## 4 Discussion
showed increased epidermal pigmentation and expression of certain inflammatory cytokines after laser irradiation, suggesting that they may be responsible for induction of PIH [5]. We found that patients with IEL had a higher probability of petechiae. This result indicated inflammation and injury to the skin tissue, and therefore a greater likelihood of developing PIH. The PIH risk was higher in patients receiving AHA peels than in those without AHA peels in this study. With application of AHA peels, \(\alpha\)-hydroxy acid may be dispersed within the basal layer melanin via epidermolysis [22]. Although AHA peels can be used to treat patients with PIH [22], they can also cause mild inflammation and injury to the skin, which can potentially exacerbate PIH [23]. The addition of laser toning can result in skin irritation, leading to PIH. To avoid irritation, care should be taken when using an AHA peel in combination with laser toning, which can cause PIH. As the epidermal melanin content is high in Asian skin, adverse pigmentary effects are more likely to occur after laser treatment [1,24]. PIH increases pigmentation secondary to an acquired inflammatory process, which occurs more easily in FSPTs III and VI and can have a considerable effect on quality of life [25]. Therefore, skin color is a significant risk factor for PIH, with deeper skin color requiring more care. A study performed in Singapore indicated that Asians with darker skin are more likely to develop PIH than Asians with lighter skin, suggesting that the degree of pigmentation may have a stronger association with PIH than race [23]. With regard to acne as a risk factor, [PERSON] et al. reported that PIH is a particularly common sequela to acne in darker skin types (Fitzpatrick phototypes III-VI) [23,25]. Acne-related PIH can be more problematic than the acne. [PERSON] et al. reported that PIH may occur without obvious clinical inflammation in patients with mild to moderate acne [6]. Acne was an important predictive risk factor for PIH in this study. PIH may be worsened by laser toning in acne patients, but data are limited regarding this issue. Physicians should take this risk factor into consideration when treating patients with Nd-YAG laser toning. With regard to melasma, FSPT and melasma facilitate melanin production. An increase in melanin can be induced by stronger laser treatment. Both [PERSON] et al. and [PERSON] et al. reported that rebound PIH occurs frequently with laser toning for treatment of melasma and skin rejuvenation [6,26]. Our results are similar to those of [PERSON] et al., who suggested that the intensity of PIH is determined by the inherent skin color, inflammatory conditions, and melanocyte stability [6]. Although PIH is an important complication in patients undergoing multisession laser toning [6], in the present study, treatment frequency was the least important predictive factor in patients undergoing laser toning. As it involves downregulation of melanogenesis (resulting in decreased melanocyte function) [27], hypopigmentation may be a consequence of cumulative phototoxic damage to melanocytes and keratinocytes caused by laser therapies [2,21,28]. Frequent use of laser toning may result in increased formation of hypopigmented macules [2,21,29]. Patients undergoing multiple sessions of laser toning should be made aware of the potential complications [29]. This study had some limitations: the treatment modalities may have differed among countries and institutes, where differences in laser toning methods may be associated with variation in PIH severity. The study population was small. To promote adoption of the PIHCP model and reduce the uncertainties, longer-duration prospective studies including larger populations are required.