Teenage boy with asymptomatic brown plaque on back of thigh

The case

A healthy 14-year-old boy developed an asymptomatic brown plaque on the back of his thigh a year ago. He just recently noted the growth of coarse dark hairs within the lesion.

Diagnosis: Becker nevus (also known as Becker melanosis, Becker pigmentary hamartoma, nevoid melanosis, or pigmented hairy epidermal nevus)

Patient's leg

Discussion

Becker nevus is a form of hamartoma of ectodermal and mesodermal derived tissues that develops during childhood or adolescence, mainly occurring in males (Figure). Becker nevus can rarely be congenital or present with familial association.¹ Typical presentation is a unilateral tan or brown hyperpigmented patch or plaque with irregular borders most commonly located on the upper trunk or shoulder, but lesions can occur anywhere on the body.^2,3 Variations include bilateral involvement and atypical locations such as the flank, lower extremities, and even the face and neck, where it can affect beard growth.^4,5

Becker nevus classically follows a block-like pattern of mosaicism, but rare reports detail nevi following Blaschko lines.⁶ Some patients may also present with hypertrichosis, which can be subtle to very pronounced and may not coincide completely with the pigmentation. Perifollicular papules are commonly noted and represent the piloerector muscles of the associated smooth muscle hamartoma. Acneiform papules may be seen. Becker nevus typically presents as a solitary lesion that may grow for the first year or two and persists indefinitely. Pigmentation generally remains stable but can fade over time.^3,7

Etiology is not clear, although genetic and androgen influence have been postulated. A hypothesis of paradominant inheritance suggests that the nevus occurs with loss of heterozygosity in a mosaic pattern.³ Alternatively, postzygotic mutations of ACTB, a protein-coding gene, have been demonstrated in patients with Becker nevus.⁸ Current literature suggests androgens play a part in Becker nevus development as well. Lesions express increased androgen receptors and receptor activity and are associated with androgen-related features, including acne, hirsutism, and the onset of puberty.^2,3

Rarely, Becker nevus may present in conjunction with other cutaneous or musculoskeletal defects, known collectively as Becker nevus syndrome. Findings include unilateral hypoplasia of the breast, asymmetric limb hypertrophy or hypoplasia, scoliosis, spina bifida occulta, and supernumerary nipples.⁹ One case series reported 5 cases of malignant melanoma co-occurring on Becker nevus lesions back in 1991, but no association between the 2 has been documented since.¹⁰

Histopathology of Becker nevus shows acanthosis, hyperkeratosis, and rete ridge elongation with increased pigmentation of the basal and suprabasal layers.^2,3 Melanocytes often appear increased in number, but there is no evidence of proliferative activity.¹¹ Bundles of smooth muscle fibers in the dermis can also be seen. Due to histological similarities with smooth muscle hamartomas, there is discussion around whether the two are part of a spectrum; one end being congenital smooth muscle hamartomas with no changes in epidermis or pigmentation and the other end being later-onset Becker nevus with acanthosis and hyperpigmentation.¹²

Differential diagnoses

Without the presence of hypertrichosis, Becker nevus may be confused with other congenital or acquired pigmented lesions. Café-au-lait macules, which also appear as tan to brown hyperpigmented patches, can present as solitary or multiple benign lesions at birth or within the first few years of life. Congenital dermal melanocytosis, commonly known as Mongolian spots, usually appears at birth or within the first few weeks of life, with blue-gray patches typically in the lumbosacral and buttocks areas and fade gradually over time. Congenital melanocytic nevus typically appears at birth or shortly after and can be accompanied by hypertrichosis, but does not exhibit the perifollicular papules seen on Becker nevus. Postinflammatory hyperpigmentation will typically have preceding inflammation or trauma. Plexiform neurofibromas are hyperpigmented lesions associated with neurofibromatosis type 1 but often present with smudged border and will become nodular over time as infiltration occurs. A diagnosis of Becker nevus is usually made clinically, but a biopsy can be performed to rule out differential diagnoses if necessary.

Management

Patients with suspected Becker nevus should also be examined for extracutaneous involvement. Treatment is primarily for cosmetic reasons and has varying effectiveness. Conservative support includes sun protection to avoid further darkening of lesions and, if desired, cosmetic camouflage for relatively exposed areas that may cause psychosocial distress among patients. Hyperpigmentation and hypertrichosis can be reduced by laser therapy, although recurrence rate is high. Hypertrichosis responds well with multiple sessions of epilation or electrolysis.^2,3 Successful treatments using spironolactone and topical flutamide (4% solution) have also been reported for associated breast hypoplasia and hyperpigmentation, respectively.2

The innocent nature of the lesion reassured the patient and his parents, and he was not interested in any type of camouflage or hair removal treatment.

Reference

1. Book SE, Glass AT, Laude TA. Congenital Becker's nevus with a familial association. Pediatr Dermatol. 1997;14(5):373-375. doi:10.1111/j.1525-1470.1997.tb00985.x

2. Ngan V. Becker naevus. DermNet NZ. 2003. Accessed July 22, 2020. https://dermnetnz.org/topics/becker-naevus

3. Patel, P, Malik, K, Khachemoune, A. Sebaceus and Becker’s nevus: overview of their presentation, pathogenesis, associations, and treatment. Am J Clin Dermatol. 2015;16:197-204. doi:10.1007/s40257-015-0123-y

4. Kiliç A, Kaya I, Gül U, Soylu S, Gönül M, Demiriz M. Becker nevus on face with asymmetrical growth of beard hair. J Eur Acad Dermatol Venereol. 2008;22(2):246-247. doi:10.1111/j.1468-3083.2007.02301.x

5. Alfadley A, Hainau B, Al Robaee A, Banka N. Becker's melanosis: a report of 12 cases with atypical presentation. Int J Dermatol. 2005;44(1):20-24. doi:10.1111/j.1365-4632.2004.02077.x

6. Molho-Pessach V, Schaffer JV. Blaschko lines and other patterns of cutaneous mosaicism. Clin Dermatol. 2011;29(2):205-225. doi:10.1016/j.clindermatol.2010.09.012

7. Kaliyadan F, Ashique KT. Becker Melanosis. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2020. https://www.ncbi.nlm.nih.gov/books/NBK435999

8. Cai ED, Sun BK, Chiang A, et al. Postzygotic mutations in beta-actin are associated with Becker's nevus and Becker's nevus syndrome. J Invest Dermatol. 2017;137(8):1795-1798. doi:10.1016/j.jid.2017.03.017

9. Danarti R, König A, Salhi A, Bittar M, Happle R. Becker's nevus syndrome revisited. J Am Acad Dermatol. 2004;51(6):965-969. doi:10.1016/j.jaad.2004.06.036

10. Fehr B, Panizzon RG, Schnyder UW. Becker's nevus and malignant melanoma. Dermatologica. 1991;182(2):77-80. doi:10.1159/000247749

11. Sheng P, Cheng YL, Cai CC, et al. Clinicopathological features and immunohistochemical alterations of keratinocyte proliferation, melanocyte density, smooth muscle hyperplasia and nerve fiber distribution in Becker’s Nevus. Ann Dermatol. 2016;28(6):697-703.doi:10.5021/ad.2016.28.6.697

12. Holst VA, Junkins-Hopkins JM, Elenitsas R. Cutaneous smooth muscle neoplasms: clinical features, histologic findings, and treatment options. J Am Acad Dermatol. 2002;46(4):477-494. doi:10.1067/mjd.2002.121358