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An 8-year-old girl presents at Beijing Children's Hospital due to concern about a dark blue band that has appeared on the nail of her right middle finger. Her parents explain that this appeared about 9 days earlier. The child says she has not injured her finger, and says she has not inserted anything under her fingernail. The discoloration is not causing her pain, nor is it tender to the touch.

Clinical examination identifies a blue-black pigmented band with a sharp regular lateral border 0.5-mm wide. The clinician who examines the nail suspects a possible nail matrix nevus.

Dermoscopic assessment reveals a dark blue, longitudinal, homogeneous, linear patch on the nail that extends from the proximal edge in a linear fashion, widening gradually toward the distal edge.

Examination from the free edge of the nail shows that the blue substance is under the nail rather than within it; microscopy shows that the color cuts off sharply at the edge of the nail. The blue-green color noted in the specimen fades over the course of about 30 minutes.

Patient History Leads to Diagnosis

On further discussion with the family, the child recounts playing with the toy known as "green slime" about 11 days earlier. Based on the brief medical history, as well as the findings of the clinical and dermoscopic examinations, clinicians diagnose the longitudinal band as exogenous pigmentation related to the child's contact with the substance, and decide to observe her status for a period of time. At her 2-month follow-up examination, they note that the pigmented band is growing out toward the distal nail edge; over time, it eventually disappears.

Discussion

Clinicians reporting this ¹ note that it may be the first reported instance of longitudinal pigmentation appearing below the nail plate from an exogenous source. They suggest that such an etiology is worthy of consideration in the differential diagnosis of recent-onset longitudinal melanonychia (LM).

The condition is a brown-black pigmented band extending from the matrix up to the distal part of the nail plate, usually induced by melanocyte activation, lentigines, nevi, and malignant melanomas.² While exogenous pigments can also cause melanonychia, they typically do not form regular linear bands.

Careful assessment of LM is important because most subungual melanomas present as LM.³ While malignant melanoma in situ has been observed in children,⁴ the most important cause in children is an innocent nail matrix nevus.⁵

Melanonychia can result from exogenous pigmentation, but often manifests as an irregular stain of the nail surface, which can be scraped away with a blade.

Likewise, the case authors cite reports of indelible ink staining that mimicked acral melanoma, leading dermatologists to suspect subungual melanoma.^6,7

The authors note that this finding must be distinguished from nail matrix nevus in children, which usually has a longer history, a brown or black color, gradual parallel widening or a wider width on the proximal end than on the distal end, and does not disappear as the nail grows.

A reported ⁸ from a 2018 American Academy of Dermatology meeting notes that in addition to findings from a patient history, discoloration of the proximal nail fold, and/or pigment that does not extend all the way proximally to the lunula also point to an exogenous source of pigment. Some common and less-common sources include tar, tobacco, henna and other hair dyes, potassium permanganate, and newspaper print.

Green slime is composed mainly of polyvinyl chloride and polypropylene, which have non-Newtonian fluid properties. It was presumed that after contact with the toy, some of the slime became embedded in the gap between the distal nail and the hyponychium, the case authors write. Because the non-Newtonian fluid has the property of a tubeless siphon, the slime entered the nail bed along the longitudinal ridge of the nail bed.

is an important diagnostic tool in helping to identify the etiology of LM and avoid misdiagnosis. Contact dermoscopy with ultrasound gel can be especially helpful in some cases. Because melanomas can bleed, photographs taken by patients or in-office can be used to track a hematoma until it is resolved.⁸

Clinicians reporting this case concluded that the experience should remind dermatologists of the importance of obtaining a comprehensive history, and of considering exogenous pigmentation in the differential diagnosis in patients with recently developed LM.

References

1. Liang Y, et al: Exogenous Pigmentation Presenting as Longitudinal Melanonychia in a Child. JAMA Dermatol 2019; doi:10.1001/jamadermatol.2019.3114

2. Mannava KA, et al: Longitudinal melanonychia: detection and management of nail melanoma. Hand Surg 2013; 18(1): 133-139

3. Haneke E, Baran R: Longitudinal melanonychia. Dermatol Surg 2001; 27(6): 580-584

4. Tosti A, et al: In situ melanoma of the nail unit in children: report of two cases in fair-skinned Caucasian children. Pediatr Dermatol 2012; 29(1): 79-83

5. Tseng YT, et al: Longitudinal melanonychia: differences in etiology are associated with patient age at diagnosis. Dermatology 2017; 233 (6):446-455

6. Prose NS: The linear nail streak: a lesson in cultural humility. JAMA Dermatol 2017; 153(10): 1064-1065

7. Stashak AB, et al: Exogenous pigmentation mimicking acral melanoma: a case of Talon d'Oyer. JAMA Dermatol 2014; 150 (10): 1117-1118

8. Oakes K: Longitudinal melanonychia: the good, the bad, and the confusing: Expert Analysis from Summer AAD 2018. MDedge Dermatology 2018.

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