What Causes a Dark Spot Under the Toenail?

It started out as a normal Friday in the brightly lit corridors of my NYC podiatry practice.  She was sitting in the chair, legs propped up on the footrest, sitting ever so daintily on the happy feet disposable foot towel.  The concerned look on her face was enough to almost stop a cab on Park Avenue.  “I’ve got a dark spot under my toenail,” she said, in a tone that was halting and plaintively pleading to get her out of there and back to work before the end of her lunch break – and I knew this dark mystery was just another case of the runner’s toe black and blues.

Most runners, from the casual to the crazy – and many other folks – have at some point developed a dark spot under a toenail, most commonly in the big toe.  In most cases this is due to bleeding under the toenail, otherwise known as a subungual hematoma, which is doctor-speak for a toe boo-boo.  This is caused by trauma to the nail, which can come in the form of a long run, an anvil dropping on the toe, or some careless and annoying fellow subway passenger with a large boot.  And in many cases the trauma is not even noticed at the time of injury.  You see, my mysterious NYC podiatry patient, the skin under the nails is very delicate and has a robust blood supply.  Even a relatively minor amount of pressure can lead to bleeding under the nail which can last for hours at a slow trickle.  Since the blood is sitting on top of the skin and under the nail, it remains in place until the nail grows out, which can take 6-9 months.  If there is a lot of blood and pain, the pressure can be relieved by inserting a needle in the nail plate to allow the blood to drain.   As horrible as that sounds, it is not any more painful than cutting the nail.  This can be done easily at home, although it is my experience that most patients would rather leave the dirty work to their friendly neighborhood NYC foot doc.  In cases of larger hematomae the nail plate may then become loose over time and lift off the nail bed (yech), after which time a new nail can regrow, usually without any long-term consequence.  Prevention consists of wearing shoes a half size bigger, in the case of runners, and not playing with anvils or riding the subway for everyone else.

One important point, in all seriousness – the majority of discoloration under the nail is due to trauma, and will resolve.  However, in rare cases, melanoma can also present as a spot under the nail (Bob Marley’s demise), and for obvious reasons, this needs to be diagnosed and treated without delay.  When in doubt, have it checked out.

So, another mystery solved, another life saved.  She was relieved and relaxed.  She got up off the happy feet paper, slipped back into her stylish yet comfortable low pumps and sauntered down the hardwood hall.  With a wave, she was gone, and I was left with a dirty foot towel, a chart, and a nominal copay.

See you in the office.

Ernest Isaacson