Dermal Hypertrophies

Introduction

All wounds heal with some degree of scar formation, but the mechanisms that govern whether the result will be a fine thin scar, a prominent hypertrophic scar, or a tumor-like keloid remain unclear. The latter represent two forms of abnormal wound healing. Both are characterized by local fibroblast proliferation and excessive collagen production in response to cutaneous injury. However, their clinical and histopathologic features differ, as well as proposed pathogeneses.

Epidemiology

There is a higher prevalence of keloids in African, African-American, Spanish and Asian populations, with an incidence ranging from 4–16% . Although the incidence of hypertrophic scars is probably higher than that of keloids, precise data are lacking . Neither entity has a gender prevalence; they occur most commonly in those 10 to 30 years of age . Individuals in this age group are more frequently subjected to trauma and their rate of collagen synthesis is higher. Younger skin also possesses greater tension, as compared to older skin which has less elasticity and is more redundant .

Etiology

Proposed etiologic factors include: genetic predisposition; depth, type and location of the skin injury or wound; degree of tension; local infection or inflammation; and hormonal influences . There is often a familial tendency to develop hypertrophic scars and keloids and possibly an autosomal dominant mode of inheritance with incomplete clinical penetrance and variable expression . A range of skin injuries can lead to abnormal scarring, from lacerations, burns, surgical excisions and skin piercings to injections (vaccines, tattoo inks) and cutaneous inflammation (e.g. acne vulgaris, insect bites) . Moreover, tension is constantly transmitted to the skin from the underlying cartilaginous and bony skeleton, and skin tension can be aggravated by loss of tissue, as occurs with surgical excisions . In addition, hormonal influences have been proposed as an explanation for the appearance of keloids at or after puberty and their resolution following menopause, as well as reports of the onset or enlargement of keloids during pregnancy . Lastly, insights should come from genetic disorders such as Goeminne syndrome and Bethlem myopathy in which spontaneous keloids are a characteristic feature .

Pathogenesis

In fetuses, cutaneous wounding during the first and second trimesters of gestation does not lead to scarring. This is thought to reflect a healing process that is occurring via tissue regenerative pathways in a sterile environment and in the absence of inflammation . The ability to heal wounds created by injuries, in particular trauma, offers an evolutionary advantage, with associated scarring being less of an issue. However, cutaneous scarring resulting from surgical procedures is viewed as an unwanted consequence. A regenerative response would be preferred, thereby conceivably providing a superior cosmetic result and avoiding the formation of hypertrophic scars and keloids .

Conventional wound healing occurs in three phases (see Fig. 141.3 ): an inflammatory phase which aims to contain the injury and prevent infection; a proliferative phase in which there is formation of granulation tissue; and a remodeling (maturation) phase in which the redness of the wound lightens and its nodularity softens and flattens due to simultaneous collagen synthesis and degradation . The latter phase can last up to one year. Greater understanding of this remodeling phase could provide targets for preventative or therapeutic interventions.

Since Chapter 141 discusses the molecular basis of wound healing, this section will focus on key mediators of tissue scarring . Within keloids, collagen synthesis by fibroblasts is markedly increased as is production of transforming growth factor (TGF)-β. TGF-β has been shown to play a pivotal role during the proliferative phase of wound healing. Expression of TGF-β1 or TGF-β2 leads to an increase in scarring , whereas expression of TGF-β3 is associated with a reduction in scarring . This has led to the introduction of therapies based upon the biologic effects of these different forms of TGF, e.g. injections of recombinant TGF-β3, injections of mannose-6-phosphate which inhibits TGF-β1 and TGF-β2 signaling (see Table 98.4 ).

In regenerative healing, there is both a lack of scarring and an absence of inflammation. Support for this association comes from studies in which proinflammatory cytokines interleukin-6 (IL-6) and IL-8 enhanced scarring, while the anti-inflammatory cytokine IL-10 decreased the amount of scar tissue . Several additional factors propagate a robust fibroblast response within the healing wound that can lead to increased scar formation, including platelet-derived growth factor (PDGF), two transcription factors – homeobox B13 and early growth response protein 1, and components of the Wnt signaling pathway . Of note, fibroblasts isolated from keloids have increased expression of receptors for PDGF and TGF-β.

Clinical Features

Although distinguishing a hypertrophic scar from a keloid clinically can sometimes be difficult, especially if the lesion is small or of recent onset, each has several characteristic features ( Table 98.1 ). Both keloids and hypertrophic scars have a smooth surface and are firm to palpation. They may be pruritic or painful and occasionally inhibit normal motion of adjacent tissues. The color can vary from pink–purple (early lesions) to skin-colored to hypo- or hyperpigmented. While keloids may be more elevated above the skin surface than hypertrophic scars, the key difference is that keloids extend beyond the boundary of the original wound into adjacent normal skin, often with claw-like extensions resembling the pincers of a crab (in Greek, chelè means claw) ( Figs 98.1 & 98.2 ). In contrast, hypertrophic scars remain confined to the site of the original injury ( Fig. 98.3 ).

Most keloids appear within one year of an injury, although the intervening interval may be up to 24 years . Hypertrophic scars usually develop within a few weeks to months after wounding, and they frequently flatten spontaneously within 1 to 2 years. Keloids, on the other hand, do not regress over time. While both favor sites of increased wound tension such as the upper trunk, shoulder, and upper outer arm, keloids can also develop in sites such as the earlobe, where there is minimal tension. In addition, keloids can form spontaneously, most often in the midchest region.

