Anomalies of the Skin Surface

Introduction

The skin surface—the visible interface between the epidermis and the external environment—provides a wealth of diagnostic information. While previous chapters focused on lesion color (flat lesions), palpability (solid lesions), and content (fluid-filled lesions), this chapter examines alterations of the skin surface itself: scaling, crusting, keratosis, erosion, ulceration, and necrosis.

These surface changes were historically termed "secondary lesions," implying evolution from a "primary lesion." However, this terminology is inaccurate—surface alterations frequently appear as the initial manifestation of disease and may exist independently. More importantly, the precise characterization of surface abnormalities directly predicts underlying epidermal pathology, as these changes reflect processes occurring in the epidermis and stratum corneum.

Normal skin is smooth and dry (except for eccrine sweat), with visible microrelief (dermatoglyphics). Gentle curetting does not produce scale. Any deviation from this baseline constitutes a surface anomaly requiring systematic characterization.

Classification of Surface Anomalies

Surface abnormalities can be classified by the nature of the alteration:

Loading diagram...

Skin Surface Is Too Thin: Atrophy

When the epidermis is thinned, the skin becomes transparent and wrinkled, with visible underlying vessels. This is epidermal atrophy, often part of dermo-epidermal atrophy.

Feature	Description
Texture	Smooth, shiny, "cigarette paper"
Transparency	Underlying vessels visible
Wrinkling	Fine, superficial wrinkles
Microrelief	Absent or reduced

[!NOTE] Atrophy is more visible than palpable when it primarily affects the epidermis. Combined dermo-epidermal atrophy produces both visible thinning and palpable depression.

Skin Surface Is Too Thick: Keratosis

Thickening of the stratum corneum produces keratosis—circumscribed or diffuse lesions that are yellow, hard, adherent, and rough.

Keratosis

Keratosis is thickening of the stratum corneum that is broader than it is tall. Key features:

Characteristic	Description
Texture	Hard, inflexible, rough
Adherence	Very adherent; curette cannot remove
Color	Yellow to brown
Microrelief	May be preserved (callus) or distorted (wart)

Common causes of keratosis:

Callus/corn (mechanical)
Actinic keratosis (photodamage)
Seborrheic keratosis
Palmoplantar keratodermas
Porokeratosis

Cutaneous Horn

A cutaneous horn (cornu cutaneum) is a keratosis that is taller than it is broad—an exophytic projection of keratin resembling a horn.

Feature	Description
Morphology	Conical, horn-like projection
Base	Examine for underlying pathology
Differential	Wart, actinic keratosis, SCC, seborrheic keratosis

[!IMPORTANT] Always examine the base of a cutaneous horn. Up to 20% have SCC at the base. Malignancy is more likely if: tenderness, induration, larger size, or location on sun-damaged skin.

Horny Plug

A horny plug is a millimeter-wide, punctate keratosis filling and covering the hair follicle. Classic example: follicular keratotic papules of discoid lupus erythematosus (carpet tack sign when removed).

Porokeratosis

Porokeratosis is characterized by a well-demarcated lesion with a distinctive keratotic collarette (cornoid lamella) at the margin—an elevated, thin, adherent ridge of keratin.

Type	Features
Disseminated superficial actinic	Multiple small lesions, sun-exposed
Mibelli	Single plaque, childhood onset
Linear	Following Blaschko's lines
Palmoplantar	Punctate pits on palms/soles

Skin Surface Is Absent: Erosion, Ulceration, Necrosis

Loss of skin surface represents disruption of epidermal continuity. The depth of the loss determines the terminology and prognosis.

Erosion

An erosion is a superficial loss of the epidermis without dermal involvement. It heals without scarring.

Feature	Description
Depth	Epidermis only
Base	Moist, red (dermal papillae visible as red dots)
Healing	Without scar
Cause	Ruptured vesicle/bulla, trauma, excoriation

Excoriation is an erosion produced by scratching—linear erosions with hemorrhagic crust, often in accessible areas.

