Palpable and Solid Lesions

Introduction

The recognition and characterization of palpable lesions represents one of the fundamental skills in clinical dermatology. Unlike flat lesions (macules and patches) which are appreciated primarily through visual inspection, palpable lesions demand tactile examination—the dermatologist's fingers must confirm what the eyes perceive. This chapter provides a comprehensive framework for understanding palpable and solid skin lesions, integrating clinical morphology, dermoscopic findings, and histopathological correlates.

Palpable lesions are defined as those that can be felt when running the examining finger across the skin surface. These lesions possess vertical dimension—either elevation above the skin surface, or a palpable mass within or beneath the dermis. The critical distinction from fluid-filled lesions (vesicles, bullae, pustules) is that solid palpable lesions contain non-fluid content: cells, matrix, or deposited material.

Understanding the mechanisms underlying palpability is essential for clinical reasoning. A lesion becomes palpable through one of several pathological processes: hyperkeratosis or acanthosis of the epidermis, infiltration of the dermis by inflammatory or neoplastic cells, deposition of abnormal substances (amyloid, mucin, lipid), edema of the papillary dermis, or proliferation of dermal components (collagen, vessels). Each mechanism produces characteristic clinical features that enable the astute clinician to predict histopathology from physical examination.

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Classification of Palpable Lesions

The classification of palpable lesions traditionally follows three axes: size (papule vs. plaque vs. nodule), depth (epidermal, dermal, subcutaneous), and surface characteristics (smooth, scaly, ulcerated). This framework enables systematic description and differential diagnosis.

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Papule

Definition and Dimensions

The papule is defined as a palpable, circumscribed, solid lesion with non-fluid content and a maximum diameter not exceeding 10 mm (or 5 mm in American and British conventions). Papules represent the smallest unit of solid, palpable pathology and are among the most common lesion types encountered in dermatologic practice.

The smaller size threshold used in American practice (≤5 mm) reflects a more stringent categorization that distinguishes small papules from larger lesions that begin to coalesce into plaques. This distinction has clinical utility: tiny papules often suggest different diagnostic possibilities than larger ones, and the behavior of conditions like lichen planus, verrucae, or molluscum contagiosum can be understood through their papular nature.

Morphological Characteristics

Papules exhibit considerable morphological variation that provides diagnostic clues:

Epidermal vs. Dermal Papules

A critical clinical distinction exists between papules arising from the epidermis versus those originating in the dermis. This differentiation can be made at the bedside and predicts histopathological findings:

Epidermal papules (e.g., seborrheic keratosis, verruca vulgaris) have several distinguishing features:

• Sharp, rectilinear borders: The lesion appears "stuck on" or "pasted on" the skin surface
• Rough texture: The proliferated epidermis often produces a papillomatous or keratotic surface
• Clearly demarcated: The transition from normal to abnormal skin is abrupt

Dermal papules (e.g., granuloma annulare, dermatofibroma) demonstrate:

• Rounded, sloping borders: The epidermis is "pushed up" by the underlying dermal process
• Smooth surface: The overlying epidermis is typically normal
• Indistinct margins: The lesion blends more gradually into surrounding skin

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Umbilicated Papules

The presence of central umbilication (a small depression at the apex) is highly characteristic of specific conditions:

[!IMPORTANT] Clinical Pearl: Multiple umbilicated papules in an immunocompromised patient should raise concern for opportunistic infections (cryptococcosis, histoplasmosis, Penicillium marneffei) that may clinically mimic molluscum contagiosum. Biopsy is mandatory.

