Configuration and Arrangement of Skin Lesions

Introduction

The spatial characteristics of skin lesions—their configuration (shape) and arrangement (relationship to other lesions)—provide powerful diagnostic information that often supersedes the nature of the primary lesion itself. A linear distribution may suggest external contact, a dermatomal pattern points to herpes zoster, and an arrangement along Blaschko lines reveals embryological mosaicism. These patterns reflect underlying pathogenic mechanisms: external factors, neural pathways, vascular anatomy, embryonic development, or immunological phenomena.

This chapter systematically examines the configurations and arrangements of dermatologic lesions, integrating clinical recognition with the pathophysiological and genetic mechanisms that produce these patterns. Understanding configuration and arrangement transforms the dermatologist from a pattern-recognizer into a mechanistic diagnostician capable of predicting etiology from morphology.

The distinction between configuration and arrangement is fundamental: configuration describes the shape of an individual lesion, while arrangement describes the spatial relationship between multiple lesions. However, these terms are often used interchangeably in clinical practice—a lesion may acquire an annular configuration through centrifugal extension, or an annular arrangement through coalescence of separate papules.

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Configuration versus Arrangement

Before examining individual patterns, clarity of terminology is essential:

A lesion may be:

• Annular in configuration: Single lesion that has expanded centrifugally with central clearing (e.g., granuloma annulare)
• Annular in arrangement: Multiple discrete lesions arranged in a ring pattern (e.g., papular warts forming a ring)

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Configurations of Individual Lesions

Annular and Arcuate

Annular (ring-shaped) lesions have a circular or oval shape with central clearing. The periphery is typically more active than the center, producing a rim of pathology surrounding relatively normal-appearing skin.

The pathophysiology of annular lesions involves either:

1. Centrifugal spread with central resolution (e.g., tinea corporis, granuloma annulare)
2. Central necrosis or atrophy (e.g., nummular eczema clearing centrally)
3. Immune-mediated processes with expanding inflammatory front (e.g., erythema migrans)

[!TIP] Dermoscopy of Annular Lesions: Tinea corporis shows peripheral scale at the advancing edge. Granuloma annulare is featureless or shows subtle pinkish structureless areas. The trailing scale of erythema annulare centrifugum is pathognomonic.

Arcuate (arc-shaped) lesions are incomplete rings—semicircular or crescentic configurations that represent partial annular expansion or arrested ring formation.

Target (Iris) Lesions

Target lesions (also called iris lesions) have at least two or three concentric zones of different color or morphology, creating a "bull's-eye" appearance. They are virtually pathognomonic of erythema multiforme when meeting strict criteria:

The concentric zones reflect pathological processes at varying stages:

• Central zone: Epidermal necrosis, interface dermatitis
• Middle zone: Edema, vasodilation
• Peripheral zone: Early inflammatory changes

Erythema multiforme-like reactions may occur in:

• HSV-associated erythema multiforme (most common)
• Mycoplasma pneumoniae infection
• Drug reactions (controversial/may represent SJS spectrum)
• Idiopathic

[!IMPORTANT] True target lesions are palpable. Flat, macular targets suggest Rowell syndrome or drug-induced reactions. The presence of mucosal involvement and atypical targets suggests Stevens-Johnson syndrome rather than classic EM.

Linear Configuration

A lesion with linear (straight-line) configuration almost always suggests an exogenous cause. Linearity reflects the geometry of contact rather than intrinsic biological processes, which tend to produce circular or irregular shapes.

Linear configurations result from:

Other Configurations

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Arrangements of Multiple Lesions

Grouped (Herpetiform)

Grouped or clustered lesions appear in localized aggregations, suggesting a common origin or triggering factor. The classic example is herpetiform grouping—clusters of vesicles on an erythematous base.

Dermatomal (Zosteriform)

Dermatomal distribution follows the sensory innervation of a spinal nerve root (dermatome). The archetypal example is herpes zoster, where VZV reactivation produces vesicles within a single dermatome, respecting the midline.

The concept of Wolf's isotopic response (or isotopic reaction) describes diverse skin conditions occurring in a previously affected dermatome (e.g., granuloma annulare appearing in a prior zoster dermatome).

Blaschkolinear

Blaschko lines are invisible developmental lines that become apparent when disease processes selectively affect cells along embryonic migration pathways. First described by Alfred Blaschko in 1901, these lines reflect the dorsoventral migration of ectodermal progenitor cells during embryogenesis.

Pattern of Blaschko Lines

The pattern differs by body region:

Genetic Basis of Blaschkolinear Disease

Blaschkolinear distribution reflects genetic mosaicism—the presence of two or more genetically distinct cell populations within a single individual. This occurs through:

The type 1/type 2 segmental mosaicism classification is particularly important for understanding autosomal dominant genodermatoses:

[!NOTE] Gene Involved in X-linked Blaschkolinear Disease: Incontinentia pigmenti follows Blaschko lines in female carriers because of random X-inactivation (lyonization). The gene IKBKG (NEMO, located on Xq28) is mutated; hemizygous males typically die in utero. The swirled pigmentation represents areas where cells with the wild-type X active vs. mutant X active interdigitate along Blaschko lines.

Conditions Following Blaschko Lines

Koebner Phenomenon (Isomorphic Response)

The Köbner phenomenon (isomorphic response) is the appearance of disease-specific lesions at sites of cutaneous trauma. First described by Heinrich Köbner in 1876 for psoriasis, it has been identified in numerous dermatoses.

The mechanism involves:

1. Trauma-induced cytokine release (TNF-α, IL-1, IL-6)
2. Activation of tissue-resident memory T cells (psoriasis)
3. Melanocyte destruction at trauma sites (vitiligo)
4. Epitope spreading of autoantibodies

The opposite phenomenon, Renbök phenomenon (reverse Köbner), describes clearing of disease at sites of trauma or epidermal damage.

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Distribution Patterns

Extent of Distribution

Photodistribution

Photodistributed eruptions affect sun-exposed areas: face (sparing nasolabial folds and upper eyelids), V of neck, forearms, and dorsal hands. The submental triangle (shaded by the chin) is characteristically spared.

Key Differentiating Features

Seborrheic Distribution

Seborrheic areas are rich in sebaceous glands: scalp, face (especially glabella, nasolabial folds, eyebrows), presternal region, and interscapular area.

Follicular Distribution

Follicular distribution shows lesions centered on hair follicles, producing a characteristic equidistant pattern. The three-language integration here is important:

Folliculotropic mycosis fungoides is a specific variant of CTCL with predilection for hair follicles, producing alopecia and follicular papules. The infiltrating T cells (CD4+) infiltrate the follicular epithelium, producing "mucinous follicular degeneration" histologically.

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Special Distribution Patterns

Intertriginous (Flexural) Distribution

Involvement of skin folds (axillae, groin, inframammary, interdigital) suggests:

Acral Distribution

Acral sites (hands, feet) have unique anatomy (thick stratum corneum, high eccrine density) that affects disease expression:

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Clinicopathological Correlations

The configuration and distribution of lesions directly reflect pathogenic mechanisms, providing a unique clinicopathological correlation at the macroscopic level:

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Clinical Pearls

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Summary Table

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Cross-References

• Volume 04, Chapter 10: Distribution
• Volume 19: Genodermatoses
• Volume 09: Infections
• Volume 18: Drug Reactions