Differential Diagnosis of Scaling

Introduction

Scaling (desquamation) refers to the visible shedding of the stratum corneum. It results from abnormal keratinization (increased or aberrant production) or increased turnover of the epidermis. The quality, color, distribution, and adherence of scales provide diagnostic clues.

40.1 Scale Characteristics

Scale Type	Appearance	Examples
Fine/Branny	Thin, powdery, easily detached	Pityriasis Versicolor, Pityriasis Alba
Silvery/Micaceous	Thick, silvery, layered "fish scales"	Psoriasis
Lamellar/Plate-like	Large, adherent sheets	Ichthyosis
Greasy/Waxy	Yellow, adherent	Seborrheic Dermatitis
Collarette	Peripheral ring of scale	Pityriasis Rosea, Secondary Syphilis
Keratotic/Thick	Hard, adherent	Actinic Keratosis, SCC

40.2 Major Causes of Scaling

40.2.1 Psoriasis

Clinical Features:
- Well-demarcated, erythematous plaques with silvery-white, micaceous scale.
- Auspitz sign: Pinpoint bleeding when scale removed (capillary loops exposed).
- Koebner phenomenon: Lesions at sites of trauma.
Distribution: Extensor surfaces (elbows, knees), Scalp, Sacrum, Nails.
Nail Changes: Pitting, Oil spots, Onycholysis, Subungual hyperkeratosis.

40.2.2 Seborrheic Dermatitis

Clinical Features:
- Erythematous patches with greasy, yellowish scale.
- Pruritus variable.
Distribution: Seborrheic areas—Scalp (dandruff), Eyebrows, Glabella, Nasolabial folds, Ears, Presternal area.
Associations: HIV (severe seborrheic dermatitis), Parkinson's disease.

40.2.3 Pityriasis Rosea

Clinical Features:
- Herald Patch: Large (2-5cm), oval, salmon-pink patch with collarette scale (fine scale at periphery).
- 1-2 weeks later: Widespread smaller oval patches on trunk.
- "Christmas Tree" distribution: Long axes of lesions follow Langer's lines.
Demographics: Young adults. Often preceded by viral prodrome.
Course: Self-limited (6-8 weeks).

40.2.4 Tinea Corporis (Ringworm)

Clinical Features:
- Annular, erythematous plaques with active, raised, scaly border and central clearing.
- Pruritic.
Diagnosis: KOH preparation shows hyphae.

40.2.5 Ichthyoses

Inherited disorders of keratinization.

Type	Inheritance	Clinical Features
Ichthyosis Vulgaris	AD	Fine, white scales. Spares flexures. Associated with atopy.
X-Linked Ichthyosis	XLR	Large, dark scales. Involves flexures. Corneal opacities.
Lamellar Ichthyosis	AR	Collodion baby → Plate-like scales. Ectropion, eclabium.
Epidermolytic Ichthyosis	AD	Blistering at birth → Thick, verrucous scales.

40.2.6 Atopic Dermatitis (Eczema)

Features: Pruritic, erythematous patches with fine scale and lichenification (chronic rubbing).
Distribution: Flexural in children/adults (antecubital, popliteal fossa). Extensor in infants.
Associations: Atopic triad (asthma, allergic rhinitis).

40.2.7 Secondary Syphilis

[!WARNING] "Great Imitator"

Features: Widespread, non-pruritic, ham-colored (copper) papules and plaques with collarette scale.
Palmoplantar Involvement: Scaling papules on palms and soles—highly suggestive.
Other Signs: Lymphadenopathy, Condyloma lata (genital warts), Patchy alopecia ("moth-eaten").
Diagnosis: RPR/VDRL, FTA-ABS.

40.2.8 Actinic Keratosis (Solar Keratosis)

Clinical Features:
- Rough, scaly macules or plaques on sun-damaged skin.
- "Feels like sandpaper."
- Adherent scale.
Significance: Pre-malignant. Can progress to SCC.

40.3 Diagnostic Algorithm

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40.4 Summary Comparison Table

Condition	Scale Type	Distribution	Key Clue
Psoriasis	Silvery, Micaceous	Extensor, Scalp	Auspitz sign, Nail pitting
Seborrheic Dermatitis	Greasy, Yellow	Scalp, Face folds	Dandruff
Pityriasis Rosea	Collarette	Trunk (Christmas tree)	Herald patch
Tinea	Annular, Active edge	Variable	KOH positive
Ichthyosis	Lamellar/Fine	Generalized	Childhood onset
Atopic Dermatitis	Fine + Lichenification	Flexural	Pruritus, Atopy
Secondary Syphilis	Collarette, Copper	Generalized + Palms/Soles	RPR+, Lymphadenopathy
Actinic Keratosis	Rough, Adherent	Sun-exposed	Sandpaper texture

40.5 Clinical Pearls

Silvery scale on extensor surfaces → Psoriasis. Check nails and scalp.
Greasy scale on face and scalp → Seborrheic dermatitis. Consider HIV if severe.
Herald patch + Christmas tree pattern → Pityriasis rosea. Self-limited.
Annular with active border → Tinea. KOH scraping.
Scaling on palms and soles → Secondary syphilis until proven otherwise. RPR.
Generalized scaling from birth → Ichthyosis. Subtypes determined by pattern and inheritance.
Rough "sandpaper" on sun-exposed skin → Actinic keratosis. Pre-malignant.