Biomedical Engineering Reference
In-Depth Information
variably hyperkeratotic, with an increase in coarseness of hair
occurring during puberty. Incidence has been calculated to be
0.5% of the population. Histologically, these lesions show
rete ridge elongation and basal layer hyperpigmentation
with variable amounts of acanthosis and hyperkeratosis.
Although no nevus cells are found, melanocytes appear to be
increased and the dermis tends to be thickened. In addition,
hypertrophied sebaceous glands and bundles of smooth mus-
cle fi bers and enlarged pilar apparatus may be present. There-
fore, this lesion is more correctly considered an organoid
hamartoma (37,38).
Treatment of Becker's nevus requires therapeutic consider-
ation of its two pathologic components: increased hair growth
and increased pigmentation. The chromophore that must be
targeted in its treatment is melanin. The fi ne clumping of mel-
anin granules in the epidermis requires shorter pulse widths
(nanosecond domain) than the larger hair follicles, which typ-
ically respond best to the millisecond pulse widths used in hair
removal lasers.
Becker's nevi are notoriously diffi cult to clear, possibly
because of their hamartomatous pathophysiology. The QS
ruby, alexandrite, and FD Nd:YAG lasers can effectively
target the increased pigmentation in Becker's nevi, but recur-
rence is common (39). The long pulsed alexandrite laser
offers the best possibility for more permanent clearing of
both pigmentation and increased hair growth. Settings are
similar to those of CALM for the surrounding pigment and
the hair removal lasers are used at their appropriate settings
for hair color and shaft size (see chap. 5). Improvement in
Becker's nevi with non-ablative fractional photothermolysis
suggests another potential treatment modality, although the
effi cacy remains to be studied (40).
which refl ects an inability to halt the underlying pathoetio-
logic mechanism. Laser is only an option as an adjunct to
strict photoprotection and/or a topical regimen. Repigmen-
tation can be minimized by strict sun avoidance and sun
protection, as well as a regular use of hydroquinone. Strict
sun avoidance and sun protection with UVA- and UVB-
blocking sunscreens are of paramount signifi cance. Treat-
ment with QS lasers (ruby, alexandrite, and Nd:YAG), IPL,
erbium, and CO
2
lasers has been reported to be of benefi t in
some patients, but subsequent repigmentation is a common
occurrence, as is postinfl ammatory hyperpigmentation
(44-52). Test sites help to predict which patient will respond
to QS laser treatment. Those with a more superfi cial compo-
nent are more likely to respond to the shorter wavelengths
and those with a dermal component may respond better to
longer wavelengths. Fractional photothermolysis has also
been studied with great interest for the treatment of melasma
(53-55); however, consistent and predictable improvement
remains elusive for this refractory condition. A test site
should be treated before undertaking treatment of the entire
affected area, as some cases may be exacerbated by the
treatment.
Nevocellular Nevi
Common Acquired Nevi
Common acquired nevi appear after the fi rst 6-12 months of
life and enlarge with body growth, reaching a peak average
count in the third or fourth decade. Although the exact fac-
tors that infl uence the natural history of common nevi are
not yet fully understood, a relationship exists between sun
exposure and the number of nevi. The number of nevi
increases with a greater concentration in sun-exposed areas.
In addition, environmental factors, such as a propensity to
burn rather than tan, a history of sunburn, a tendency to
freckle, and a lifestyle involving increased sun exposure, are
also related to the number of benign pigmented nevi in chil-
dren. Unfortunately, an increased number of benign melano-
cytic nevi serves as an independent risk factor for malignant
melanoma. Therefore, any treatment of common acquired
nevi without histologic evaluation should be undertaken
cautiously.
The effi cacy of QS laser treatment of nevi is variable.
Smaller and thinner nevi respond better; all three QS laser
systems (QS ruby, alexandrite, and Nd:YAG lasers) are effec-
tive with minimal side effects. Unfortunately, many nevi
recur or only partially respond. Long pulsed lasers (alexan-
drite and diode) have been shown to more effectively eradi-
cate these nevi in fewer treatment sessions, using fl uences of
40-60 J/cm
2
, and 8- to 12-mm spot sizes; however, hyper-
trophic scarring can occur (56). Cooling may be turned off
for better effi cacy as long as the surrounding skin is pro-
tected. Settings should be determined with consideration of
the nevus color, in the context of the patient's skin type.
Ablative resurfacing with an Er:YAG laser has been reported
in a small cohort to result in cosmetically satisfactory
results (57).
Melasma
Melasma is an acquired, usually symmetric, light- to dark-
brown facial hypermelanosis that develops slowly. It may be
idiopathic or more often associated with pregnancy or inges-
tion of oral contraceptives. This disorder, with a reported inci-
dence of 50-70% in pregnant women, usually occurs in the
second and third trimesters. It has an incidence of 8-29% in
women taking oral contraceptives. The etiology and patho-
genesis of melasma are unknown; however, a genetic predis-
position is supported by a 21% familial occurrence in one
series (41-43).
Two types of melasma exist on histologic examination. In
the epidermal type, the major sites of melanin deposition are
in the basilar and suprabasilar layers. The melanocytes have
been found to contain highly melanized melanosomes. The
dermal type is characterized by melanophages in the superfi -
cial and deep dermis in addition to the epidermal hyperpig-
mentation. A simple clinical differentiation between the two
types can be made by using a Wood's lamp, which shows
enhancement of pigmentation in the epidermal type and not
in the dermal type.
Melasma is frustrating for both the patient and physician.
Improvement in pigmentation is diffi cult and recurs easily,
