Eckart Haneke, Robert Baran, Rodney PR Dawber, Antonella Tosti
The term 'chromonychia' indicates an abnormality in colour of the substance or the surface of the nail plate and/or subungual tissues. Generally, abnormalities of colour depend on the transparency of the nail, its attachments and the character of the underlying tissues. Pigment may accumulate due to overproduction (melanin) or storage (haemosiderin, copper, various drugs), or by surface deposition. The nails provide a historical record for up to 2 years (depending on the rate of linear nail growth) of profound temporary abnormalities of the control of skin pigment which otherwise might pass unnoticed. Colour is also affected by the state of the skin vessels, and by various intravascular factors such as anaemia and carbon monoxide poisoning.
There are some important points to note concerning the examination of abnormal nails for colour changes. The nails should be studied with the fingers completely relaxed and not pressed against any surface. Failure to do this alters the haemodynamics of the nail and changes its appearance. The finger tips should then be blanched by pressing them on an even surface to see if the nail bed is grossly altered; this may help to differentiate between discoloration of the nail plate and its vascular bed. If the discoloration is in the vascular bed, it will usually disappear. Further information can be gleaned by transillumination (diaphanoscopy)
Complex internal diseases may sometimes be diagnosed solely by colour changes in the nail apparatus of the nail using a pen torch placed against the pulp. If the discoloration is in the matrix or soft tissue, the exact position can be identified more easily and dark material or a nontransparent foreign body will give a dark shadow. Furthermore, if a topical agent or superficial infection is suspected as the cause, one can remove the discoloration by scraping or cleaning the nail plate with a solvent such as acetone. If the substance is impregnated more deeply into the nail or subungually, microscopic studies of potassium hydroxide preparations, tangential or punch biopsy specimens using special stains may be indicated. Wood's lamp examination is sometimes useful.
When the discoloration is of exogenous origin, for example from nail contact with occupationally derived agents or topical application of therapeutic agents, the discoloration typically follows the contour of the proximal nail fold when the discoloured nail grows out. If the proximal margin of the discoloration corresponds to the shape of the lunula an internal cause is likely.
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