A black mark under the toe nail

Melanoma skin cancer may appear under a finger or toe nail, but it is uncommon. The main signs to look out for are a dark brown or black line down the length of the nail which slowly gets wider from the base, pigment spreading out into the skin next to the nail (Hutchinson’s sign) a lump, and destruction of the nail. These are all ‘red flag’ warning signs requiring urgent specialised medical attention.

The next 2 images are of a typical case of a black mark under a toenail for several weeks. there was no recollection of trauma. The patient had gone on line and worried themselves sick that they had a melanoma.

The picture is slightly out of focus but clearly shows irregular pigment under part of the big toe nail. What could it be? Dermoscopy was useful.

It’s blood. Study the deep blue globules, this is what blood looks like under the nail. It grows out slowly. This could not possibly be a melanoma as there is a clear gap between the pigment and the proximal nail fold (the base of the nail). There are no melanocytes in the skin underneath the nail plate, if a melanoma is there it has to have grown in from outside.

Blood under the nail can result from the most minimal trauma. The blood grows out very slowly. I have it on the authority of two top nail experts, Dr David de Berker of Bristol, and Professor Luc Thomas of Lyon, France, that if you can see a gap between the pigment and the proximal nail fold, it can’t be a melanoma.

I wish all doctors knew that, it would save a lot of avoidable anxiety. You don’t even need a dermoscope to demonstrate the gap, just a good light and a magnifying lens. Take a photo and observe, no need to worry.

For a more thorough tutorial on examination of the toenail in case of suspicious pigmentation, check out this great article on dermnetnz. and this other article by my friend Ivan Bristow.