Given the over 1-year history of an unchanging longitudinal band of pigment without extension to the proximal or lateral nailfolds or any other nail findings, the most likely diagnosis is benign longitudinal melanonychia.
Longitudinal melanonychia, also known as melanonychia striata, describes a brown to black streak of pigment extending from the nail matrix to the free edge of the nail.1,2
This disorder can occur secondary to a wide variety of benign and pathologic causes including lentigines, nevi, melanoma, chronic trauma, inflammatory skin diseases, systemic diseases, iatrogenic causes, and genetic syndromes.3 In melanocytic causes of longitudinal melanonychia, either melanocytic activation or hyperplasia drive pigmentary development leading to the brown to black band seen in the nail.4 Benign causes of longitudinal melanonychia include benign melanocyte activation, lentigo, and benign nevus.1
What’s the differential diagnosis?
The differential diagnosis for longitudinal melanonychia can include a wide variety of local and systemic causes. For our discussion, we will limit our differential to other locally involved disorders of the nail including subungual melanoma, subungual hematoma, onychomycosis, and glomus tumor.
Subungual melanoma is a rare subtype of acral lentiginous melanoma that most often presents as longitudinal melanonychia. Subungual melanoma is more common in those aged 50-70 years, individuals with personal or family history of melanoma or dysplastic nevus syndrome, and persons with African American, Native American, and Asian descent. Longitudinal melanonychia features that can be concerning for subungual melanoma include the presence of multiple colors, width greater than or equal to 3 mm, blurry borders, rapid increase in size, and extension to the proximal or lateral nailfolds (Hutchinson’s sign). Biopsy is required to make the diagnosis of subungual melanoma but is not necessary for melanonychia without atypical features.
Treatment of subungual melanoma depends on disease stage and can range from wide local excision of the nail apparatus to amputation of the affected digit and management with a medical oncologist. Given the absence of concerning neoplastic findings or personal or family history of melanoma, subungual melanoma is unlikely in this patient.
Subungual hematoma is an accumulation of blood underneath the nail plate that is typically the result of acute or chronic trauma to the distal phalanx. It can present as purple, red, pink, brown, or black discoloration under the nail plate and is most commonly found on the first toe. With acute trauma, pain is usually present upon initial injury. Subungual hematomas typically resolve on their own with normal nail growth. The absence of a history of trauma or pain, and the linear appearance of the lesion in our patient are inconsistent with a subungual hematoma.
Onychomycosis is a fungal infection of the nail caused by dermatophytes, nondermatophytes, or yeasts. It may present with longitudinal melanonychia; however, it more often presents with other nail abnormalities such as nail thickening, yellow discoloration, onycholysis, splitting, subungual hyperkeratosis, and nail plate destruction, which are not present in this patient. Furthermore, onychomycosis is more common in adults than children. Diagnosis is usually made with potassium hydroxide (KOH) preparations, histopathologic examination of nail clippings with a periodic acid-Schiff stain, fungal culture, or PCR.
Glomus tumor is a rare, benign neoplasm originating from cells of the glomus body. It is often found in the subungual region, in addition to other areas rich in glomus bodies such as the fingertips, palms, wrists, and forearms. Subungual glomus tumors present as a red, purple, or blueish lesions under the nail plate. Distal notching or an overlying longitudinal fissure may be present. Subungual glomus tumors are typically associated with pinpoint tenderness, paroxysmal pain, and cold sensitivity, features that are not present in our patient. The history and examination of our patient are much more consistent with benign longitudinal melanonychia.
It appears that melanoma associated with longitudinal melanonychia is very rare in children. According to one review published in 2020, only 12 cases of pediatric subungual melanoma have been reported.5 Recent series have observed longitudinal melanonychia in large sets of children, with findings that demonstrate that the vast majority of longitudinal melanonychia either stops progressing or regresses. These investigations therefore recommend serial observation of longitudinal melanonychia except in rare circumstances.6,7
Given the lack of troubling findings or concerning history, our patient was managed with observation. On follow-up 6 months later, he was found to have no change in his nail pigmentation.
Dr. Haft is an inflammatory skin disease fellow in the division of pediatric and adolescent dermatology; Ms. Sui is a research associate in the department of dermatology, division of pediatric and adolescent dermatology; and Dr. Eichenfield is vice chair of the department of dermatology and professor of dermatology and pediatrics, all at the University of California and Rady Children’s Hospital, San Diego. They have no relevant disclosures.
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