Ji Su Lee MD^a, Sungbum Kim MD^a, Dong Hyo Kim MD^a, Ji Won Lee MD^a, Je-Ho Mun MD, PhD^a,b,c, Si-Hyung Lee MD, PhD^a,b,c,*

^aDepartment of Dermatology, Seoul National University Hospital, Seoul, Korea
^bDepartment of Dermatology, Seoul National University College of Medicine, Seoul, Korea
^cInstitute of Human-Environmental Interface Biology, Medical Research Center, Seoul National University, Seoul, Korea

^*Corresponding author.

Background
Large studies on the clinical features and natural course of pediatric longitudinal melanonychia (LM) are lacking.

Objective
To investigate the clinical features and natural course of pediatric LM.

Methods
Retrospective cohort analysis of pediatric patients (age ≤ 18 years) with LM.

Results
We examined 703 LM lesions in 381 children. Single, narrow, and homogeneously pigmented fingernail lesions were most frequently observed. Our results suggested that within 3, 4.5, and 9.5 years after onset, approximately 3%, 5%, and 10% of LM lesions, respectively, will completely regress and that single, left-sided, and homogeneously pigmented lesions are more likely to disappear completely. The age of onset, sex, finger/toe position, Hutchinson’s sign, and nail dystrophy were not associated with complete regression. During follow-up, most cases demonstrated no change in color or width between the first and last visit, and early darkening/widening before stabilization or lightening/narrowing was common. The lightning of pigmentation was associated with complete regression, whereas change in width was not.

Limitations
Retrospective study at a tertiary center.

Conclusion
Our results suggest that clinicians ought to follow pediatric patients with LM without intervention for several years even if lesions grow darker or wider. Single, left-sided, and homogeneously colored lesions are more likely to regress.