	Expanding ring at tick bite site. Central clearing. "Bull's eye" appearance.
Erythema Annulare Centrifugum	Slowly migrating annular plaques with a "trailing scale" at the inner border.
Tinea Corporis	Annular, scaling plaque with raised, active border. KOH positive.
Granuloma Annulare	Annular or arcuate papules, NON-scaly. Dermoscopy: focally thickened collagen.
Subacute Cutaneous Lupus (SCLE)	Photosensitive, polycyclic, annular lesions on upper trunk. Ro/SSA antibody.

17.3.2 Reticulate Erythema (Livedo)

Net-like, lacy pattern due to sluggish blood flow in dermal venules.

Livedo Reticularis (Physiologic): Symmetric, reticulated, on lower extremities. Reversible with warming. Benign.
Livedo Racemosa (Pathologic): Asymmetric, broken, irregular network. Does NOT reverse with warming. Associated with:
- Antiphospholipid Syndrome: Recurrent thrombosis, miscarriages.
- Sneddon Syndrome: Livedo racemosa + stroke.
- Polyarteritis Nodosa: Livedo + subcutaneous nodules + systemic vasculitis.

17.3.3 Figurate Erythema

Gyrate, polycyclic, or arcuate patterns.

Erythema Gyratum Repens: Rapidly moving, concentric, "wood-grain" pattern. PARANEOPLASTIC (often lung cancer).
Erythema Marginatum: Evanescent, annular, non-pruritic. Seen in Acute Rheumatic Fever.
Necrolytic Migratory Erythema: Migratory, erosive, crusted, annular erythema. Seen with Glucagonoma.

17.4 Classification by Color Nuance

The shade of red provides diagnostic clues.

Color	Implication	Examples
Bright Red (Scarlet)	Acute inflammation, toxin-mediated	Scarlet Fever, TSS, Sunburn
Dusky Red / Violaceous	Ischemia, venous congestion, dermatomyositis	Cutaneous LE, Dermatomyositis, Chilblains
Orange-Red / Salmon	Mycosis Fungoides, Pityriasis Versicolor	MF (patch stage), PV
Pink / Pale Red	Early inflammation, drug eruption	Morbilliform drug rash
Brown-Red	Hemosiderin staining from old hemorrhage	Stasis dermatitis, Pigmented Purpuric Dermatosis

17.5 "Do Not Miss" Diagnoses in Erythema

[!CAUTION] Red Flags Requiring Urgent Action

Necrotizing Fasciitis / Deep Infection:
- Pain OUT OF PROPORTION to clinical findings.
- Rapid spread of erythema.
- Systemic toxicity (fever, tachycardia, hypotension).
- Crepitus on palpation.
- Action: IV antibiotics + URGENT surgical debridement.
Erysipelas / Cellulitis with Systemic Signs:
- Fever, chills, lymphangitis.
- Monitor for progression despite antibiotics.
Mycosis Fungoides:
- Any fixed erythematous patch that fails to respond to topical steroids.
- Especially on non-sun-exposed areas (buttocks, medial thighs).
- Action: Skin biopsy.
Angiosarcoma of the Face/Scalp:
- Violaceous plaque or "bruise" on the scalp of an elderly patient that doesn't heal.
- Action: Biopsy urgently.
Erythema Migrans (Lyme Disease):
- Expanding annular erythema after potential tick exposure.
- Action: Empiric doxycycline. Do not wait for serology.

17.6 Diagnostic Flowchart: Approach to Erythema

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17.7 Summary Table: Key Differentials of Erythema

Feature	Erysipelas	Cellulitis	Erythema Migrans	Lupus (ACLE)	Mycosis Fungoides
Border	Well-demarcated, raised	Poorly demarcated	Expanding annular	Malar (butterfly)	Well-demarcated
Location	Face, Lower leg	Lower leg	Tick bite site	Face	Bathing trunk
Texture	"Peau d'orange"	Edematous	Flat	Flat	May have subtle scale/atrophy
Systemic Signs	Fever, chills	Fever	May have arthralgias, fatigue	Arthritis, nephritis	None early
Diagnosis	Clinical	Clinical	Clinical + Serology	ANA, dsDNA	Skin Biopsy

17.8 Contiguous Inflammation: Hidden Cause

Erythema can be the surface manifestation of deep-seated infection or inflammation.

[!WARNING] Always consider underlying structures.

Periorbital/Facial Erythema: Rule out Sinusitis (Ethmoiditis), Cavernous Sinus Thrombosis.
Overlying a Joint: Consider Septic Arthritis, Osteomyelitis.
Breast Erythema: Inflammatory Breast Carcinoma vs. Mastitis.
Perineal Erythema (Child): Perianal Streptococcal Disease.