Becker melanosis (Becker's Nevus) [BM] is a form of acquired hyperpigmentation. S. William Becker first described the condition in 1949, in two reported cases, as "concurrent melanosis and hypertrichosis in the distribution of nevus unius lateris."
The exact etiology is not clear. BM is considered to be a benign, late-onset type of epidermal nevus. Associated features like peri pubertal development, male preponderance, hypertrichosis, and acneiform lesions suggest a role for androgens. An increase in the number of androgen receptors has been reported.
Males are more commonly affected (male to female ratio of about 5:1). BM has an estimated prevalence of 0.5% among males. The lesion usually presents during puberty, but rarely cases of BM presenting at birth or early childhood have been reported. Familial occurrence of BM has been reported. Some studies have shown a higher incidence among people with darker skin.
Androgen sensitivity and stimulation have been suggested as one of the main factors involved in the pathogenesis of Becker nevus. The onset of the lesions during adolescence, male preponderance, associated features like hypertrichosis and acneiform lesions, have all been linked to the role of androgens in the pathophysiology. Some studies have reported an increased amount of androgen receptors in the lesional skin. Post-zygotic mutations in beta-actin have been reported in association with both Becker nevus and Becker nevus syndrome.
BM presents as a well-defined, unilateral, hyperpigmented tan or brown patch which increases in size, gradually often developing into a geographic pattern. Some studies have reported the right side of the body to be more commonly affected. The most common sites involved are the shoulder, scapular area, and the upper arms. Rarely, BM may present with bilateral lesions and on atypical sites like the lower limbs. Significant hair growth over the lesions is usually seen some time after the hyperpigmentation is established and may take months to years to develop. Acneiform lesions may develop over the affected area. BM may be associated with other findings related to ectodermal abnormalities known as Becker nevus syndrome. Reported associations include smooth muscle hamartomas, hypoplasia of the breast, pectoral muscle and fat, limb hypertrophy, adrenal gland hyperplasia, and accessory scrotum. A rare association with melanoma has been reported. Once developed, the natural course is for the lesions to persist indefinitely. The hypertrichosis typically develops after the hyperpigmentation and the hairs become progressively coarse with time. Some recent studies have suggested that hypertrichosis may not be associated with a majority of the case of BM. 
The diagnosis is mainly clinical. A skin biopsy shows mild acanthosis and hyperkeratosis. Increased melanin is seen in the basal layer although the number of melanocytes tends to be normal. Dermal melanophages can be seen. Other features reported in histopathology include elongation of the rete ridges, smooth muscle, and sebaceous gland hyperplasia. The hyperproliferation of keratinocytes, melanocytes, arrector pili muscle and dermal nerve fibers have been reported in recent studies. Some immunohistochemical studies have shown increased expression of some markers like epidermal Ki-67, melan-A, and keratin 15 in the lesional and perilesional skin as compared to normal skin. Dermal nerve fiber length and expression of smooth muscle actin have also been reported to be higher in the lesional skin of Becker melanosis.
The hyperpigmentation of Becker melanosis usually remains stable; although, there are reports of the pigmentation fading spontaneously in some cases. Treatment is mainly indicated for cosmetic reasons, especially the rapid transformation of the lesion during adolescence. The hyperpigmentation and the hypertrichosis respond to lasers. Different types of lasers have been found to be effective in BM like the Q-switched Ruby laser, Q-switched Nd YAG, long-pulsed Alexandrite and various types of fractional ablative lasers. The most commonly used lasers are the q-switched ruby and the Q-switched Nd YAG lasers. However, both are associated with a high rate of recurrence. Hypertrichotic lesions have been reported to respond to combinations of fractional lasers (like the 1550 nm non-ablative laser) with hair removal lasers. Multiple sessions are required for optimum results. Electrolysis has also been reported to be effective in treating the hypertrichosis associated with Becker melanosis. Sun protection is advised as sun exposure might make the lesions appear darker. Acneiform lesions have been found to respond to topical retinoids. In patients with associated breast hypoplasia, a novel yet effective treatment is breast lipofilling which is fat grafting to treat the cosmetic defect related to ipsilateral breast hypoplasia, which can be associated with Becker melanosis.Cosmetic camouflage can be useful in addressing the psychosocial issues and quality of life in patients who have lesions in relatively exposed areas
Although the typical presentation is in the form of a single, unilateral, hyperpigmented or tan-colored macule over the shoulder or pectoral area, BM has been reported to manifest in various atypical presentations.
Associated abnormalities include unilateral hypoplasia of the breast which can vary in magnitude, affecting the whole breast area or the nipple/areola alone. Supernumerary nipples can also occur as an association with aplasia of the ipsilateral pectoralis major muscle, ipsilateral limb shortening, localized lipoatrophy, spina bifida, scoliosis, pectus carinatum, quadriparesis, osteoma cutis, congenital adrenal hyperplasia, and accessory scrotum. BM has been found to occur in association with phakomatosis pigmentovascularis and neurofibromatosis. BM occurring in association with nevus depigmentosus has been described as a possible example of twin spotting. A case report has mentioned the occurrence of basal cell carcinoma occurring on the site of BM in a sun-protected area. Hypohidrosis associated with BM has been reported. Pityriasis versicolor localized to the area of BM has been described.
Multiple and bilateral lesions have rarely been reported. A case of giant, bilateral Becker melanosis simulating the armor of a gladiator's arm has been described. The same patient also had marginal osteophytes over the cervical vertebrae. Becker melanosis associated with short stature, skeletal deformities, and mental retardation. Bilateral, congenital BM has been described. There are reports of Becker melanosisBM occurring in siblings. Atypical sites of involvement include the lower limbs. Many cases may present with later age of onset and absence of hypertrichosis.
It is important to look for extra cutaneous involvement in all cases of suspected Becker's melanosis. Patients must know the possibility of increased hair growth over the lesions and that the treatment is essentially for cosmetic purposes only. Advanced surgical treatment will require a coordinated effort from dermatologists and plastic surgeons. Lasers are as of the main evidenced based management option for better cosmesis in Becker's melanosis. 
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