Pathology

Distinguishing between a keloid and a hypertrophic scar is important when contemplating treatment, especially more aggressive therapies such as radiation. In some patients, histologic examination is required to make this distinction ( Table 98.3 ). In hypertrophic scars, there is an increase in both the number of fibroblasts and the density of collagen fibers within the dermis, both of which are oriented parallel to the skin surface ( Fig. 98.4 ). Keloids are characterized by the presence of whorls and nodules of strikingly thick, glassy, homogeneous collagen bundles that are composed of densely packed fibrils and oriented haphazardly throughout the dermis (keloidal collagen) ( Fig. 98.5 ). Early on, there are abundant deposits of fibrillary collagen within the reticular dermis of keloids, while mature lesions often have the characteristic thick sclerotic collagen. In longstanding keloids, there may be a return to the earlier fibrillary pattern.

Table 98.3

Histopathologic features of hypertrophic scars and keloids.

Shading indicates differences.

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It is noteworthy that keloidal collagen may be absent in up to 45% of keloids . In scars with no detectable keloidal collagen, histologic features that favor a keloid include: no flattening of the epidermis; a lack of fibrosis within the papillary dermis; a tongue-like advancing edge as the scar tissue extends through the reticular dermis; a horizontal cellular fibrous band within the upper reticular dermis with a sharp demarcation from the normal-appearing papillary and reticular dermis; and prominent fascia-like fibrous bands in the deeper portion of the scar .

Attempts to differentiate hypertrophic scars from keloids via immuno­histochemistry has led to conflicting results . In one study, nodules of α-smooth muscle actin (α-SMA)-positive cells (myofibroblasts) were only observed in hypertrophic scars , whereas another investigation found α-SMA expression in both hypertrophic scars (70%) and keloids (45%) . Expression of cyclooxygenase (COX)-1 was observed in 100% of keloids, but it was also expressed in ~50% of hypertrophic scars ; thus COX-1 expression favored, but did not specifically identify, keloids.

Differential Diagnosis

For keloids, the clinical differential diagnosis includes the sclerotic form of xanthoma disseminatum, lobomycosis (keloidal blastomycosis; lacaziosis), and the keloidal forms of scleroderma and morphea. Rarely, carcinoma en cuirasse can present as keloidal nodules. Patients with the vascular type of Ehlers–Danlos syndrome may occasionally develop keloidal plaques on their lower extremities, and spontaneous formation of keloids can be a clinical clue to other rare inherited syndromes such as Bethlem myopathy, Rubenstein–Taybi syndrome, and Goeminne syndrome (see below).

Histopathologically, keloids and hypertrophic scars need to be differentiated from spindle cell tumors, primarily dermatofibroma (DF), dermatofibrosarcoma protuberans (DFSP), desmoplastic melanoma, and the scar-like variant of squamous cell carcinoma (SCC). Immunostaining for CD34 and factor XIIIa, which is usually positive in DFSP and DF, respectively, is negative in keloids and hypertrophic scars . While there is minimal or no immunostaining for S100 protein in scars, desmoplastic melanomas usually demonstrate strong expression of this marker . Finally, the unusual scar-like variant of SCC may only have focal keratin expression, and a complete panel of keratin stains (to increase sensitivity) may be required for its detection .

Treatment

The management of keloids and hypertrophic scars continues to challenge clinicians, and there is no universally accepted treatment al­gorithm. Potential evidence-based treatments are outlined in Table 98.4 . Prevention remains the best strategy in predisposed patients, including avoidance of nonessential surgery in high-risk anatomic sites and attention to postsurgical wound care . There is limited evidence to support over-the-counter scar reduction products such as silicone sheeting and topical creams containing vitamin E . While potent topical corticosteroids are sometimes used to address pruritus, intralesional triamcinolone is the major first-line therapy for flattening hypertrophic scars and keloids . Its utility was confirmed in a recent meta-analysis . An alternative, especially for needle-averse individuals, is clobetasol propionate 0.05% cream under silicone dressing occlusion . Intralesional or topical corticosteroids may be combined with intralesional 5-fluorouracil .

Table 98.4

Treatment options for hypertrophic scars and keloids.

There are also reports of the use of topical retinoic acid, calcineurin inhibitors, imiquimod (deemed ineffective in controlled trials), and tamoxifen; botulinum toxin A; mitomycin C (topical or intralesional); and systemic methotrexate and calcium channel blockers . Treatments reserved for the treatment of keloids are shaded mid-blue and experimental treatments are shaded dark blue. Key to evidence-based support: (1) prospective controlled trial; (2) retrospective study or large case series; (3) small case series or individual case reports. TGF, transforming growth factor.

Adapted from Tziotzios et al. J Am Acad Dermatol. 2012;66:13–24.

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Surgical revision of both hypertrophic scars and keloids must be undertaken with caution given the high recurrence rates. Cryosurgery has been used successfully and is often combined with intralesional triamcinolone. Pulsed dye laser therapy has become popular as a modality for postsurgical scar reduction and is associated with few side effects . Due to theoretical concerns regarding carcinogenesis, radiotherapy is reserved primarily for treatment-resistant keloids. However, current postoperative protocols limit total exposure and vary from 12–16 Gy in 3–4 fractions to 20 Gy in 5 fractions, beginning within 24–48 hours following surgery (see Ch. 139 ).

Potential therapies based upon insights into pathogenesis (see above) include recombinant TGF-β3 (avotermin), human recombinant IL-10, and mannose-6-phosphate which is a potent inhibitor of TGF-β1 and TGF-β2 signaling .