Clinical Classification of melasma based on distribution pattern. (a) Centrofacial melasma, (b) malar melasma, and (c) mandibular melasma
6.2 Wood’s Lamp
Wood’s lamp is an ultraviolet light A of long wavelength which emits wavelength between 340 and 400 nm with a peak at 365 nm [6, 7]. It is the most widely used method of melasma classification by highlighting the difference in pigmentation of the affected skin into four subtypes: epidermal, dermal, mixed, and indeterminate [2, 3, 8].
The pigmented areas that are enhanced when viewed under a Wood’s lamp imply increased epidermal melanin content (epidermal subtype) (Fig. 6.2b), whereas those that are not enhanced imply an increased in dermal melanin content (dermal subtype) (Fig. 6.3b). The pigmented areas that have both enhancing and non-enhancing areas imply a mixed subtype of melasma (Fig. 6.4b). For those with darker skin type V and VI, their melasma will not be evident under Wood’s lamp examination, and hence, this is classified as indeterminate subtype. Many patients have a mixture of these subtypes of melasma [1–3].
(a) Epidermal melasma on the right cheek. (b) Wood’s lamp showed enhanced pigmented areas. (c) Dermoscopy showing dispersed reticular brown pigmentation
(a) Dermal melasma on the right cheek. (b) Wood’s lamp showed non-enhanced pigmented areas. (c) Dermoscopy showing diffuse dark brown to grayish pseudoreticular pigmentation
(a) Mixed type of melasma on the right cheek. (b) Wood’s lamp showed both enhanced and non-enhanced pigmented areas. (c) Dermoscopy showing both diffuses reticular brown pigmentation with sparing of follicular openings and diffuses dark brown to grayish pseudoreticular pigmentation
Recent histopathological studies suggested that Wood’s light examination may not be accurate in determining the depth of pigment [2, 8–10]. Despite Wood’s lamp evaluation indicating epidermal melasma in some patients, the biopsy specimens from both the lesional and perilesional skin examined by Grimes et al. and Kang et al. showed that there was increased melanin deposition in both epidermis and dermis. Therefore, patients with apparent epidermal melasma after a Wood’s lamp examination may have significant melanin in the dermis. The presence of dermal melanin and melanophages in these studies may explain the difficulty in treating patients with apparent epidermal melasma [1, 2, 9, 10].
Dermoscopy is a noninvasive optical equipment that permits a variable magnification from 6 to 400 times. It is a proven reliable tool for direct visualization of skin pigmentation including melasma as it has been shown to be useful to observe pigment components as well as their position on the skin layers [11–14]. The color intensity of melanin and the regularity of the pigment network depend on the quantity or density and the location of melanin.
When the melanin is located in the stratum corneum, it appears as black or dark brown in color with well-defined network. When the melanin is located in the lower layers of the epidermis, the color will appear as shades of light brown, and the pigment network will appear irregular. Dermoscopy of melanin located in the dermis will appear blue or bluish-gray color with pseudoreticular network. The follicular openings are spared throughout the level of pigmentation [12–15]. Therefore, melasma is considered epidermal when a regular pigment network, with a brownish homogeneous pigmentation, is observed (Fig. 6.2c), and it is considered dermal when an irregular and mixed network with bluish-gray pigmentation is observed (Fig. 6.3c). Finally, the melasma is considered a mixed subtype when the areas show both features (Fig. 6.4c).
Additionally, dermoscopy allows observation of a vascular component (Fig. 6.5), which is present in many patients with melasma, as reported in recent literature [15–17]. An immunohistochemistry study demonstrated a significant increase in the number and size of dermal blood vessels in the lesional skin of melasma . It has also been suggested that the number of vessels is positively related to the degree of pigmentation, which is also reported in other studies which state that the deoxyhemoglobin contributes significantly to the skin color [16–18].
Epidermal melasma with vascular component. Dermoscopy on the left cheek melasma showing diffuse brown pigmentation with fine telangiectasias
Based on the principles of dermoscopic examination, some dermatologists may consider this method more appropriate and helpful for routine diagnosis, assessment, and monitoring of patients with melasma when compared to Wood’s lamp examination. This may be due to the fact that dermoscopy examination allows an objective classification of melasma by providing an accurate observation of the color of melanin which is not affected by factors such as the patient’s skin phototype, vascular and collagen changes, or the use of topical products .
6.4 Reflectance Confocal Microscopy
In the recent years, various noninvasive imaging tools such as high-frequency ultrasonography, optical coherence tomography, magnetic resonance imaging, and reflectance confocal microscopy (RCM) have been developed to provide additional information that is not readily available through mere clinical inspection. All these imaging tools are noninvasive and can provide both real-time diagnostics and also the possibility of following the progression of skin lesions or conditions over time.
Among these new techniques, RCM has emerged to be a novel noninvasive imaging technique that can provide real-time in vivo examination of the skin up to the level of the papillary dermis while providing a cellular resolution comparable with histology. In a confocal microscope, near-infrared light from a diode laser (830 nm) is focused on a microscopic skin target. As this light passes between cellular structures with different refraction indexes generated mainly by keratin, melanin, hemoglobin, and cellular organelles, it is naturally reflected, captured, and recomposed into a two-dimensional gray scale image by computer software [19–21]. This technique has been used for the evaluation of several inflammatory, neoplastic, and melanocytic skin conditions and may constitute an excellent alternative to invasive skin biopsy in the diagnosis of several skin disorders [22–26].
As melanin is the best endogenous contrast agent in the pigmented skin, which causes strong backscattering, this would allow precise identification of melanocytes, pigmented keratinocytes, and melanophages. A few studies reported the role of RCM in the classification of melasma and found good correlation with histology [27–30]. The distributions of melanin were found to be located at all the different levels of the melasma lesion (epidermis, dermoepidermal junction (DEJ), and upper dermis). At the level of the epidermis, increased pigmentation or melanin was observed as highly refractile keratinocytes distributed within the spinous layer and basal layer. At the DEJ, the presence of strongly visible papillary rings around the dermal papillae composed by a sequence of bright cellular structures was observed, corresponding to activated melanocytes and junctional keratinocytes receiving packed melanosomes. At the superficial dermis, an abnormal presence of round or polygonal refractile structures within dermal collagen bundles was observed. These structures were consistent with melanophages filled with melanin originating in the DEJ [27, 29, 30].
Based on the distribution of melanin, RCM has classified melasma into epidermal type if the distribution of increased melanin was observed only in the epidermis (Fig. 6.6a, b) and mixed type if the distribution of increased melanin was observed in both epidermis and dermis (Fig. 6.7a, b). No genuine dermal type of melasma has been found in RCM studies [27, 29, 30]. This differs from the traditional Wood’s lamp classification of epidermal, dermal, mixed, and indeterminate subtypes. Wood’s lamp technique is not microscopic, but based on the quantification of the different levels of fluorescence according to the depth of the pigment, and does not take into account the contributions of different layers. Hence, it has been shown to have questionable accuracy when compared histologically [2, 8–10, 27]. Ardigo et al. also found no correlation between the classification of melasma using Wood’s lamp and RCM . For example, there were cases classified as an epidermal melasma using the Wood’s lamp, but RCM imaging revealed pigmentation both in the epidermis and dermis.