A 48-year-old man is sent to dermatology for evaluation of “fungal infection” of one fingernail. The condition, present for months, has been unresponsive to two months of treatment with terbinafine prescribed by his primary care provider. In the past, other fingernails have had a similar appearance to the one currently affected but have always reverted to normal within a few months.

The patient denies any changes to his toenails. He has not experienced any skin problems on his elbows, knees, or scalp. He has seasonal allergies and has had “sensitive skin” since early childhood, a trait he shares with his mother and a sibling. This sensitivity has manifested over the years with the occasional transient rash on his hands and fingers.

Transverse ridges are seen on his right index fingernail, regularly spaced and roughly parallel, but with no discoloration or onycholysis noted. There is clear separation of the cuticle from the nail plate, but only on this finger. Examination elsewhere fails to show any signs of psoriasis, and all his other nails appear unaffected.

ANSWER
The correct answer is eczema (choice “a”), a very common trigger for fingernail dystrophy. Lichen planus (choice “b”) can certainly cause nail dystrophy, onycholysis, or even onychorrhexis, but the changes seen on this man’s nail were much more uniform than those seen in lichen planus. The lack of response to the most effective oral antifungal medication available effectively ruled out onychomycosis (choice “c”). That medication would have had no effect on a candidal infection (choice “d”), either, but the lack of perionychial edema and redness spoke rather loudly against that possibility in the first place.

DISCUSSION/TREATMENT
Atopic dermatitis, in all its manifestations, is seen many times every day in the average clinical dermatology practice, and odds are that’s what was affecting this man’s nail. Two possible mechanisms have been advanced to explain the very common nail ridging and dystrophy seen in atopic patients. One is the separation of dry, thin cuticle from the nail plate, which exposes the matrix to environmental insults sufficient to disrupt production of normal nail. We see this same phenomenon in a number of other conditions, including chronic candidal paronychia—but while that condition is quite common in women, it is almost never seen in otherwise healthy men.

The other proposed mechanism for nail dystrophy is simply scratching, which is arguably what atopic patients do best. This tends to polish the nail as well, producing an unusually shiny nail surface, which is exactly what we see with this patient. Obviously, this scratching promotes cuticular separation as well.

One other item that belongs in this differential is chronic alopecia areata, which usually produces uniform pitting of nails but can result in a number of different patterns of dystrophy, including ridging.

Perhaps the most useful take-home message from this case is: Be appropriately skeptical about the diagnosis of “fungal infection” of fingernails, because even though it most assuredly exists, studies show it is roughly 18 times less likely than fungal infection of toenails—yet another example of why having a differential is so critical.