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Ask about contact with known allergens (i.e., nickel, fragrances, neomycin, and poison ivy/oak).
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- Nickel exposure is often related to the wearing of rings, jewelry, and metal belt buckles (Figures 146-3, 146-4, 146-5, and 146-6).
- Fragrances in the forms of deodorants and perfumes (Figure 146-7).
- Neomycin applied as a triple antibiotic ointment by patients (Figures 146-8 and 146-9).
- Poison ivy/oak in outdoor settings. Especially ask when the distribution of the reaction is linear (Figures 146-10 and 146-11).
- Ask about occupational exposures, especially solvents. For example, chemicals used in hat making can cause ICD on the hands (Figure 146-2).
- Tapes applied to skin after cuts or surgery are frequent causes of CD (Figure 146-12).
- If the CD is on the feet, ask about new shoes (Figures 146-13 and 146-14).
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A detailed history of products used on the skin may reveal a suspected allergen. In Figure 146-15, this truck driver was using baby wipes to clean his skin during long drives. Patch testing ultimately revealed that he was allergic to one of the ingredients in those wipes.
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All types of CD have erythema. Although it is not always possible to distinguish between ICD and ACD, here are some features that might help:
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- ICD:
- Location—usually the hands.
- Symptoms—burning, pruritus, pain.
- Dry and fissured skin (Figure 146-2).
- Indistinct borders.
- ACD:
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Both ICD and ACD may be complicated by bacterial superinfection showing signs of exudate, weeping, and crusts.
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Toxicodendron (Rhus) dermatitis (poison ivy, poison oak, and poison sumac) is caused by urushiol, which is found in the saps of this plant family. Clinically, a line of vesicles can occur from brushing against one of the plants. Also, the linear pattern occurs from scratching oneself and dragging the oleoresin across the skin with the fingernails (Figures 146-10 and 146-11).
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Systemic CD is a rare form of CD seen after the systemic administration of a substance, usually a drug, to which topical sensitization has previously occurred.6
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The diagnosis is most often made by history and physical examination. Consider culture if there are signs of superinfection and there is a concern for methicillin-resistant Staphylococcus aureus (MRSA). The following tests may be considered when the diagnosis is not clear.
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- KOH preparation and/or fungal culture if tinea is suspected.
- Microscopy for scabies mites and eggs.
- Latex allergy testing—This type of reaction is neither ICD (nonimmunologic) nor ACD. The latex allergy type of reaction is a type I, or immunoglobulin (Ig)E-mediated response to the latex allergen.
- Patch testing—Common antigens are placed on the skin of a patient. The T.R.U.E. Test comes in three tape strips that are easy to apply to the back (Figure 146-16). There is no preparation needed to test for the 29 common allergens embedded into these strips (Table 146-1 for a list of the 29 allergens). The strips are removed in 2 days and read at that time and again in 2 more days (Figure 146-17). The T.R.U.E. Test website provides detailed information on how to perform the testing and how to counsel patients about the meaning of their results. Any clinician with an interest in patch testing can easily perform this service in the office.
- A metaanalysis of the T.R.U.E. Test shows that nickel (14.7% of tested patients), thimerosal (5.0%), cobalt (4.8%), fragrance mix (3.4%), and balsam of Peru (3.0%) are the most prevalent allergens detected using this system.5
- Critics of the T.R.U.E. Test state that it misses other important antigens. There are a number of dermatologists who create their own more extensive panels in their office. If the suspected allergen is not in the T.R.U.E. Test, refer to a specialist who will customize the patch testing. Also, personal products, such as cosmetics and lotions, can be diluted for special patch testing.
- A metaanalysis of children patch tested for ACD showed the top five allergens to be nickel, ammonium persulfate, gold sodium thiosulfate, thimerosal, and toluene-2,5-diamine (p-toluenediamine).7 Only two of these five allergens are in the T.R.U.E. Test, so it may be best to not use this standardized patch testing for children.
- Once the patch test results are known, it is important to determine if the result is “relevant” to the patient's dermatitis. One method for classifying clinical relevance of a positive patch test reaction is: (a) current relevance—the patient has been exposed to allergen during the current episode of dermatitis and improves when the exposure ceases; (b) past relevance—past episode of dermatitis from exposure to allergen; (c) relevance not known—not sure if exposure is current or old; (d) cross-reaction—the positive test is a result of cross-reaction with another allergen; and (e) exposed—a history of exposure but not resulting in dermatitis from that exposure, or no history of exposure but a definite positive allergic patch test.6
- Punch biopsy—When another underlying disorder is suspected that is best diagnosed with histology (e.g., psoriasis).
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