The nail plate is a semi-transparent keratinous structure and is not normally pigmented. It may appear discolored because of pigment on top of the nail plate, within the nail plate or on top of the nail bed. The pigment may arise from the distal or proximal nail matrix.

Longitudinal melanocyhia is due to activation of melanocytes in the nail matrix. It is more common in darker-skinned individuals and may affect one or several nails. The band may occasionally extend across the entire nail (total melanonychia). Transverse melanonychia is rare.
There are various causes of longitudinal melanocychia.

  • Exogenous pigment e.g., silver nitrate, tobacco, henna
  • Ethnic pigmentation is known as Racial Melanonychia
  • Inflammatory skin disease (psoriasis, lichen planus)
  • Trauma (nail biting, friction from shoes, radiotherapy)
  • Infections (paronychia, onychomycosis especially when due to molds; pigmentation is nonmelanocytic)
  • Drug reactions (hydroxyurea, antiretrovirals, antimalarials, metals)
  • Endocrine disease (Addison disease, Cushing syndrome)
  • Nonmelanocytic tumors (squamous cell carcinoma in situ, onychomatricoma, myxoid cyst, viral warts)
  • Melanocytic naevus of nail matrix
  • Lentigo / benign melanocytic hyperplasia
  • Malignant melanoma

Nails grow slowly – about 1mm a month, taking months to reach the distal edge, and longitudinal melanonychia reflects melanin deposition rather than the site of its production.