Biomedical Engineering Reference
In-Depth Information
13
Use of lasers on Asian skin
Woraphong Manuskiatti
introduction
The approximately 4.2 billion Asians in China, India, Japan,
the Middle East, Southeast Asia, and elsewhere represent the
majority of the world's population (1). Asian population is a
diverse group with various skin phototypes ranging from Fitz-
patrick types III to V. Racial differences in skin pathophysiol-
ogy have been well documented (2-4). The high risk of
pigmentary alterations and scarring following any procedure
that produces infl ammation of the skin continues to infl uence
physicians to exercise caution with this group of patients. This
caution also applies to laser therapy. Even with the highly selec-
tive characteristics of current laser and pulsed light therapy,
when results are expected to be similar between the races, they
are not. Both genetic background and environmental factors
are involved in these differences.
Skin laser surgery for Asians is different from that for Cau-
casians in several important characteristics. Asian skin is often
more pigmented than Caucasian skin, resulting in interference
by epidermal melanin when using lasers to treat dermal
lesions. Consequently, adverse pigmentary reactions, espe-
cially postinfl ammatory hyperpigmentation, are more likely to
develop following laser surgery (5,6). Another important issue
is the differences in the biological behavior of melanocytes
among patients from different genetic backgrounds. A contro-
versial deleterious effect of laser exposure is malignant trans-
formation. Unlike the Caucasian population, melanoma is
uncommon among Asians, and differences in skin types are
uncertain to be the main explanation. Therefore, the risk of
using laser for the removal of nevomelanocytic lesions in
Asians differs from that in Caucasians.
Race is also a critical factor in the response of the skin to
infl ammation. Asians are far more likely than Caucasians to
develop keloids. Some conditions such as nevus of Ota or
acquired bilateral nevus of Ota-like macules (ABNOMs, Hori's
nevus) are more commonly seen in Asians (5,6). Furthermore,
photoaging in Asians tends to occur at a later age and have
more pigmentary problems but less wrinkling than that in
Caucasians (7).
melanin. Although there is no difference in the quantity of
melanocytes between the two groups, the larger and more
melanized melanosomes in nonwhite skin compared with
white skin have been well documented (8). In addition, the
degradation rate of melanosomes within the keratinocytes of
dark skin is slower than that of white skin. The larger and
more melanized melanosomes of black skin absorb and scatter
more energy, thus providing higher photoprotection. Con-
versely, the melanocytes and mesenchyma in darker skin seem
to be more vulnerable to trauma and infl ammatory conditions
than those in white skin (9).
The majority of cutaneous laser wavelengths have signifi cant
overlap with the absorption spectrum of melanin (see chap. 1).
Therefore, nonwhite skin presents a signifi cant challenge
because of greater absorption of laser energy and resulting
damage to melanin-laden cells, increasing the risk of adverse
complications, including hypopigmentation, hyperpigmenta-
tion, and depigmentation. Interestingly, alterations in pigmen-
tation may not be apparent for several months after laser
therapy. Thus, when treating nonwhite skin, test sites and
long-term follow-up should be considered.
Asian and black skins have a thicker dermis than white
skin, the thickness being proportional to the intensity of pig-
mentation. This increased dermal thickness, along with pho-
toprotection from an increase in the size and number of
melanosomes, may account for a lower incidence of facial rhyt-
ides in Asians and blacks. Increased mesenchymal reactivity
may result in hypertrophic scars and keloids. Like black skin,
Asian skin has a greater tendency toward hypertrophic scar-
ring. Asians may also have a greater tendency toward prolonged
redness during scar maturation than whites do (10,11).
Fitzpatrick developed the classifi cation of skin phototypes
based on response to ultraviolet (UV) irradiation of the Cau-
casian population (Box 13.1) (12). However, it has often been
found that a patient with skin phototype I or II may have
genetic origins of skin phototypes III-VI. Given the same
clinical expertise in a specifi c cosmetically sensitive proce-
dure, such as laser surgery, the result would be signifi cantly
different in clinically similar patients if one had considered
more distant ancestry. Lancer proposed the so-called Lancer
Ethnicity Scale (Table 13.1), factoring in this additional his-
torical information to provide a method to presurgically
determine skin type of the patients and more clearly predict
the outcome (13,14). Goldman proposed a “universal classi-
fi cation of skin type” that considers genetic racial heritage in
the response of melanocytes to both UV light and infl amma-
tion (Box 13.2) (15).
differences between pigmented
and white skin
The surgeon who considers performing laser surgical proce-
dures in non-Caucasian patients should have an understand-
ing of the morphological differences between white and
nonwhite skin, especially in patients of Asian and black
descent. The major determinant of differences in skin color
between nonwhite and white skin is the amount of epidermal
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