Fissure

A fissure is a fine, linear, superficial loss of substance without erosion of the dermis. Fissures occur in:

Hyperkeratotic skin (palmoplantar)
Angular cheilitis (lip commissures)
Interdigital tinea
Perianal dermatitis

Ulceration

An ulceration is a deeper loss of cutaneous substance affecting both epidermis and dermis. Ulcerations heal with scarring.

Feature	Description
Depth	Through dermis ± subcutis
Base	Fibrin, granulation tissue, or necrotic
Borders	Undermined, punched-out, sloping, rolled
Healing	With scar formation

Border Characteristics

The borders of an ulcer provide diagnostic clues:

Border Type	Appearance	Suggests
Punched-out	Vertical, sharp edges	Arterial, neuropathic
Undermined	Overhanging edge	Pyoderma gangrenosum
Sloping	Gradual transition	Venous
Rolled/raised	Elevated, pearly	BCC, SCC
Violaceous	Purple/dusky	Vasculitis, PG

Base Characteristics

Base Type	Description	Significance
Granulating	Beefy red, friable	Healthy healing
Fibrinous	Yellow-white coating	Slough, needs debridement
Necrotic	Black eschar	Devitalized tissue
Clean	Pink/red, minimal exudate	Well-healing

Ulcer Classification

An ulcer (as distinct from acute ulceration) is a chronic loss of substance (>1 month) without tendency to spontaneous healing.

Ulcer Type	Location	Features
Venous	Gaiter area (medial malleolus)	Shallow, irregular, lipodermatosclerosis
Arterial	Distal, pressure points	Punched-out, pale base, painful
Neuropathic	Pressure points (plantar)	Painless, callused margins
Pressure (decubitus)	Bony prominences	Staging system applies
Pyoderma gangrenosum	Any location	Undermined, violaceous, pathergy

Loading diagram...

Skin Surface Is Broken/Interrupted

Some conditions produce discontinuities in the skin surface that are not true losses of substance—the epidermis is present but punctuated by openings.

Comedo

A comedo is a dilated hair follicle filled with keratin, producing a visible "blackhead" (open comedo) or "whitehead" (closed comedo).

Type	Appearance	Pathophysiology
Open comedo	Black dot, dilated pore	Oxidized keratin/melanin
Closed comedo	Flesh-colored papule	Keratin plug without opening
Giant comedo	Large, dilated cyst	Dilated infundibular cyst

Sinus Tract and Fistula

A sinus tract or fistula is a channel connecting a deeper structure to the skin surface, with an external opening that may discharge fluid.

Condition	Connection	Discharge
Pilonidal sinus	Sacrococcygeal cavity → skin	Purulent
Dental fistula	Tooth abscess → skin	Purulent
Hidradenitis suppurativa	Abscesses → skin	Purulent, interconnecting
Crohn's fistula	Bowel → skin	Feculent (enterocutaneous)

Rhagade

A rhagade is a fine depression (<1 mm) without true loss of substance, occurring in inflammatory or hyperkeratotic skin. Common in:

Angular cheilitis
Palmoplantar psoriasis
Chronic hand eczema

Skin Surface Is Covered: Scales and Crusts

When substances accumulate on the skin surface, they produce visible coverings that modify the surface appearance.

Scale

Scale consists of lamellae of stratum corneum cells at the skin surface. Unlike keratosis, scales are easily removed (though adherence varies).

Types of Scale

Type	Description	Examples
Pityriasiform	Fine, white, floury	Seborrheic dermatitis, pityriasis versicolor
Psoriasiform	White, silvery, micaceous	Psoriasis
Ichthyosiform	Large, polygonal, "fish scale"	Ichthyosis
Scarlatiniform	Large sheets, peeling	Post-scarlet fever, TSS, Kawasaki
Collarette	Peripheral rim of scale	Pityriasis rosea, secondary syphilis
Lamellar	Stacked, plate-like	Exfoliative dermatitis

Special Scale Patterns

Collarette scale (trailing scale): A rim of scale at the periphery of a lesion, with the free edge pointing inward. Classic for:

Pityriasis rosea
Tinea corporis
Secondary syphilis
Superficial pemphigus

Scarlatiniform desquamation: Large sheet-like peeling, occurring after:

Scarlet fever
Staphylococcal toxic shock syndrome
Kawasaki disease
Drug reactions

Crust

A crust is dried exudate, secretion, blood, or necrotic material on the skin surface. Crusts are adherent but removable with curetting.

