Segmental lentiginosis with ipsilateral nevus depigmentosus: another example of twin spotting?

ARTICLE

Segmental lentiginosis (SL) is defined as a circumscribed grouping of small pigmented macules on one side of the body. The grouping may be small or large and is arranged in a segmental pattern. Histological examination of the macules reveals increased numbers of melanocytes in elongated epidermal rete ridges, with no nests of melanocytes or cellular inflammation [1]. This disorder is also known as partial unilateral lentiginosis, agminated lentigines, lentiginous mosaicism and zosteriform lentiginous nevus [1]. Nevus depigmentosus (ND) is defined as a congenital, non-progressive, hypopigmented macule or patch. It is caused by reduced melanosome synthesis or defective transfer of melanosomes to neighboring keratinocytes [2, 3]. Both disorders manifest at birth or in early childhood [1-3]. While there are usually no associated disorders, rare cases of SL have been reported in combination with systemic abnormalities [1]. To our knowledge, simultaneous occurrence of SL and ND has been reported in only one patient to date. In that case, SL and ND occurred on either side of the body [4].

We report a case of concomitant SL and ND on the same side of the body, and speculate that this paired disorder may represent a twin-spot phenomenon.

Case report

An 18-year-old girl had color changes present since birth on her neck and left arm. On the left side of the patient's neck, there was a 5 x 6 cm cluster of brown macules on a background of normal-appearing skin (Fig. 1). The macules had a regular margin, and ranged in size from 2 to 4 mm. In addition, multiple hypopigmented macules and patches with ill-defined borders were noted in close proximity to the lentiginous area. These lesions were arranged along Blaschko's lines on the left side of her neck and also extended onto the left arm (Fig. 2). The hypopigmented lesions varied in size and shape. Under the Wood lamp they showed an off-white accentuation, in contrast to the chalky-white accentuation noted in vitiligo.

Histopathologic examination of a hyperpigmented macule revealed increased melanocyte density with elongation of the rete ridges. There were no nests of melanocytes. No pathological features were observed in hematoxylin-eosin stained sections of a hypopigmented lesion. On the basis of these clinical and histopathological findings, we diagnosed the hyperpigmented lesions as SL and the hypopigmented lesions as ND. There was no evidence of any other cutaneous or extracutaneous abnormalities. The patient was advised to return annually for follow-up.

Discussion

The clustering of hyperpigmented macules in this case led us to consider SL and nevus spilus (speckled lentiginous nevus) in the differential diagnosis. Dermatologic and Wood's lamp examinations revealed pigmented macules in normal-appearing skin, and histopathologic examination indicated no nests of melanocytes; thus, these lesions were consistent with SL.

ND is an uncommon, stable hypomelanosis that is usually congenital. Various clinical forms of this condition have been described either as isolated or systematized lesions, and arranged either along the lines of Blaschko or in other patterns [3]. The hypopigmented lesions of our case were arranged along the lines of Blaschko. The terms ND and hypomelanosis of Ito are today best taken as synonym. However, cases of hypomelanosis of Ito without extracutaneous anomalies are so far often categorized in the literature as ND, whereas cases of ND associated with extracutaneous anomalies are usually categorized as examples of hypomelanosis of Ito [5].

SL is sometimes seen in combination with somatic abnormalities, including cerebrovascular hypertrophy, neuropsychiatric disturbance or café-au-lait spots [1, 6]. Our patient had no other cutaneous or extracutaneous abnormalities.

We present a case of concomitant SL and ND. These conditions have traditionally been recognized as separate sporadic entities; however, a recent report by Alkemade and Juhlin [4] described a patient with SL and contralateral ND. Besides, Bolognia et al. have reported the development of lentigines within segmental achromic nevus which is close but not exactly the same as in our case [7].

It is highly unlikely that the simultaneous occurrence of SL and ND in our patient was a chance event. We believe that this finding fits well with the concept of twin spotting, which was recently introduced to explain the phenomenon of vascular twin nevi [8]. Twin spotting is a particular form of loss of heterozygosity [9, 10]. The hypothesis of twin spotting assumes that two different, independent, recessive mutations occur on the same chromosome, and manifest visibly only in the rare event of somatic recombination occurring at an early developmental stage. The result is the emergence of two different homozygous daughter cells that form the stem cells of two different mutant patches. The paired mutant areas may be localized on the same side or on opposite sides of the body, and they may or may not follow Blaschko's lines [10, 11].

The concept of twin spotting provides a plausible explanation for a number of paired skin disorders. Other examples of skin conditions that arise from twin spotting will probably be identified in the years to come.

Article accepted on 7/4/02

REFERENCES

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