Blaschko Lines:Following "Lines of Evidence" to a Rash
Published on Psychiatric Times
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Blaschko Lines:Following "Lines of Evidence" to a Rash
Diagnosis
September 01, 2008
By Rachel D. Rosenbaum, MD [1], William G. Wilson, MD [2], and Gregory F. Hayden, MD [3]
Many factors can be considered in attempting to establish the cause of a skin disorder. These
include the color, morphology, and location of the lesions; associated symptoms, such as itching and
fever; and exposure to drugs or to other children who have a rash. Linearity of the lesions may also
suggest the diagnosis.
Many factors can be considered in attempting to establish the cause of a skin disorder. These include
the color, morphology, and location of the lesions; associated symptoms, such as itching and fever;
and exposure to drugs or to other children who have a rash. Linearity of the lesions may also
suggest the diagnosis.
Common examples of linearity include:
Linear vesicular dermatitis caused by exposure to poison ivy.
Painful, vesicular lesions of herpes zoster.
Pigmentary demarcation (known as Futcher or Voigt lines) often seen along the lateral edge
of the biceps in persons with dark skin. Oval patches in a "Christmas tree" distribution over
the back seen in pityriasis rosea.
Some skin disorders follow known anatomic linear patterns. The lesions of herpes zoster follow the
generally predictable pattern of dermatomal lines (Figure 1). The lesions of pityriasis rosea follow
the distribution of skin cleavage lines (or Langer lines) (Figure 2). Langer lines are thought to
represent the principal axis of orientation of collagen fibers in the dermis; incisions along these lines
have been thought to result in the least amount of scarring.
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Blaschko Lines:Following "Lines of Evidence" to a Rash
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Figure 1 Herpes zoster on the upper torso of a 5-month-old girl is shown. The virus was acquired in utero as a
result of maternal chickenpox during pregnancy. The rash can involve several sensory dermatomes,
is unilateral, and rarely crosses the midline.
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Blaschko Lines:Following "Lines of Evidence" to a Rash
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Figure 2 - Pityriasis rosea on the back of a 17-year-old girl is shown. The most common presenting
sign is a herald patch (inset) usually found on the trunk-as in this patient-and less often on the neck
and proximal extremities. The long axes of the lesions are along skin cleavage lines (Langer lines).5
Figure
Other disorders follow a linear distribution that does not correspond with dermatomes or skin
cleavage lines. For instance, the distribution of poison ivy dermatitis is often linear, reflecting a linear
deposition of Rhus antigen (or urushiol) on the person's skin.
Still other skin disorders are linear but not explainable in any of these ways. Could Blaschko lines
explain the linearity of these disorders?
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Blaschko Lines:Following "Lines of Evidence" to a Rash
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HISTORICAL OVERVIEW
In 1901, the German dermatologist Alfred Blaschko described a system of cutaneous lines that
represented the typical pattern that linear nevi appeared to follow.1 Because many skin disorders
that followed these lines were present at birth, Blaschko suspected that they had an embryonic
origin.
About 60 years later, British cytogeneticist Mary Lyon proposed that the mottled appearance of
female mice heterozygous for coat color genes might be explained by the random inactivation of 1
female X chromosome, leading to functional mosaicism.1 This theory was extended to the coloration
of tortoiseshell and calico cats, and Lyon suggested that X-linked human skin diseases showed
similar patterns. About 15 years later, Jackson and Happle proposed that functional X-chromosomal
mosaicism by means of random X inactivation was the mechanism underlying cutaneous diseases
that follow Blaschko lines.2,3 Current thinking suggests that Blaschko lines depict the directed route
of embryonic ectodermal cell migration, which illustrates how epidermal cells originate in the neural
crest and move toward the periphery in embryos by directional proliferation in keratinocytes and
migration in melanocytes.3 Cells expressing a normal X chromosome make up normal skin, whereas
cells expressing an abnormal X chromosome constitute contrasting, abnormal skin in the distribution
of Blaschko lines.
Other causes of mosaicism (such as postzygotic somatic mutations occurring early in
embryogenesis) can also result in linear skin disorders following Blaschko lines. The extent of skin
involvement depends on the proportion and distribution of cells with the genetic abnormality and on
the developmental stage when the mutation took place.4 The skin involvement and patterning are
more widespread with mutations that occur earlier in embryogenesis but may be confined to a single
anatomical region with later mutations.4
DISTRIBUTION OF PIGMENTATION
Blaschko lines track distinct patterns (Figure 3). They follow a dorsal "V" shape over the spine and
back, an "S" shape over the lateral and anterior areas of the trunk, an arc from the chest to the
shoulders, a spiral shape on the scalp, and a linear longitudinal pattern on the extremities. On the
face, Blaschko lines typically run perpendicularly on the forehead, along the lateral aspects of the
nose, and down to the chin. They do not correspond to dermatomes, cranial nerve distribution, skin
cleavage lines (Langer lines), skin tension lines, embryonic clefts, the lines of lymphatic drainage or
blood supply, or hair tracts.
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Blaschko Lines:Following "Lines of Evidence" to a Rash
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Figure 3 - This
diagram demonstrates the distribution of Blaschko lines.
ASSOCIATED SKIN DISORDERS
A variety of skin disorders can follow Blaschko lines, including inflammatory, dysplastic, dyskeratotic,
and pigmentary conditions. Figure 4 illustrates pigmentary mosaicism following Blaschko lines in a
6-month-old baby with hypomelanosis of Ito. Figure 5 shows a pigmented epidermal nevus along
Blaschko lines in the axilla of a 4-year-old girl with Proteus syndrome.
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Blaschko Lines:Following "Lines of Evidence" to a Rash
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manifestation of pigmentary mosaicism, is shown.
6
Figure 4 - Hypomelanosis of Ito, a
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Blaschko Lines:Following "Lines of Evidence" to a Rash
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manifestation of Proteus syndrome, is shown.6
Figure 5 - An epidermal nevus, a
When examining a child with a skin disorder that follows a linear pattern, try to determine whether it
follows dermatomal lines or the lines of skin cleavage but remember to consider Blaschko lines.
Recognizing a linear pattern that corresponds to Blaschko lines may help you follow the "lines of
evidence" and make the correct diagnosis.
References:
1. Traupe H. Functional X-chromosomal mosaicism of the skin. Rudolph Happle and the lines of
Alfred Blaschko. Am J Med Genet. 1999;85:324-329.
2. Happle R. X-chromosome inactivation: role in skin disease expression. Acta Paediatr Suppl.
2006; 95:16-23.
3. Moss C. Cytogenetic and molecular evidence for cutaneous mosaicism: the ectodermal origin
of Blaschko lines. Am J Med Genet. 1999;85:330-333.
4. Siegel DH, Sybert VP. Mosaicism in genetic skin disorders. Pediatr Dermatol. 2006;23:87-92.
5. Leung AKC, Kong JCW. What's your diagnosis? Pityriasis rosea. Consultant For Pediatricians.
2008; 7:116-120.
6. Williams GP, Dean S. Hyperpigmented macules. Consultant For Pediatricians.
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Blaschko Lines:Following "Lines of Evidence" to a Rash
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2005;4:314-323.
Source URL:
http://www.psychiatrictimes.com/articles/blaschko-linesfollowing-lines-evidence-rash-diagnosis
Links:
[1] http://www.psychiatrictimes.com/authors/rachel-d-rosenbaum-md
[2] http://www.psychiatrictimes.com/authors/william-g-wilson-md
[3] http://www.psychiatrictimes.com/authors/gregory-f-hayden-md
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