Wednesday, 17 September 2014

Differential diagnosis of Becker’s naevus (melanosis)

Becker’s naevus is a relatively common anomaly, affecting most racial groups, and about five times more frequent in males than females. The basal and suprabasal keratinocytes are heavily pigmented, and melanocyte density is variably reported as increased or normal, with a few melanophages in the upper dermis. There are no junctional or intradermal naevus cells. It may present in childhood, but is usually first noticed during adolescence, initially pale in colour and becoming more conspicuous after sun-exposure. Classically, Becker naevus is unilateral but it is rarely bilateral. The usual site is shoulder, anterior chest or scapular region, but lesions may occur on any area of the body. It starts as an area of irregular macular pigmentation, which spreads to a diameter of several centimetres, new macules developing beyond the margin and fusing with it, giving a geographical contour. The skin may thicken towards the centre of the lesion. Increased terminal hairs may appear on and around the lesion.

Becker’s naevus has been reported to follow Blaschko’s lines, but the usual lack of conformity to Blaschko’s lines may be due to the relatively late occurrence of a causative mutation. The mutant skin clone is presumably predisposed not only to pigmentation but also to androgen sensitivity, since Becker’s naevus is prone to acne and hypertrichosis (increased expression of androgen receptors within lesional skin). Many dermatologists would not regard Becker’s naevus as an epidermal naevus.  Once present, Becker’s naevus remains indefinitely. Almost all epidermal naevi follow the pattern of lines documented by Blaschko from drawings of epidermal naevi. The pattern is attributed to the lines of migration and proliferation of epidermal cells during embryogenesis. The lines do not correspond to any known nervous, vascular or lymphatic structures. The bands of abnormal skin represent clones of cells carrying a mutation in a gene expressed in the skin. Mosaicism impact on the phenotype relates to the proportion of cells that harbour the mutation and their distribution. Mosaic describes an art form in which pictures are produced by joining together tiny pieces of different coloured stone or glass. The term is used in genetics to describe individuals composed of cells of different genotypes, such as patients with Turner syndrome who have both 45, XO and 46, XX cells. The pattern may vary according to cell type and timing of mosaicism. In the skin, genetic mosaicism classically appears as Blaschko's lines. The term "pigmentary mosaicism" encompasses different phenotypic expressions of common pathogenic process caused by genetic mosaicism that specifically disrupt expression of pigmentary genes.

Cutaneous conditions have been reported to colocalize with Becker’s naevus and include e.g. pityriasis versicolor (I have seen one case), granuloma annulare and lichen planus. The term Becker’s naevus syndrome has been proposed to describe the association of a Becker’s naevus with ipsilateral non-cutaneous abnormalities.

A well-developed Becker’s naevus is unmistakeable.

Differential diagnosis of less developed lesions includes:

  • Progressive cribriform and zosteriform hyperpigmentation: This might be a non-hypertrichotic variant of Becker’s naevus or a localised form of linear and whorled naevoid hypermelanosis. 
  • (Diffuse) linear and whorled naevoid hypermelanosis
  • Idiopathic eruptive macular hyperpigmentation: The lesions disappear gradually over several months to years.
  • Acquired smooth muscle hamartoma: It has similar clinical and histopathological features, but in different proportions, with less pigmentation and more smooth muscle. Lesions described in the literature as ‘congenital Becker’s naevi’ could have actually been congenital smooth muscle hamartomas. However, it is possible that in some cases there is clinical/histopathologic overlap with Becker’s naevus.
  • Partial unilateral lentiginosis/agminated lentiginosis: Two similar conditions if not identical. 
  • Café-au-lait macule/spot/patch: Café-au-lait macules are common (found in approximately 15% of individuals), but increased numbers of such lesions are a component of many disorders.
The condition has been misdiagnosed by a dermatologist as pityriasis versicolor for several months.

The condition has been misdiagnosed by a dermatologist as pityriasis versicolor for several months.

This page was last updated in September 2014. 

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Main Works of Reference List (The first eight are my top favourites)

  • British National Formulary
  • British National Formulary for Children
  • Guidelines (BAD - BASHH - BHIVA - Uroweb)
  • Oxford Handbook of Genitourinary Medicine, HIV, and Sexual Health
  • Oxford Handbook of Medical Dermatology
  • Rook's Textbook of Dermatology
  • Simple Skin Surgery
  • Weedon's Skin Pathology
  • A Concise Atlas of Dermatopathology (P Mckee)
  • Andrews' Diseases of the Skin
  • Andrology (Nieschlag E FRCP, Behre M and Nieschlag S)
  • Bailey and Love's Short Practice of Surgery
  • Davidson's Essentials of Medicine
  • Davidson's Principles and Practice of Medicine
  • Fitzpatrick's Colour Atlas and Synopsis of Clinical Dermatology (Klaus Wolff FRCP and Richard Allen Johnson)
  • Fitzpatrick’s Dermatology in General Medicine
  • Ganong's Review of Medical Physiology
  • Gray's Anatomy
  • Hamilton Bailey's Demonstrations of Physical Signs in Clinical Surgery
  • Hutchison's Clinical Methods
  • Lever's Histopathology of the Skin
  • Lever's Histopathology of the Skin (Atlas and Synopsis)
  • Macleod's Clinical Examination
  • Martindale: The Complete Drug Reference
  • Oxford Handbook of Clinical Examination and Practical Skills
  • Oxford Textbook of Medicine
  • Practical Dermatopathology (R Rapini)
  • Sexually Transmitted Diseases (Holmes K et al)
  • Statistics in Clinical Practice (D Coggon FRCP)
  • Stockley's Drug Interactions
  • Treatment of Skin Disease: Comprehensive Therapeutic Strategies
  • Yen & Jaffe's Reproductive Endocrinology