The patient was a 19-year-old woman with no past history of interest. She came to our outpatients for an asymptomatic brownish macule on the left leg that had appeared at puberty and had progressively increased in size with the patient's growth, until it stabilized. The patient had applied emollients with no improvement.

In the left pretibial region, extending onto the medial and lateral surfaces of the leg, there was a macular lesion with a homogeneous light-brown color and irregular well-defined borders, measuring approximately 20cm in diameter (Fig. 1).

Figure 1

(0,06MB).

Histology of the biopsy revealed an epidermis with orthokeratotic hyperkeratosis, papillomatous hyperplasia, and prominent pigmentation of the basal layer (Fig. 2), with no alterations of the dermis (Fig. 3).

Figure 2.

Hematoxylin-eosin, original magnification ×20.

(0,19MB).

Figure 3.

Hematoxylin-eosin, original magnification ×4.

(0,14MB).

What Is Your Diagnosis?

Pretibial Becker nevus.

Becker nevus is a pigmented hamartoma of the skin first described by Becker in 1969. It mainly affects men and typically appears during adolescence. It usually occurs sporadically, though familial cases have been reported. The estimated prevalence is 1% in the general population, but a higher incidence has been recorded in the white population.1

The etiology is unknown, but androgens have a recognized effect on the development of these lesions. Compared with normal skin, Becker nevus has been shown to have a larger number of androgen receptors.2

Becker nevus usually presents as a single, hyperpigmented macule of several centimeters in diameter, with a unilateral distribution. The lesions tend to arise on the upper region of the trunk and on the proximal areas of the upper limbs. Hypertrichosis is observed in only approximately 50% of cases and typically appears after the hyperpigmentation.

Histology reveals acanthosis, papillomatous hyperplasia, hyperkeratosis, and hyperpigmentation of the basal layer of the epidermis, with no melanocytic proliferation. Hypertrophy of the erector pili muscle may be observed, in addition to fascicles of smooth muscle in the dermis not associated with skin adnexa, but these changes are not always present, as in our patient.

Becker nevus is uncommon on the lower limbs, but it has not been possible to establish whether the low prevalence in this region is genuine or is due to underdiagnosis or to a lack of reports in the literature. Twelve cases of Becker nevus arising in the proximal regions of the lower limbs (above the knees) have been described in the English-language literature3; the patients were 11 men and 1 woman, and hypertrichosis was observed in only 2 cases. There is only 1 previous report in the literature of Becker nevus arising below the knee. That was in a 16-year-old patient with mental retardation who presented 3 lesions (right forearm and both pretibial regions) clinically and histologically compatible with Becker nevus, with a microdeletion of chromosome 5.4

We have presented a patient with a lesion clinically and histologically compatible with Becker nevus in the pretibial region. This is the second case of Becker nevus at this site described in the literature. It is not possible to determine the true prevalence of this condition at this site, nor its prognostic value as a marker for other abnormalities, for which it would be necessary to perform larger studies. We consider that Becker nevus at this site may be less uncommon than is thought, and both diagnostic error and a failure to publish could explain a possible under-recording of such cases.