DermDx: A widening and darkening streak in the nail

A brown or black pigmented line beginning at the nail matrix and extending to the top of the nail plate is referred to as longitudinal melanonychia.  Longitudinal melanonychia is due to the deposition of melanin into the growing nail and...

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A brown or black pigmented line beginning at the nail matrix and extending to the top of the nail plate is referred to as longitudinal melanonychia. 

Longitudinal melanonychia is due to the deposition of melanin into the growing nail and can be the result of several different conditions, the most concerning of which is melanoma.

Longitudinal melanonychia may be a normal finding, and is common in individuals of darker skin colors. It is also normally found in increasing frequency as patients age. If longitudinal melanonychia appears in children, it is usually due to a benign cause.

The pathogenesis of this condition is melanocytic activation, melanocyte hyperplasia, or non-melanocytic tumors. If a patient presents with multiple bands, then the pathogenesis is likely from melanocytic activation.

Melanocytes can be activated by a variety of causes, including trauma, postinflammatory hyperpigmentation (commonly seen in psoriasis and lichen planus), onychomycosis, drugs (especially chemotherapy agents), pregnancy, Peutz-Jegher syndrome, racial variations, and HIV infection/treatment.

Subungual hematoma or hemorrhage can enter into the differential if irregularly shaped violet or red spots are seen on close examination or dermoscopy. However, the same finding may also be seen with melanoma.

Hutchinson’s sign refers to the appearance of black or brown pigmentation in the periungual skin or the cuticle. When combined with longitudinal melanonychia, this suggests a melanoma.

Pseudo-Hutchinson’s sign describes where the pigmented nail plate can be seen though the transparent nail fold, which suggests a nail matrix nevus, Bowen’s Disease, or a variety of other non-melanocytic tumors.

Other non-melanocytic tumors which should be included on the differential of longitudinal melanonychia include basal cell carcinoma, squamous cell carcinoma, subungual keratosis, verrucae, and myxoid cyst.

Benign melanocyte hyperplasia can also cause longitudinal melanonychia. This condition includes a lentigo or benign nevus of the nail matrix.

It is recommended to biopsy the nail matrix when a patient presents with a single new longitudinal melanonychia that is not due to the common causes of melanocytic activation. Ruling out melanoma is a main concern.

A useful mnemonic to remember the risk factors for subungual melanoma is the first six letters of the alphabet, ABCDEF.

• A stands for age (peak in 5th to 7th decade), African Americans, Asians, and Native Americans.
• B stands for a band on the nail bed that is brown or black with a breadth of 3mm or more and with a blurred border.
• C stands for change, which can indicate a rapid change in the nail bed due to the melanoma or no change in the nail bed despite treatment for other likely causes of longitudinal melanonychia.
• D stands for the digit mostly likely involved, which is the thumb of the dominant hand.
• E stands for extension of the pigment to the proximal or lateral nail fold, which is Hutchinson’s sign.
• F stands for family (or personal) history of melanoma or dysplastic nevus.

Dermoscopy can assist in assessing for melanoma. A close examination of the pigment in the nail plate can reveal brown or black lines with irregular color, spacing, or thickness, which suggest melanoma.

When the lines appear parallel, this suggests a benign cause. In this case, a nail matrix biopsy was performed.  Fortunately, the pathology demonstrated a benign melanocytic nevus. 

Jason Preissig, BS is a medical student at Baylor College of Medicine.
Adam Rees, MD is a graduate of the University of California Los Angeles School of Medicine and a resident in the Department of Dermatology at Baylor College of Medicine in Houston.