Becker naevus (Becker melanosis)

Definition

Becker naevus is a benign cutaneous hamartoma with overgrowth of the epidermis, melanocytes and hair follicles (a late-onset epidermal naevus). Clinically it presents as a large, circumscribed brown patch, often with increased hair. Synonyms: Becker melanosis, Becker pigmentary hamartoma.

Occurrence & epidemiology

Males are affected about five times more often than females; the estimated prevalence in males is 0.5%. It occurs in all skin types, develops in childhood or adolescence and is usually noticed in the second or third decade of life.

Aetiopathogenesis

Most patients carry postzygotic (mosaic) mutations in the ACTB gene (beta-actin); in one study more than 20% had no ACTB mutation, suggesting genetic heterogeneity. Androgens are thought to be a possible trigger of its appearance at puberty. Familial occurrence is rare.

Clinical features

• Large, unilateral, well-defined brown patch, usually on the shoulder or upper trunk, sometimes covering half of the upper back or chest.
• The skin may be slightly thickened; after puberty it often becomes darker and markedly hairy (hypertrichosis); occasionally acne develops within the naevus. In darker skin it appears darker and leathery.
• Becker naevus syndrome (rare): additional malformations, e.g. smooth muscle hamartoma, underdevelopment of the breast, pectoral muscle, fat, limb, chest wall or spine.
• Malignant change has not been reported.

Histology

Elongated and fused rete ridges, keratotic plugging, sebaceous hyperplasia, smooth muscle hyperplasia and increased pigmentation of the basal and suprabasal epidermis.

Diagnosis

• Clinical diagnosis based on history (onset in childhood or adolescence, increasing hair) and examination.
• A biopsy is only occasionally needed, for instance to distinguish it from a café-au-lait macule.

Differential diagnoses

• Café-au-lait Spots
• Nevus sebaceus
• Port-wine stain (naevus flammeus)
• Solar lentigo (age spot)

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References (selection)

1. DermNet: Becker naevus
2. PubMed: An update on Becker's nevus: Pathogenesis and treatment (PMID 35502558)
3. PubMed: ACTB Mutations Analysis and Genotype-Phenotype Correlation in Becker's Nevus (PMID 34944694)

Cross-references