Special Types of Papules

Acuminate (pointed) papules are cone-shaped with a sharp apex and broad base. They are characteristically follicular and seen in:

• Pityriasis rubra pilaris: Reddish-orange follicular papules with "nutmeg grater" texture
• Keratosis pilaris: Follicular papules on extensor surfaces
• Lichen spinulosus: Grouped follicular spiny papules

Depressible papules are unusual and suggest specific diagnoses:

• Neurofibroma: Classic "buttonhole" sign (invaginates with pressure)
• Anetoderma: Outpouching lesion that herniates on pressure
• Piezogenic papules: Pedal fat herniation, visible only on weight-bearing

Dermoscopic Features of Papular Lesions

Dermoscopy enhances the evaluation of papular lesions, providing subsurface architectural details:

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Plaque

Definition

A plaque is a palpable lesion greater than 10 mm (or 5 mm in some conventions) in diameter, characterized by horizontal (superficial) rather than vertical spread. In essence, a plaque is a "table-top" lesion—broad and relatively flat, as opposed to the hemispheric or dome-shaped nodule.

Formation of Plaques

Plaques may arise through two mechanisms:

1. De novo: The lesion originates as a plaque-sized process from inception
2. Confluence of papules: Multiple papules merge to form a larger plaque

This distinction has clinical relevance. In psoriasis, for example, new plaques often begin as small papules that coalesce, whereas in mycosis fungoides, early patches may evolve into plaques without passing through a papular stage.

Plaque Variants

Plaques demonstrate the same morphological variations as papules, with additional features specific to their larger size:

Lichenification

Lichenification represents a specific plaque pattern characterized by:

• Thickening of the skin
• Exaggeration of skin markings (dermatoglyphics become prominent)
• Superimposed papules within the accentuated skin lines
• Hyperpigmentation (often brownish or violaceous)

Lichenification is the consequence of chronic scratching and represents the skin's response to repeated mechanical trauma. It is commonly seen in chronic atopic dermatitis, lichen simplex chronicus, and prurigo nodularis. The process involves epidermal acanthosis and elongation of rete ridges, with increased thickness of the papillary dermis.

[!NOTE] Pathophysiology: Lichenification results from the "itch-scratch cycle." Pruritus leads to scratching, which induces keratinocyte proliferation and neural sensitization, perpetuating the itch and leading to further scratching.

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Nodule and Tumor

Definition and Distinction

A nodule is a palpable mass with non-fluid content exceeding 10 mm (5 mm in USA) in diameter, characterized by three-dimensional expansion—the lesion extends in depth as well as breadth. Unlike plaques that spread horizontally, nodules are hemispheric or spherical.

A tumor is traditionally defined as a nodule exceeding 20 mm in diameter. The term implies a mass lesion with potential for continued growth. While "tumor" in dermatology does not necessarily imply malignancy, it carries the connotation of neoplasia.

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Depth Assessment

The depth of a nodule provides critical diagnostic information and can be assessed clinically:

Deep-Seated Nodules (Nouures)

The French dermatologic term "nouure" (plural: nouures) describes a specific type of nodule: large (often >5 cm), deep-seated, inflammatory, and extending into the subcutaneous fat. The hallmark condition is erythema nodosum, where the pathology resides in the septal panniculitis.

Clinical features of deep-seated nodules:

• Often barely elevated above the skin surface
• Warm, tender, and erythematous
• Detected by deep palpation with fingertips hooked into the subcutaneous tissue
• Contusion-like color evolution (red → purple → blue → green → yellow → brown)

[!TIP] Examination Technique: To assess deep nodules, use the thumb and forefinger in a "hook" grip, pressing firmly into the subcutis. Feel for induration that extends beyond what is visible on the surface.

Surface Changes

Nodules and tumors commonly exhibit surface alterations that provide diagnostic clues:

Gumma

A gumma is a specific nodular lesion classically associated with tertiary syphilis but also seen in other chronic granulomatous infections. The gumma progresses through characteristic stages:

1. Crude phase: Deep-seated, firm, non-tender nodule
2. Softening phase: Central liquefaction begins
3. Ulceration phase: Breakdown with discharge of "gummy" viscous material
4. Healing phase: Atrophic or cribriform scarring

Modern gummas are rare but may be seen in immunocompromised patients or those with delayed syphilis treatment.

