The nails are part of the skin, so it is perhaps not surprising that a skin disease such as psoriasis can affect the nails. No one knows why some people get nail involvement and others don’t. Nails grow from the nail root (matrix), which is just under the cuticle. In people who develop nail psoriasis it is involvement of the nail root that causes pitting and ridging of the nails.
These are the most common changes in nail psoriasis:
Pitting of the nails – the surface of the nail develops small pits, looking rather like the surface of a thimble. The number of pits can vary from one to dozens.
Onycholysis – the nail becomes detached from the underlying nail bed and a gap develops under the nail. When it starts there is a white or yellowish patch at the tip of the nail, and this then extends down to the cuticle. The gap between the nail and the nail bed can become colonised by particular bacteria, such as pseudomonas, which can then produce a dark green pigment. The nail can become infected and discoloured and can easily be mistaken for melanoma under the nail.
Subungual hyperkeratosis – a chalky substance accumulates under the nail. The nail becomes raised and can become tender, especially when the surface of the nail is pressed. Subungual hyperkeratosis of the toenails can be particularly uncomfortable because when wearing shoes the nail may be put under constant pressure.
Discolouration – this may be seen as unusual nail colouration, such as yellow-brown.
Onychomycosis – a fungal infection that can cause thickening of the nails. This could be present alongside nail psoriasis and can be confused in diagnosis. If diagnosed correctly it can be treated with systemic antifungal medication. It is estimated that approximately 35% of people who have nail psoriasis may also have a fungal infection that could make the nails worse. Treating the fungal infection may not have any effect on the clearance of nail psoriasis.
Some nail changes are caused by using systemic retinoid medication, which can help the skin but may result in formation of very thin nails which do not appear normal. These nail changes can take several months to grow out only after retinoids are stopped.
