Contact dermatitis (dermatitis venenata), a type IV delayed hy-persensitivity reaction, is an acute or chronic skin inflammation that results from direct skin contact with chemicals or allergens. There are four basic types: allergic, irritant, phototoxic, and photoallergic (Table 53-4). Eighty percent of cases are due to excessive exposure to or additive effects of irritants (eg, soaps, detergents, organic solvents) (Tierney et al., 2001). Skin sensi-tivity may develop after brief or prolonged periods of exposure, and the clinical picture may appear hours or weeks after the sen-sitized skin has been exposed.
Symptoms include itching, burning, erythema, skin lesions (vesi-cles), and edema, followed by weeping, crusting, and finally dry-ing and peeling of the skin. In severe responses, hemorrhagic bullae may develop. Repeated reactions may be accompanied by thickening of the skin and pigmentary changes. Secondary inva-sion by bacteria may develop in skin abraded by rubbing or scratching. Usually, there are no systemic symptoms unless the eruption is widespread.
The location of the skin eruption and the history of exposure aid in determining the condition. In cases of obscure irritants or an unobservant patient, however, diagnosis may be extremely diffi-cult, often involving many trial-and-error procedures before the cause is determined. Patch tests on the skin with suspected of-fending agents may clarify the diagnosis.