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Vegetations and Verrucosis

Vegetations

Vegetations are filiform, digitated (finger-like), or lobulated excrescences with a characteristic morphology:

• Soft consistency (unlike the firm verrucous lesion)
• Pink, moist surface with thinned epidermis
• "Cauliflower" or "cockscomb" pattern
• Bleeds easily when traumatized
• May be eroded or ulcerated

The classic example is condyloma acuminatum (venereal warts), where HPV infection produces exuberant epithelial proliferation with elongated papillae and minimal keratinization.

Other conditions with vegetating morphology include:

• Pemphigus vegetans: Bullae evolve into vegetating plaques
• Iododerma/bromoderma: Halogenodermas with pustules becoming vegetating
• Pyoderma vegetans: Often associated with inflammatory bowel disease

Verrucosis

Verrucosis (verrucous lesions) share the filiform, papillomatous architecture of vegetations but differ in having:

• Firm, keratotic surface (thick, grayish-white coating)
• Dry appearance rather than moist
• "Warty" texture

Examples include:

• Verruca vulgaris: Common warts caused by cutaneous HPV types
• Verrucous carcinoma: Low-grade SCC with exophytic, verrucous morphology
• Seborrheic keratosis: May have verrucous surface

Vegetating vs. Vegetations

The term "vegetating" describes a specific plaque variant that should be distinguished from true vegetations:

• Vegetating plaques: Erythematous-erosive plaques with small papules and/or pustules, often with serpiginous tracks of confluent pustules at the margin
• Vegetations: The filiform/lobulated excrescences described above

Vegetating lesions are classically seen in:

• Pemphigus vegetans (PV subtype)
• Bullous pemphigoid (vegetating variant)
• Chronic inflammatory bowel disease (pyodermatitis-pyostomatitis vegetans)

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Other Palpable Lesions

Cord

A cord is a linear, palpable lesion that feels like a string or rope beneath the skin. Cords are often more palpable than visible and have a characteristic sinuous or linear configuration.

Furrow

A furrow is a tiny intradermal tunnel, typically only a few millimeters in length, representing the burrow of a parasite. The classic example is the scabies burrow, where the female Sarcoptes scabiei mite excavates a track within the stratum corneum.

Clinical features of the scabies furrow:

• 2-10 mm length, slightly elevated
• Grayish-white or skin-colored linear track
• May contain dark dot at one end (mite or eggs)
• Predilection sites: finger web spaces, wrists, male genitalia
• Associated with erythematous papules (hypersensitivity)

[!TIP] Dermoscopic identification: The "delta-wing jet" sign (also called "jet with contrail") is pathognomonic—a dark triangular structure (the mite) with a trailing line (the burrow).

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Clinicopathological Correlations

Understanding why a lesion is palpable links clinical examination to histopathology. The mechanisms can be categorized by the anatomical level of the abnormality:

Epidermal Anomalies

When palpability results from epidermal pathology:

• Hyperkeratosis/parakeratosis: Creates rough, scaly surface (keratosis)
• Acanthosis: Thickened epidermis elevates the surface (psoriasis)
• Tunneling by parasites: Creates the furrow of scabies

Clinical correlate: Sharp, rectilinear borders; lesion appears "stuck on" the skin.

Dermal Anomalies

Multiple pathological processes in the dermis produce palpable lesions:

Clinical correlate: Rounded borders; epidermis is "pushed up" by underlying process.

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Subcutaneous and Deeper Anomalies

Deep palpable lesions result from pathology in the hypodermis or underlying structures:

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Summary Table: Differential by Lesion Type

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Clinical Pearls

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Examination Algorithm for Palpable Lesions

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Cross-References

• Volume 04, Chapter 3: Flat Lesions
• Volume 04, Chapter 5: Fluid-Filled Lesions
• Volume 06: Dermatopathology
• Volume 07: Dermoscopy
• Volume 22: Cutaneous Oncology