Becker Naevus

Becker naevus (Becker melanosis) is an acquired, hyperpigmented, often hairy patch, usually on the shoulder, upper chest or back, that appears in adolescence or young adulthood. It is a benign, androgen sensitive hamartoma.

It affects about 0.5% of young men and is 5 times commoner in men than women. Onset is usually between 10 and 20 years, often after sun exposure.

Becker naevus syndrome (about 3% of patients) is the association with an ipsilateral hypoplastic breast or other anomalies of muscle, skeleton and skin.

Pathophysiology.

Post-zygotic mosaic variants in ACTB (beta actin) are found in the lesions. They cause a smooth muscle hamartoma, with basal hyperpigmentation, epidermal thickening and increased androgen receptors in the lesion.

This explains the onset at puberty, the hair growth and the male predominance.

• A unilateral, brown patch, 1 to 20 cm or more, with an irregular, geographic border, often with darker hair after a year or two.
• Sites: the shoulder, upper anterior chest, scapular area, and occasionally the face, neck or lower limbs.
• The patch may be acneiform or have mild papules from hair follicles. Pigment may stop spreading in adult life.
• In Becker naevus syndrome: breast hypoplasia or areolar abnormality, pectoralis or limb hypoplasia, spina bifida occulta, scoliosis and pectus anomalies.

• Clinical diagnosis from age at onset, site, pigment and hypertrichosis.
• Dermoscopy shows a faint reticular brown pattern with perifollicular hypopigmentation. Biopsy shows acanthosis, basal hyperpigmentation and sometimes smooth muscle bundles in the dermis.
• Examination for breast, chest wall and spine anomalies when the patch is on the trunk. Imaging only if abnormalities are found.

Differential diagnosis.

• Congenital melanocytic naevus and smooth muscle hamartoma (present at birth).
• Café-au-lait macule, naevus spilus and epidermal naevus.
• Melasma, post-inflammatory hyperpigmentation and plexiform neurofibroma.

• Reassure that it is benign and that malignant change is not a recognised risk.
• No treatment is required. For appearance, Q-switched or fractional lasers, or intense pulsed light, give partial and unpredictable clearance, and pigment often recurs. Test patches first.
• Hair: laser hair removal (alexandrite, diode or neodymium doped yttrium aluminium garnet) or electrolysis. Pigment lightening may be needed separately.
• Anti-androgen therapy is not effective for the naevus.
• Camouflage cosmetics and sun protection.

Special groups. Patients with Becker naevus syndrome need review by plastic surgery or orthopaedics for breast or skeletal problems.

Monitoring. Review if the lesion changes.

Referral and escalation. Dermatology for laser; orthopaedics or plastic surgery for syndromic features.