Don’t Scratch That: Rashes
Presented by: David Peng, MD, MPH
Clinical Associate Professor of Dermatology
Stanford University Medical Center
November 18, 2010
Lecture Overview:
- The skin is an immune system organ, and the system can be activated by a number of external causes.
- The most common cause of itching is dry skin. Other widespread causes include fungus, insect bites, and contact allergies.
- Water (especially hot water) can actually worsen the problem of dry skin by removing normal, protective skin oils.
- Contact allergens include rubber, adhesives, nail polish, hair dye, and metal-particularly nickel, which is used in earrings, belt buckles, waistband snaps, and watchbands.
- A dermatologist can provide a test to find the trigger of your allergic response and prescribe the most appropriate treatment.
Your mother was right: Don’t scratch! Tempting as it is, one of the worst things you can do to an itch is to scratch it. While a good scratch can sometimes be the ultimate pleasure, the fact is an itch is a physical response that involves the skin’s receptors, your immune system, and your brain. Unchecked scratching can often damage the skin and not address the source of the physical stimulus, said David Peng, MD, a clinical associate professor of dermatology, who spoke at a presentation sponsored by the Stanford Health Library..
The most common cause of itching is dry skin, known clinically as xerosis. Most people experience a normal seasonal variation in skin dryness caused by low environmental humidity. For many, the first step is a moisturizer, but Dr. Peng warns that not all moisturizers are the same and vary in strength depending on the ratio of water to oil in the mixture. Solutions have the highest proportion of water, which is like trying to moisturize with pure water. Water alone can actually worsen the problem of dry skin by removing the normal, protective skin oils.
Next in strength are lotions, which feel good on the skin but evaporate rapidly, making the skin drier. Creams and ointments have more oil content-they tend to moisturize and don’t evaporate. While this attribute is good for the skin, it may cause lifestyle complications, said Dr. Peng, referring to the mess a common ointment like Vaseline Petroleum Jelly can make. Moisturizers should be reapplied liberally during the day and evening, especially to areas prone to dryness (hands, arms, legs) and when your skin feels itchy.
“”Extremely dry skin may require moisturization with an ointment,” said Dr. Peng. “Otherwise most people will find that a cream will get them through the day.”
Hot, soapy water depletes the natural skin oils and may be dry skin’s worst enemy. Dr. Peng recommended using lukewarm-not hot-water and patting dry-not rubbing. Follow the three-minute rule for showers, applying moisturizer within three minutes after getting out of the water.
“The top layer of skin is like a sponge. You have about three minutes until it evaporates. Applying moisturizer within that window of time traps water inside the skin, where you want it,” he said.
Another common cause of itching can be a fungus. Athlete’s foot, the most common skin fungus, affects about 10 percent of the entire U.S. population every year and can involve other parts of the body besides the feet. Skin fungus infections cause an inflammatory response by the body’s immune system, which brings on itching, flaking, redness, and thickened skin. The condition is treated by antifungal ointments like Lamisil (available over the counter) or Clotrimazole (by prescription).
Insect bites can affect people in different degrees, from simple irritation to hives and swelling. Bedbugs and fleas are common pests that can cause a reaction from their saliva.
Mosquitoes are attracted to the lactic acid in your sweat: Use DEET to prevent them from biting and hydrocortisone to treat the bites afterward, said Dr. Peng. Scabies, a skin infection caused by a parasitic mite, tend to create a circular pattern of itching that includes the nipples, armpits, wrists, bellybutton, and genitals. Most cases can be treated with Elimite cream, he said.
Allergic contact dermatitis is caused by direct physical contact with your skin. Unlike food allergies, which are usually identified with a prick test, Type IV hypersensitivity is tested with stickers applied directly to the skin. A full-spectrum antigen test uses a grid of patches containing common household products to induce a reaction. Common allergens include rubber, adhesives, nail polish, hair dye, and metal-particularly nickel, which is used in earrings, belt buckles, waistband snaps, and watchbands.
More than half of Americans are allergic to poison ivy, which can last for months on clothing after initial exposure. Many people develop contact dermatitis from fragrances used in detergent, toothpaste, perfume, or lotions.
The first step in treating any itch should be a good medical evaluation. Your dermatologist or allergist may discover the “trigger factors” in your environment and encourage you to minimize or avoid them. “The most important thing is to find what you are allergic to and avoid it,” said Dr. Peng. “Your dermatologist should go on a hunt to find your trigger-a systematic process of identification and elimination.”
Medical care will focus on diminishing the itchy sensation, using various classes of topical steroids. Treatments range from Class I, which includes clobestasol to treat thick plaque, to Class 5, which is prescribed for short-term daily maintenance, to Class 7, which includes over-the-counter creams like hydrocortisone.
“There are side effects to topical steroids so these treatments should not be used for the long term,” said Dr. Peng.
About the Speaker
David Peng, MD, MPH, is a clinical associate professor of medicine and director of the dermatology residency program. He was an assistant professor of clinical dermatology, director of the dermatology residency training program, director of the Pigmented Lesion Clinic, and director of Contact and Occupational Dermatitis at University of Southern California before joining the Stanford faculty in 2010. Dr. Peng received his medical degree from University of California, San Diego, where he completed his residency in dermatology, and he earned his master’s in public health from UCLA. Dr. Peng is Board Certified by the American Board of Dermatology.
For More Information:
Stanford Health Library can do the searching for you. Send us your medical questions.
About Dr. Peng
http://med.stanford.edu/profiles/dermatology/faculty/David_Peng/
Stanford Hospital Dermatology Clinic
http://stanfordhospital.org/clinicsmedServices/clinics/dermatology/team/
Stanford Health Library
http://healthlibrary.stanford.edu/
Stanford Department of Dermatology
http://dermatology.stanford.edu/
