Contact dermatitis is an eczematous disease caused by exposure to substances in the environment. Clinical presentation of contact dermatitis, which has characteristic distribution patterns, is a critical guide in the diagnosis of the disease. These patterns help, firstly, in diagnosing whether the condition is due to external or internal reasons and secondly in detecting the offending agent.

Clinical presentation shape and location

The shape and site of the inflammation is often an important clue to the identity of the allergen. Inflammation on the scalp and ears may be due to shampoo, hair dyes and topical medications among others and facial inflammation may be caused by cosmetics, acne medications and air-borne allergens, like ragweed. Sometimes the shape and location of the inflammation is exactly the same as the shape of the offending object. Therefore, the offending agent is easily identified when the inflammation appears restricted to the area under the objects, such as, a watch strap, shoes, or other clothing articles. Cotnact with allergen inducing plants like poison ivy produce linear patterns of inflammation.

But most inflammation in contact dermatitis is not confined to the area in contact with the allergen. If the allergen is present in a cosmetic or any facial make-up, then the rash is often patchy rather than diffuse and spread over all the facial areas where the cosmetic was applied. Allergens may also spread, accidentally, to areas where they were not meant to be applied. For example, hair dyes may spread to the ears and face. The scalp, palms and soles of the feet are more resistant to contact dermatitis, since the skin is thicker at these sites. But affected individuals may develop minimal inflammation if the adjacent areas of skin are affected. Aeroallergens (in the air) affect only the exposed areas of the skin and spare areas covered by clothing.

Recent studies show that contact dermatitis produced by clothing allergens are not that rare as was earlier thought. Textile contact dermatitis produces clinical patterns which are similar to patterns occurring in other types of contact dermatitis, but also produce patterns that are unusual and located at unusual places, such as, shins and inner thighs. The inflammation is sometimes pruritic. Diagnosis of such cases presents special problems to a doctor and a correct diagnosis can be difficult.

Intensity and patterns

The inflammation intensity and pattern depends on the degree of sensitivity of the person and the concentration of the allergen. Allergens which are strong sensitizers, like, oleoresin of poison ivy plants, can produce intense inflammation in low concentrations, whereas weak sensitizers may produce only erythema. The clinical presentation also depends on the area of skin affected and duration of exposure to the allergen.

Acute inflammation shows macular erythema, edema and vesicles, while chronic inflammation features lichenification, scaling and fissures in the skin. Contact dermatitis may also exhibit non-eczematous characteristics, such as, cellular-like appearance of dermal contact hypersensitivity, lichenoid variants and contact leukoderma.

Direct versus air-borne contact

Acute and chronic contact dermatitis of exposed parts of the body, particularly the face, may be due to chemicals in the environment, such as, sprays, perfumes, chemical dusts and plant pollen, especially ragweed pollen. The air-borne allergens tend to produce inflammation which is more diffuse and. Air-borne allergens collect easily on the upper eyelids, and the eyelid skin is particularly vulnerable to dermatitis because it is relatively thin. Photo dermatitis also has a diffused distribution of the rashes it causes. Volatile substances can collect in clothing.

Clinical presentation of rhus contact dermatitis

A type of contact dermatitis, known as rhus dermatitis, outnumbers, in its incidence, all other types of contact dermatitis in the USA. It is caused by contact with the substance oleoresin present in the plant poison ivy and those belonging to the same family, such as, mango trees and cashew trees. All parts of these plants contain oleoresin.

The contact dermatitis occurs when the skin contacts the leaf or internal parts of the root or stem. The clinical presentation depends on the quantity of oleoresin contacted, the pattern in which the contact was made, susceptibility of the skin and regional variations in skin reactivity. Large amounts of oleoresins cause intense inflammation whereas small amounts produce only mild skin erythema (redness).

The linear patterns of inflammation that is very typical of rhus dermatitis, is produced when a plant part is drawn across the skin or from streaking the oleoresin while scratching the itchy skin.

Diffuse or unusual patterns of inflammation are produced when the oleoresin is contacted from contaminated animal hair or clothing or from smoke while burning the plant. The eruption may appear in as quickly as 8 hours after contact or may appear a week or more later. The appearance of new lesions a week after contact may be confusing to the patient, who may attribute new lesions to the spread of the disease by touching active lesions or to contamination with blister fluid. Blister fluid does not contain the oleoresin and, contrary to popular belief, cannot spread the inflammation.