A 45-year-old man came to see me because of a rash on his nose. It had appeared the day before. It hurt a little and itched a bit, though it was more annoying than anything else. He had no other skin concerns and felt fine otherwise. No fevers, chills, night sweats, or weight loss. Nothing.

I examined the skin on his nose: There were perfectly demarcated oval patches of redness on the sides of both nostrils, completely symmetrical. There was some blistering and yellowish crusting, as well.

The diagnosis was obvious: impetiginized contact dermatitis - an allergic reaction to something he had gotten on his skin and then infected with bacteria that normally live there. But what had caused it?

He said he hadn't tried any new soaps, aftershaves, or other skin- or personal-care products. No new shampoos or anything new for his hair. No new laundry detergents or dryer sheets. No new bedding or clothing. He hadn't eaten, drunk, smoked, or otherwise consumed anything new or out of the ordinary. He lived alone, so no one had brought anything else into the house.

But he did recall working with some carpet-cleaning products that he said were somewhat strong-smelling. He thought he might have blown his nose with a paper towel that could have been contaminated with cleaning solution. That could have been it, except that those spots were such perfect ovals. They looked like nose clips, but he said he hadn't used any. I didn't think the dirty paper towel was the likely culprit, but I couldn't come up with anything better.

I treated him with some topical antibiotic cream for the infection and told him it should clear up promptly. If it hadn't been infected, I would also have offered him a mild steroid cream, but steroids at this point could have interfered with the healing process.

We continued wracking our brains for a few more minutes, but nothing occurred to us. I sent him home and told him to keep thinking about it. Four days later, he called me.

Solution

The cause of contact dermatitis is a frequent real-life medical mystery because the rash generally appears 12 to 24 hours after contact with the offending substance. Many times, the answer is obvious. Sometimes, it takes some heavy-duty detective work to help the patient remember all the specific details of the last day or so. There also are times we never figure it out.

I like to say dermatology is like real estate: "Location, location, location." When there's a red, itchy, blistery eruption around the base of a finger, it's probably due to nickel-containing jewelry. Other locations, unsurprisingly, are earlobes, necks, and wrists, from earrings, necklaces, and bracelets, respectively.

Latex rubber, used in many elastics, can produce reactions around the waistline and in the groin, following the trim of undergarments.

When the redness and blisters appear in streaks or lines, typically along the hands and arms, bet on rhusdermatitis: poison ivy, poison sumac, and poison oak. Men can spread it to their genitals as well if they're not careful. We can't tell precisely, based on the appearance of the rash, which plant caused it, but whichever, the course and treatment is the same.

There's a form of contact dermatitis that also requires sunlight. It's called phytophotodermatitis, and it can be caused by exposure to the sun's ultraviolet (UV) radiation after contact with lemons, limes, carrots, wild parsnips, and celery, among other foods.

What I find most interesting about diagnosing contact dermatitis is that, once I've explained it to the patient, he or she is usually the one who comes up with the answer - almost always after leaving my office. Sometimes, several days later, they remember something that had completely slipped their minds during the visit, even if they denied it at the time.

My patient recalled that in response to the strong odor of his cleaning solution, he had, indeed, found some old nose clips and used them to block out the smell.

Bingo!

Lucy Hornstein is a family physician in Phoenixville. She blogs at http://dinosaurmusings.wordpress.com.