Abstract: 【Abstract】 Objective To analyze clinical and histopathological characteristics of infantile congenital melanocytic nevi （ICMN）. Methods Clinical and pathological data were collected from 126 infants with confirmedly diagnosed congenital melanocytic nevi in Department of Dermatology, Xijing Hospital from January 2015 to January 2020, and were retrospectively analyzed. Chi-square test was used for comparisons of enumeration data. Results Among the 126 patients with ICMN, 68 were males and 58 were females; 109 （86.5%） presented with skin lesions at birth; 73 （57.9%） were 2 - 3 years old at the first clinic visit. The skin lesions occurred on the head and face （76 cases, 60.3%）, trunk （24 cases, 19.1%） or extremities （26 cases, 20.6%）. There were 36 （28.6%） patients with small congenital nevi, 68 （54.0%） with M1-type medium-sized nevi, 13 （10.3%） with M2-type medium-sized nevi and 9 （7.1%） with giant nevi. Of 126 cases of ICMN, 121 （96.0%） had solitary lesions, 5 （4.0%） had multiple lesions, 44 （34.9%） had nevi with coarse hairs, 15 （11.9%） had nevi complicated by papules or hyperplastic nodules, and 6 （4.8%） had satellite lesions. Pathological subtypes included compound nevus （120 cases, 95.2%）, intradermal nevus （4 cases, 3.2%）, and junctional nevus （2 cases, 1.6%）. Under the microscope, the depth of the skin lesions was < 1 mm in 38 （30.1%） cases, 1 - 2 mm in 61 （48.4%） and > 2 mm in 25 （19.8%）, and 45 （35.7%） cases showed nevus cells infiltrating the subcutaneous fat layer or deeper tissues. Among the 126 ICMN lesions, common pathological features included nevus tissue maturation （100%, 2 cases of junctional nevi were excluded）, pigment granules in the stratum corneum （53 cases, 42.1%）, disordered/asymmetric distribution of nevus cells （80 cases, 63.5%）, scattered epidermal nevus cells （91 cases, 72.2%）, pagetoid spread of epidermal nevus cells （67 cases, 53.2%）, melanophages in the dermis （71 cases, 56.4%）, and nevus cells distributed along hair follicles/sebaceous glands（82 cases, 65.1%）. Special pathological features included nevus cells embedded in the vascular/lymphatic vessels （42 cases, 33.3%）, nevus cell lysis （45 cases, 35.7%）, fibromatous changes （25 cases, 19.8%）, involvement of the arrector pilli muscles （31 cases, 24.6%）, and mast cell infiltration （30 cases, 23.8%）. Pathological patterns of ICMN with different clinical features: the incidences of infiltration depth > 2 mm, pigment granules and columnar pigment granules in the stratum corneum were significantly higher in the giant nevi than in the small and medium-sized nevi （χ2 = 7.93, 10.76, 5.89 respectively, all P < 0.05）; the incidences of infiltration depth > 2 mm, epidermal spongiosis with scattered nevus cells, nevus cell nests distributed along the hair follicles/sebaceous glands, fibromatous changes and mast cell infiltration were significantly higher in the skin lesions with coarse hairs than in those without （χ2 = 28.29, 8.11, 6.22, 7.92, 8.19 respectively, all P < 0.01）; the incidences of pagetoid spread of epidermal nevus cells and atypical nevus cells were significantly higher in the skin lesions with papules/hyperplastic nodules than in those without papules/hyperplastic nodules （χ2 = 4.92, 6.30 respectively, both P < 0.05）. Conclusions The clinical and histopathological characteristics of ICMN are unique, and atypical nevus cells are common in ICMN. The diagnosis and treatment of ICMN need to be based on the combination of clinical and pathological characteristics.

Key words: Infant, Nevi and melanomas, Skin manifestations, Pathologic processes, Congenital melanocytic nevi