Allergic contact dermatitis

Allergic contact dermatitis is an allergic reaction which only occurs in people whose skin has been exposed to and previously sensitised by an allergen. Subsequent contact with the antigen elicits a specific cell-mediated sensitisation (e.g. nickel, cosmetics, perfumes, hair dye, dyes and plants) of the immune system to a specific allergen/ s with resulting dermatitis or exacerbation of pre- existing dermatitis. Phototoxic, photoallergic and photo-aggravated contact dermatitis occur when allergens are photo-allergens but it is not always easy to distinguish between photo-allergic and phototoxic reactions (Bourke et al., 2001). Phototoxic reactions result from direct damage to tissue caused by light activation of the photosensitising agent. Photoallergic reactions are a cell-mediated immune response in which the antigen is the lightactivated photosensitising agent. Some patients with atopic dermatitis and other chronic inflammatory skin conditions become photosensitive (DermNetNZ, 2009).

Acute dermatitis will follow a single exposure to an irritant or an allergen, and the allergic and eczematous manifestations observed in the skin are defined by Burns et al. (2004). The manifestations are as follows:

• Vasodilatation of the dermis causes inflammation;
• Peri-vascular infiltrate of lymphocytes and polymorphs;
• Intra-epidermal vesicles form by accumulation of fluid in cells, and may coalesce to form bullae;
• Vesicles and bullae will rupture to oozing, crusting, scaling and healing.

Patients with allergic contact dermatitis usually present with acute dermatitis at the body site where the allergen has been in direct contact with the skin. Severe allergic contact reactions may extend outside the contact area or it may become generalised.

Diagnosis of irritant and contact dermatitis
The key to diagnosing irritant and/or allergic contact dermatitis is the detection of the irritant sensitising agent respectively. Sensitisation may suddenly occur following years of trouble-free contact with the allergen. An examination and skin/occupational history will reveal clues and common sites of involvement, e.g.:

• nickel – ear lobes/nape of neck (jewellery)
• leather – wrists (watch straps)
• tanning agents/adhesives – soles of feet (shoes)

Patch testing
Patch testing identifies whether a substance that comes in contact with the skin is causing inflammation of the skin (contact dermatitis) and confirms or excludes an allergen.

The guidelines for managing contact dermatitis (Bourke et al., 2001) recommend that patients with persistent eczematous eruptions should be patch tested. Patch testing should include at least to an extended series of allergens. In addition, patch testing should be undertaken by an individual who has had training in the investigation of contact dermatitis, prescribes the appropriate patch tests and performs patch test readings at day 2 and day 4 for patients undergoing diagnostic patch tests. The dual time sequenced readings allow time for the immunological response to evolve sufficiently.

Patch testing involves testing patients with the suspected substance (prepared allergens) and the European standard battery (the European standard of allergens most widely used in patch testing) plus other likely allergens (English, 1999). The procedure for patch testing is as follows.

1. Suspected allergens are placed within small metal chambers called Finn chambers and are the placed on the patient’s back.
2. The patch tests are left in place for 48 hours and then removed (the patient is instructed not to wash their back for 5 days).
3. Results are read at 48 and 96 hours using a four-point scale to score the skin reaction to the patch tests. The scale is as follows:
   • O = no reaction
   • + = a palpable oedematous reaction develops
   • ++ = the reaction becomes vesicular
   • +++ = the reaction is very strong and spreads beyond the boundary of the patch.
   
   
4. The skin reactions from patch testing are interpreted with consideration to the patient’s history, lifestyle and occupation. Further details on conducting patch test are illustrated in Radcliffe (1998). Clinical images depicting the process are also given in Figure 9.10.

Figure 9.10 Patch testing: (a) metal cups containing allergens; (b) positive patch test reactions. (Source: Reprinted from Graham-Brown and Burns, 2006.)

When photo-allergic dermatitis is suspected, photo-patch testing involves application of the photo allergen series and any other suspected allergens in duplicate on the back. One set of patch tests is irradiated with ultraviolet light (5 J cm^-2), and after 48 and 96 hours the patch tests are read in parallel (Bourke et al., 2001).