The laser hair removal treatment in some patients has been found to result in a terminal growth of hair in untreated areas in close proximity to the treated ones [46-50]. In most cases, this paradoxical hair growth occurs at a site adjacent to the treatment area that has high vellus growth, and is relatively free of terminal hair. Face is the most common site where the terminal hair growth stimulation has been noted. It is considered a rare side effect of laser treatment, and the most susceptible population is of Mediterranean descent with darker skin types.
In a retrospective analysis of 750 patients of Mediterranean ancestry, with 4374 subjects administered laser treatments over a 7-year period, terminal hair induction was noted in 30 (4%) subjects [51]. In 28 of the 30 cases, the terminal hair growth was on the face. There was no relation as to which laser system or energy fluence was used. The majority of the subjects who experienced hair stimulation were of skin types III or IV.
In another retrospective study of 543 patients treated over five years in a dermatology clinic in Spain, the incidence rate for hair growth stimulation was much higher at 10.5% [52]. The treatments were administered on the face: cheek, chin, and neck. Increased terminal hair growth was noted both within the treatment area and in areas adjacent to the treated site. Majority of the subjects in this study were Type III (68%), although Type II (13%) and Type IV (19%) were also present. There was no correlation between the hair growth stimulation and the skin type. Researchers in this study believe that the hair stimulation was related to subtherapeutic dose of the laser reaching the follicles bordering the treatment area. To this end, the researchers treated additional 200 subjects over a two-year period with concomitant use of cold packs placed on areas surrounding the treatment site, and have not had a single incidence of hair stimulation since.
Unlike the studies from Greece [51] and Spain [52], a retrospective study from Canada involving a similar patient group size of 489 the incidence rate for terminal hair stimulation was found to be only 0.6% (3/489 patients) [53]. All three subjects had Type IV skin. The study concluded that though hypertrichosis is real, it is a rare occurrence in the Canadian population studied.
In a study of 49 patients with facial hirsutism with polycystic ovarian syndrome, 5 of the 49 subjects treated with an IPL source showed terminal hair stimulation [46]. All subjects had Type III skin and the study was conducted in Spain. This 10% incidence rate was similar to the larger study from Spain by Willey [52]. Though paradoxical hair growth has mostly been seen on face, a case study after diode laser treatment on a man’s back was reported by Bernstein [54].
Isolated cases of localized hypertrichosis have been noted under conditions that result in dermal trauma, stress, or inflammation. Increased localized hair growth has been reported after fracture cast application [55], bug bites [56], site surrounding a burn [57], or scar [58], and site of chronic inflammation [59,60]. An excellent review on different forms and causes of hypertrichosis has been written by Wendelin [59]. Since the laser-induced paradoxical hair growth has mostly been associated with the Mediterranean population, it is not clear what the underlying cause is for this stimulation.
