Environmental risk factors have not been identified. Approximately 60 cases were reported worldwide between 1970 and 1991. The histopathologic features of Laugier-Hunziker syndrome are indistinguishable from those of physiologic melanoplakia. It is located most commonly on the thumbs, with onset often during infancy or puberty. Essential melanotic pigmentation typically develops during early to middle adulthood, between ages 20-40 years, but can occur as late as the sixth or seventh decades.
Macules are brown, black or gray with a smooth surface.
They may be solitary or confluent. Round, lenticular and linear lesions have been described, and they may be well defined or indistinct. Subsequent reports have also described onset of oral pigmentation following melanonychia. Typically multiple nails from both the fingers and toes are involved. The degree of pigmentation may vary between streaks on the same patient.