Type	Appearance	Indicates
Serous crust	Amber, honey-colored	Dried serum
Hemorrhagic crust	Brown, black	Dried blood
Honey-colored crust	Yellow-gold	Impetigo (S. aureus)
Thick, tenacious crust	Brown-black "rupia"	Tertiary syphilis, ecthyma

[!TIP] Always remove crusts to examine the underlying lesion. Crusts hide erosions, ulcerations, and even tumors.

Skin Surface Is Necrotic

Necrosis represents cell death and tissue devitalization. Necrotic skin is cold, insensitive, and progresses through characteristic color changes.

Clinical Features of Necrosis

Feature	Description
Temperature	Cold
Sensation	Absent (anesthetic)
Color evolution	Pink → white → blue → black
Demarcation	Groove forms between viable and non-viable tissue
Texture	Initially soft, then leathery (eschar)

Causes of Cutaneous Necrosis

Category	Examples
Vascular occlusion	Arterial thrombosis, embolism, calciphylaxis
Vasculitis	Large vessel involvement
Pressure	Decubitus ulcer
Infection	Necrotizing fasciitis, ecthyma gangrenosum
Drug-induced	Warfarin necrosis, heparin necrosis
Cold injury	Frostbite
External compression	Compartment syndrome

Eschar vs. Slough

Term	Appearance	Composition
Eschar	Black, dry, leathery	Dried necrotic tissue
Slough	Yellow, moist, soft	Fibrin + necrotic debris

Clinicopathological Correlations

Surface anomalies directly reflect epidermal pathology, providing immediate clinicopathological correlation:

Surface Finding	Histopathology
Scale (fine)	Parakeratosis
Scale (thick, silvery)	Confluent parakeratosis, Munro microabscesses
Scale (ichthyosiform)	Retained stratum corneum, ↓ granular layer
Keratosis	Orthokeratotic hyperkeratosis
Cutaneous horn	Massive hyperkeratosis
Erosion	Epidermal loss, intact basement membrane
Ulceration	Full-thickness epidermal + dermal loss
Necrosis	Cell death, coagulative/liquefactive necrosis
Crust	Dried serum, fibrin, inflammatory cells

Clinical Pearls

Topic	Pearl
Scale types	Psoriasiform = silvery, micaceous; Pityriasiform = fine, floury
Collarette scale	Think: PR, tinea, syphilis, pemphigus
Scarlatiniform peeling	Superantigen-mediated: scarlatina, TSS, Kawasaki
Cutaneous horn	Always biopsy the base—may be SCC
Ulcer borders	Undermined = PG; Rolled = BCC/SCC; Punched = arterial
Remove crusts	Always examine what lies beneath
Honey crusts	Impetigo until proven otherwise
Necrosis color	Evolution: pink → white → blue → black
Eschar	Do not debride eschar on stable heel (protective)
Horny plug	Carpet-tack sign = discoid lupus

Summary Table: Surface Anomalies

Anomaly	Definition	Key Features	Examples
Atrophy	Thin epidermis	Transparent, wrinkled	Lichen sclerosus, corticosteroid
Keratosis	Thick stratum corneum	Hard, adherent, rough	Callus, actinic keratosis
Cutaneous horn	Exophytic keratosis	Taller than wide	Wart, SCC
Scale	Loose stratum corneum	Removable lamellae	Psoriasis, eczema
Crust	Dried exudate	Adherent, covers lesion	Impetigo, erosion
Erosion	Superficial loss	Heals without scar	Post-vesicle, excoriation
Ulceration	Deep loss	Heals with scar	Venous ulcer, PG
Necrosis	Tissue death	Cold, black, demarcated	Frostbite, arterial occlusion

Cross-References

Volume 04, Chapter 3: Flat Lesions
Volume 04, Chapter 4: Palpable and Solid Lesions
Volume 04, Chapter 5: Fluid-Filled Lesions
Volume 35: Wound Healing