The ruby laser can reduce deep pigmentation and the number of thick, dense hairs in congenital nevi. Whether repeated laser irradiation poses a risk of malignant change is unknown. Imayama and Ueda obtained biopsies of laser-treated congenital nevi in 10 patients to evaluate histologic changes after ruby-laser treatment.

Patients in the study received one to eight laser treatments, and follow-up was a mean of four years, nine months (range: one hour to eight years, 10 months). Cosmetic results were classified as excellent in one patient, good in seven, fair in one and poor in two. In two patients, overall results were not assessed because of the short (seven days or less) follow-up. The patients with poorly responsive lesions exhibited persistent deep pigmentation even after multiple laser treatments. Specimens were obtained from three patients to evaluate short-term histologic changes following a single laser treatment.

The congenital nevi in eight patients with excellent and good cosmetic results exhibited a marked reduction in deep pigmentation and the number of coarse hairs. The surface of the skin was smooth and unscarred. Histology revealed residual nevus cells below the surface. The development of a microscopic scar about 1 mm thick was required to mask the underlying residual pigmentation. Improvement in skin texture was attributed largely to the presence of the microscopic scar. Although hair follicles tended to regenerate after each laser treatment, the hair shafts became increasingly thinner as the hair bulb was repeatedly exposed to the thermal damage of the laser. The reduction in dense hair contributed greatly to the desired cosmetic effect.

The congenital nevi in the patients with poor cosmetic results exhibited deep pigmentation and a rough surface even after multiple treatments. Histology revealed scattered nests of pigmented nevus cells in the epidermis. Unlike the patients with good cosmetic results, patients with poor results had relatively abundant melanin in basal keratinocytes. The authors believe the nests of pigmented nevus cells presumably represent the regeneration of nevus cells that contained little melanin and were thus spared from thermal damage.

Immediately following laser treatment the congenital nevi in two patients showed a scorched appearance similar to that of a chemical burn. The epidermal cells showed evidence of acute thermal injury. Within 24 hours, the damaged epidermis had become eroded and by the fourth day, the epidermis had begun to regenerate and accumulate layers.

Regardless of the cosmetic results, no mitotic or atypical cells were found in the residual nevus cells in any of the 10 patients who were followed for two to eight years. No patients exhibited atypical mitotic figures in the cells of the epidermis, hair follicles, sweat glands or surrounding connective tissue.

The authors conclude that no malignant transformation occurred during long-term follow-up of congenital nevi treated with the ruby laser. However, they recommend further follow-up to determine whether regenerated nevus cells of poorly responsive lesions or residual nevus cells in responsive lesions are associated with a risk of malignancy. Routine palpation of the area is recommended to try to detect any changes below the microscopic scar. The microscopic scar could mask such changes and obscure the remaining nevi.