When it comes to solving mysteries, a super sleuth like Sherlock Holmes has nothing on a good dermatologist.

Consider the case of the woman whose 21-year-long bout of eczema of the right palm was traced by her dermatologist to a carving knife she received as a wedding present 21 years before. The knife's wooden handle was the culprit causing her allergic rash.

Or the lady who had eczema of the palms for six years. The dermatologist deduced that the guilty party was the car she had bought six years earlier — the steering wheel contained a compound to which she was allergic.

Or the recent widower who suddenly developed an itchy rash on his hands. His dermatologist determined that it was not a psychosomatic disorder brought on by grief, but a sensitivity to oils in the skin of oranges, which he was squeezing for himself for the first time.

Allergies

The red, raw hands that tormented these people resulted from the skin's reaction to a substance (allergen) to which they had become sensitized, or allergic. This type of eczema, or superficial skin inflammation, is called allergic contact dermatitis. Normally, the skin doesn't react the first time it meets up with an allergen. Sometimes it occurs with the second exposure. But in other cases, it takes years and many exposures for hypersensitivity to a particular substance to develop. Once sensitized, the skin will usually become inflamed within hours or days after contact.

Irritants

Unlike allergic contact dermatitis, irritant contact dermatitis — a more common type — is a nonallergic inflammatory skin reaction caused by exposure to irritating substances that actually damage the skin. Not everyone develops allergies, but everybody's skin can become irritated if abused. Contact with strong irritants, such as acid or lye, can result in blisters, erosion and ulcers within minutes or hours. For weaker irritants, such as soaps or detergents, exposure over days or weeks may be necessary before eczema develops. Any substance can act as an irritant if it is concentrated enough and if the skin is exposed to it long enough.

The eczema usually starts with a red, itchy rash and progresses to tiny blisters that ooze.

Doctors can usually distinguish contact dermatitis from other types of dermatitis by its unusual pattern. The eruption often appears with clear-cut margins, acute angles, and geometric outlines, although poison ivy and other poison plants cause lines or groups of blisters.

Though the configuration of the rash aids in diagnosis, it's not so easy to determine whether an allergy or irritant is involved. The skin reaction produced by either, especially when mild, frequently looks the same. Redness or an itchy rash may be the first sign. However, blisters that weep or form a crust, along with swelling, are more likely to appear in allergic dermatitis, such as poison ivy. As the inflammation lessens, the skin may scale and become temporarily thickened. When the dermatitis becomes chronic, the skin becomes dry, thickened and cracking.

Sometimes, if the inflammation with mild irritants continues for a long time, the original irritation disappears because the skin becomes hardened.

Finding the Source

A person may know what caused the inflammation — for example, recent hand contact with a corrosive, such as an oven cleaner — or may not have a clue. The patient's hobbies, diet, occupation, sports activities, clothing and cosmetics, as well as medications that are taken internally — all come under suspicion.

An irritant or allergen at home or at work may be responsible. In one particularly puzzling case, Walter B. Shelley, M.D., Professor of Dermatology, Medical College of Ohio in Toledo, decided to visit the home of a patient who had been hospitalized twice for severe contact dermatitis. All tests had been negative, but after seeing her stainless steel kitchen, the cause of the dermatitis was apparent — the stainless steel polish she used to keep her kitchen shiny.

The location of the rash will sometimes tell the tale — except for hands, which are into everything. Anything on the scalp, face and neck suggests cosmetics such as hair sprays, shampoos, makeup, sunscreens, perfumes, shaving cream, acne medications — the list is endless. Eyelid dermatitis is often traced to nail polish, which can cause an allergic reaction if nails touch the eye area before the polish dries completely, a two-hour process. Lips can become sensitive to ingredients in lipsticks, toothpastes or chapped lip medications. Armpits can become allergic to or irritated by ingredients in deodorants or antiperspirants.

Dyes, elastic materials, fabric finishes such as sizing and permanent press, laundry detergents, and fabric softeners can cause dermatitis on the torso and arms and legs. "Some of those antistatic laundry products are really mean," says Shelley. "They can cause terrible problems. They set off itching, and people scratch, of course, which only makes things worse. It takes a lot of laundering to get that stuff out of the clothes." The feet can be affected by dyes, rubber compounds, and leather-tanning products in shoes, or elastic fibers in hosiery. Nickel — used in jewelry, bra fasteners, eyelash curlers, metallic eyeglass frames, and many other products — can produce inflammation wherever it touches the body. People may become sensitized to nickel from ear-piercing instruments and from nickel-plated earrings inserted after piercing.

Fragrances in cosmetics are frequent offenders. Redness under the ears may be caused by perfume dermatitis. Some components of fragrances may interact with sunlight or other sources of ultraviolet light to produce dermatitis by a process called photosensitization. When dermatitis occurs on the left side of a man's face and neck — the side exposed to the sun while driving a car — photosensitization caused by aftershave lotion fragrances should be suspected.

Betrayed by Medications

And the unkindest cut of all — people can become sensitized to the very medications that they're using to relieve a skin problem, such as acne. If a skin product does not seem to be working, if things are getting worse instead of better, there's a chance that an allergy to some ingredient in the medication has developed, especially if puffiness, redness and itching appear.

Dermatitis may be caused by topical (externally applied) medications, such as anesthetics containing benzocaine, dibucaine and other chemicals that end in "caine," topical antibiotics such as neomycin and streptomycin, topical antihistamines such as diphenhydramine and promethazine, and topical mercury compounds such as mercurochrome and merthiolate.

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Tags: Skin Care

About the Author

FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.