Rash is a generalized, non-medical term that refers to irritation or discoloration of the skin. The causes of and treatments for rashes vary. Accurate diagnosis depends on the assessment of the appearance of a rash, any accompanying symptoms, and exposure to substances that can irritate the skin or cause allergies.

Contact Dermatitis and Patch Testing

Contact dermatitis is the sudden onset of a skin rash as a result of an allergic reaction to a substance that comes in contact with the skin’s surface. The rash develops at the point of skin contact, and the rash will become worse with each future exposure. The rash can look red, flaky, blistery, or can be open or weeping.  Occasionally, pigmentation or brown discoloration can occur following skin healing from the initial dermatitis. Common substances or allergens that can cause an allergic contact dermatitis include nickel, poison ivy, topical antibiotics like neomycin, fragrance and formaldehyde.

Simple removal from the aggravating substance will cure the rash or a prescription for topical steroid may be needed. A chronic rash, dermatitis or eczema, may require patch testing which will investigate if your skin is sensitive to one or more of  >30 common allergens including chemicals found in jewelry, makeup and hair dye.

A specific type of contact dermatitis related to plants is termed phytophotodermatits.  Phyto- means plant, photo- means light, and dermatitis means inflammation of the skin.  Therefore, this reaction occurs when the skin comes in contact with juices from certain fruits and plants, followed by exposure to sun or light, causing an inflammatory reaction on the skin.  Lime (from fruit juices or margaritas) are a common culprit, although parsley, parsnip, fennel, mustard, fig leaves, and St. John’s wort may also cause the reaction.  The skin often exhibits redness and swelling, and sometimes a blistering reaction.  Once this subsides, brown discoloration often appears, which may last for weeks to months.  Beyond treatment of the skin eruption, sunscreen plays an important role in minimizing pigmentation following the rash.