2. Pathophysiology and histopathology of laser toning-induced hypopigmentation/dyspigmentation (LTID)
In the previous article, we discussed LT techniques that are associated with a higher likelihood of hypopigmentation.1 So, what is the pathogenesis of post-LT hypopigmentation at the skin or cellular level? Little is known about the etiological mechanism of LTID at the cellular level. From the earliest to the most recent studies focusing on the occurrence of mottled hypopigmentation, most speculated that hypopigmentation might arise due to phototoxicity (excessive fluence level or toxic level of total cumulative energy) that damaged melanocytes.2,22,23 The cellular factors which are involved in hypopigmentation have not been hypothesized more scientifically beyond this supposition.
Despite lack of evidence in its pathogenesis, analyzing histologic features of LTID can help us infer it at the cellular level. Regarding this matter, one can ask two questions: Is hypopigmentation caused by destruction of melanocytes due to phototoxic damage? Or, is it driven by functional impairment of melanocytes? In 2010, Kim et al.24 published the first report regarding this topic. They performed biopsy in one patient with melasma and LTID. The patient had developed punctate hypopigmentation after the seventh treatment session of LT with 1,064 nm Q-switched (QS) Nd:YAG laser (7 mm spot size, 2.5J/cm2) given over three-week intervals. The sample tissue was taken from the hypopigmented lesion. Hematoxylin and eosin staining showed no special findings, however, Fontana-Masson staining revealed a small number of melanin granules. Melan-A staining also revealed a small number of melanocytes. Therefore, the authors concluded that “LT-induced loss of melanocytes” might cause mottled depigmentation after LT, resulting in “melanocytopenic” hypopigmentation, rather than “melanopenic”.24
It was not until 2013 that three follow-up studies were published in a row. Kim et al.21 examined clinical and histological outcomes of LT in melasma and included histological analysis of hypopigmentation. Electron microscopy of samples obtained from two patients with severe mottling (Grade 4) showed that the number of melanosomes was markedly decreased while the number of melanocytes was unchanged. These results indicated that the leucoderma lesions were “melanopenic”, not melanocytopenic, thus LTID might be caused by reduction of melanogenesis.
In 2013, Ryu et al.25 also published histological findings from one case of hypopigmentation arising after LT carried out in melasma. They found that the number of melanocytes did not decrease in the hypopigmented lesion and suggested LTID was regarded as melanopenic condition without destruction of melanocytes, refuting the first report.
In 2015, Jang et al.26 performed biopsy in two patients with hypopigmentation. They found the number of melanocytes was similar to that in adjacent normal tissues and the site of melasma lesion. They supported the melanopenic argument and criticized Kim et al.24 for not comparing the histologic properties of the lesion to those of the surrounding skin.
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-To be continued
References
21. Kim JE, Chang SE, Yeo UC, Haw S, Kim IH. Histopathological study of the treatment of melasma lesions using a low-fluence Q-switched 1064-nm neodymium:yttrium-aluminium-garnet laser. Clin Exp Dermatol 2013;38:167-71.
22. Vachiramon V, Sahawatwong S, Sirithanabadeekul P. Treatment of melasma in men with low-fluence Q-switched neodymium-doped yttrium-aluminum-garnet laser versus combined laser and glycolic acid peeling. Dermatol Surg 2015;41:457-65.
23. Sugawara J, Kou S, Yasumura K, Satake T, Maegawa J. Influence of the frequency of laser toning for melasma on occurrence of leukoderma and its early detection by ultraviolet imaging. Lasers Surg Med 2015;47:161-7.
24. Kim T, Cho SB, Oh SH. Punctate leucoderma after 1,064-nm Q-switched neodymium-doped yttrium aluminum garnet laser with low-fluence therapy: is it melanocytopenic or melanopenic? Dermatol Surg 2010;36:1790-1.
25. Ryu HJ, Kim J. A case of mottled hypopigmentation after low-fluence 1,064-nm Q-switched neodymium-doped yttrium aluminum garnet laser therapy. J Cosmet Laser Ther 2013;15:290-2.
26. Jang YH, Park JY, Park YJ, Kang HY. Changes in Melanin and Melanocytes in Mottled Hypopigmentation after Low-Fluence 1,064-nm Q-Switched Nd:YAG Laser Treatment for Melasma. Ann Dermatol 2015;27:340-2.
