Prurigo nodularis confined to Becker’s naevus

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Becker’s naevus (BN), also called Becker’s melanosis, is an organoid epidermal hamartoma characterised by circumscribed hyperpigmentation and hypertrichosis, typically presenting unilaterally on the shoulder, chest, scapula, or upper arm. It may be congenital or acquired, often becoming more conspicuous during adolescence. Studies have reported androgen receptor overexpression in lesional skin, suggesting a role for androgen sensitivity. This may explain associated features such as hypertrichosis, acneiform changes, sebaceous hyperplasia, and histological findings like acanthosis, rete ridge elongation, and proliferation of androgen-responsive arrector pili muscles. Various dermatological conditions such as acne, lichen planus, lichen striatus, eczema, psoriasis, ichthyosis, granuloma annulare, scleroderma, hypohidrosis, and pityriasis versicolor, have been reported to co-localise with BN. Proposed mechanisms for these associations include Wolf’s isotopic response, androgen hypersensitivity, and underlying mosaicism within the naevus.^1-3

A 65-year-old man presented with severely itchy lesions on a large hyperpigmented, hairy patch affecting the right side of his chest. The patch, which appeared in adolescence, was initially asymptomatic. Over the past 7 years, itchy skin lesions developed on the patch. The patient denied any history of irritant application, herpes zoster, or surgical or laser procedures. There was no personal or family history of atopy. Examination revealed a well-defined, 17 cm x 12 cm well-defined hyperpigmented patch with hypertrichosis and irregular borders. Lichenified, excoriated papules and nodules, numbering 10-15 and measuring 5 to 10 mm in diameter were scattered over the patch, interspersed with areas of depigmentation and scarring [Figure 1]. Polarised dermoscopy of nodules showed pearly white areas, red globules, microhaemorrhages, along with peripheral brown dots [Figure 2a], while the hyperpigmented patch demonstrated irregular pigment network with brown reticular lines, focal interfollicular and perifollicular hypopigmentation, and increased terminal hair [Figure 2b]. A 4-mm skin biopsy from a papule showed irregular acanthosis, papillary dermal fibrosis, and a mild to moderate perivascular and interstitial lymphohistiocytic infiltrate [Figures 3a and 3b], confirming prurigo nodularis (PN). PN confined to BN was diagnosed. Treatment with topical mometasone furoate 0.1% cream, oral hydroxyzine 25 mg at night, and emollient was initiated, resulting in partial resolution and symptomatic improvement within 4 weeks. The patient was advised to reduce the frequency of steroid application and continue regular use of emollient.

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Figure 1:

Irregular hypertrichotic, hyperpigmented patch with overlying multiple, excoriated papules and nodules localised to the right side of the chest.

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Figure 2a:

Dermoscopy [AM7115MZT Dino-Lite Edge 3.0 digital microscope] of a nodule showing pearly white areas (green circles), red globules (blue arrows), microhaemorrhages, along with brown dots (yellow arrows) in the periphery (polarised, 35x).

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Figure 2b:

Dermoscopy [AM7115MZT Dino-Lite Edge 3.0 digital microscope] of hyperpigmented patch showing irregular pigment network with brown reticular lines, focal areas interfollicular and perifollicular hypopigmentation, and increased terminal hair (polarised, 35x).

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Figure 3a:

Histopathology showing irregular acanthosis, papillary dermal fibrosis, and mild to moderate perivascular and interstitial infiltrate of lymphocytes and histiocytes (Haematoxylin and eosin, 100x).

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Figure 3b:

Higher magnification showing irregular acanthosis and papillary dermal fibrosis (Haematoxylin and eosin, 400x).

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Wolf’s isotopic phenomenon, characterised by the development of new dermatoses at sites of healed lesions, has been implicated in lichen planus, eczema, and PN co-localisation with BN.³ Viral, immunological, neural, and vascular hypothesis have been proposed to explain the phenomenon, without any conclusive evidence.³ However, it’s noteworthy that this explanation may not fully apply to BN, as it is generally considered a non-inflammatory lesion and does not necessarily undergo a healing process at any developmental stage. Recruitment of memory T-cells and a Th2-polarized T-cell response characterise chronic pruritic conditions like eczema and prurigo. Altered immune response within the lesion may also contribute to the immunological hypothesis of isotopic response.³ PN and eczema occurring over the naevus are rare with only three documented cases [Table 1].^4-6 Kim et al., investigated the expression of neuropeptides and innervation in BN and found that neuropeptide Y, galanin, and neurotensin, along with their corresponding receptors were overexpressed in the lesional skin.⁷ This dysregulation of neuropeptides, together with increased innervation, may contribute to the development of pruritus and secondary skin lesions such as PN and lichen simplex chronicus overlying the naevus. According to the neural hypothesis of Wolf’s isotopic response, local inflammation damages nerve fibres, releasing neurotransmitters and neuropeptides that subsequently trigger an abnormal local immune response.³ Androgen-induced alterations in the immune or barrier milieu may also be contributory. BN harbours postzygotic mutations in the ACTB gene, which encodes β-actin, leading to cytoskeletal dysfunctions, impairing cell adhesion and tight junctions. This may plausibly compromise epidermal barrier integrity, increasing transepidermal water loss and facilitating allergen or microbial penetration that could trigger inflammation. ACTB dysregulation may interfere with immune synapse formation and cytokine signalling, promoting localised immune dysfunction. Given BN’s tendency to follow Blaschko’s lines, a neuroectodermal mosaic origin suggests actin-related neural signalling disturbances underlying pruritus.^1,2 However, these mechanisms remain unclear. Age-related neural and barrier changes may precipitate PN in genetically mosaic BN skin. Comprehensive studies highlighting BN’s neuroimmunological properties are needed to corroborate these pathogenic relationships.

In conclusion, Becker’s naevus is an organoid epidermal hamartoma with various co-localised dermatological conditions. Immunological and neural factors likely contribute to associated pruritus and secondary skin lesions.