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Table 5 Features of longitudinal melanonychia compared with those of subungual bleeding-all features are generally true, but there can be individual exceptions

From: Clinical guidelines for the recognition of melanoma of the foot and nail unit

Melanoncyhia Subungual bleeding
The duration of history is from 3-6 months upwards to 20 years or more The duration of history is rarely more than 6 months and is typically shorter
A history of trauma is quite common A history of trauma or precipitating activity is quite common
Lateral margins within the nail are mainly straight and longitudinally oriented Lateral margins may be irregular
Where margins merges with the nail fold, pigment may spread onto nail fold (Hutchinson's sign) Pigment rarely extends from beneath the nail plate
There are rarely any detectable transverse features There may be a proximal transverse groove and/or transverse white mark within the nail
In the absence of clinical tumour, nail plate pigmentation is in continuity with a single zone Haemorrhage may be broken up into a number of zones
Dermoscopy reveals Dermoscopy reveals
   • continuous pigment between proximal nail fold and distal free edge    • Pigment may not be continuous in the longitudinal axis, with clear nail at either the proximal or distal margin
   • in the transverse axis, pigment may vary-whereas in the longitudinal axis it remains largely constant    • Pigment may vary in any axis
   • There may be longitudinal flecks of darker pigment within the background pigment of the nail    • Droplets of blood may be seen separated from the main zone of pigmentation
   • Pigment is mainly brown black    • Blood may be seen as a discrete layer of material on the lower aspect of the nail plate at the free margin
     • Pigment may be purple black, with increasing red hues at margins. It is rarely brown