Differential Diagnosis for Vertical Linear Hyperpigmentation in the Thumb Nail Bed

• Single most likely diagnosis
  • Melanonychia: This condition is characterized by a brown or black pigmentation of the nail plate due to the activation or proliferation of melanocytes in the nail matrix. The fact that the hyperpigmentation is vertical and linear, and has been present for 4 years, increasing in width, supports this diagnosis. Melanonychia can be caused by a variety of factors including trauma, infection, and systemic diseases, but in many cases, it is benign.
• Other Likely diagnoses
  • Nail melanoma: Although less common than melanonychia, nail melanoma is a significant consideration due to its potential for malignancy. It can present similarly to melanonychia but may have additional features such as nail plate distortion, pigmentation of the periungual skin (Hutchinson's sign), or a rapidly changing lesion.
  • Subungual hematoma: This condition, caused by bleeding under the nail, can sometimes present as a linear pigmentation. However, it typically has a more acute onset and may be associated with trauma.
• Do Not Miss diagnoses
  • Subungual melanoma with nail matrix involvement: This is a critical diagnosis to consider due to its potential for serious morbidity and mortality if not recognized and treated early. Any new or changing pigmentation in the nail, especially if accompanied by other signs such as nail dystrophy or pigmentation of the surrounding skin, warrants careful evaluation.
• Rare diagnoses
  • Laugier-Hunziker syndrome: A rare condition characterized by longitudinal melanonychia associated with mucosal pigmentation. It is usually benign but can be a source of concern due to its appearance.
  • Peutz-Jeghers syndrome: Although primarily known for its gastrointestinal polyps and mucocutaneous melanin deposits, it can rarely present with nail changes, including longitudinal melanonychia. This diagnosis would be considered in the context of a family history or other systemic manifestations.