Despite the fact that the scalp is constantly exposed to various everyday allergens, ACD is uncommon at this site. The greater thickness of the epidermis, the absence of folds and wrinkles, and the abundance of pilosebaceous glands all act as the perfect barrier to allergens. Even if a very potent allergen makes contact with the scalp, the most likely finding is that the patient will present symptoms and signs of dermatitis at another site, such as the face, eyelids, or neck.

Two of the 3 eczema patterns discussed below actually apply to sites other than the scalp. These are discussed in this section for practical reasons, as they are the areas most affected by the allergen.

Figure 1.

Clinical patterns of allergic contact dermatitis affecting the scalp.

A, Rinse-off pattern: eczematous plaques on the sides of the face (preauricular and mandibular) and neck; B, C, and D, pattern along the hairline. B, Forehead and area above the ears. C, Occipital and retroauricular area.

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Rinse-off pattern

Eczematous plaques on the sides of the face (preauricular and mandibular) and neck. The lesions are produced by the allergen running along the side of the face. The pattern is typical of shampoos, conditioners, and other products that are applied temporarily to the scalp and make brief but recurrent contact with the skin of the face (Fig. 1A and Fig. 2).

Figure 2.

A and B. Allergic contact dermatitis caused by shampoo affecting both sides of the face (rinse-off pattern).

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Hairline pattern

Eczematous plaques found at the limit between the scalp and the skin of the face, including the forehead, retroauricular region, nape of the neck, and the area above the eyebrows. The pattern is typical of dyes and perming solutions (Figs. 1B, C, and D and Fig. 3).

Figure 3.

A-D, Allergic contact dermatitis caused by paraphenylenediamine in black hair dye. A, Involvement of the forehead and eyelids. B and C, Involvement of the retroauricular area and nape of the neck. D, Scarce involvement of the scalp.

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Geographic pattern

Eczematous plaques confined to the area of contact with the allergen. Typical of objects (Fig. 4).

Figure 4.

A and B, Allergic contact dermatitis caused by the earpiece of the patient's glasses. C, Positive patch test result with scrapings from the glasses.

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The products most frequently described in ACD affecting the scalp are hair dyes, followed by shampoos and conditioners.1 In the case of hair dyes, the most common allergen is paraphenylenediamine, which is an oxidative coloring agent present in many dyes. Its concentration is higher in dark-colored dyes, although it is also found in light-colored dyes and has even been found in some dyes in which the manufacturer does not indicate its presence on the label.2 The symptoms of ACD caused by paraphenylenediamine are very typical and consist of acute edematous dermatitis with considerable involvement of the face, eyelids, and neck and minimal involvement of the scalp3 (Fig. 3). Shampoos and conditioners are uncommon causes of ACD, since rinsing means they have little contact with the skin. They are even well tolerated by sensitized patients.4,5 The most common allergens are fragrances, cocamidopropyl betaine, and preservatives such as quaternium-15.1

Special mention must be made of lotions containing minoxidil, where the most frequent allergens are propylene glycol, which is used as a vehicle, followed by minoxidil itself.6,7 Typical clinical findings include erythema, pruritus, desquamation, and dryness of the scalp, thus obliging us to make the differential diagnosis with seborrheic dermatitis or psoriasis. Table 1 summarizes the main allergens causing ACD of the scalp.

The allergens causing the bilateral patchy pattern are generally found in hygiene and moisturizing products.9,10 The list of potential allergens is very long and can be divided into 2 major groups: preservatives and fragrances. Within the preservatives, Kathon CG seems to be the most frequently involved in all series.9,10 Kathon CG is a 3:1 mixture of methylchloroisothiazolinone and methylisothiazolinone. This very efficient preservative is present in many products, such as moisturizing creams, gels, and shampoos.11 Sensitization to Kathon CG has become increasingly common in recent years. Therefore, in 2005, use of the preservative methylisothiazolinone alone was proposed to reduce sensitivity. However, this approach was unsuccessful, and methylisothiazolinone became Allergen of the Year in 2013 in Europe.12 With respect to fragrances, it is noteworthy that these substances are found not only in perfumes, but also in cosmetic products to make them more pleasant. The main allergens are summarized in Table 2.9–13 It is important to remember that the standard European series only detects some 50% of cases of ACD to cosmetics,9,10 thus making it necessary to perform patch tests with the patient's own products.

The allergens causing the rinse-off pattern are addressed in the section on the scalp. The geographic pattern can be produced by several allergens, depending on the object that comes into contact with the skin.

Most cases involving the airborne pattern are occupational, with the main allergens being those present in medicines, plants, and resins.14 Airborne ACD to medicines generally affects health professionals and carers, who are responsible for crushing tablets for patients with difficulty swallowing.14 The main allergens in plants are sesquiterpene lactones from the Compositae family, which mainly affect people who work in the open air.15 In the case of resins, the main allergen is epoxy resin, which has high sensitizing potency. Resins have several uses and are found in paints and finishing products, immersion oils, adhesives, coating used in electrical equipment, and material used in sports articles, aeronautical applications, and even in medical products such as dental prostheses and pacemakers. ACD mainly affects workers in the industries where these products are manufactured.14

The allergens most frequently involved in the photoallergic pattern are topical anti-inflammatory drugs such as ketoprofen and etofenamate and sunscreens such as octocrylene and benzophenone-3.16–18

It is important to remember that eyelid ACD occurs more frequently after application of products on the scalp or face than after direct application on the eyelids.10,23 In addition, symptoms rarely appear at the sites of direct application, thus further hampering diagnosis. Therefore, when assessing a patient with eyelid eczema, in addition to considering patterns of direct application, we should include an evaluation of patterns on the scalp and face and the airborne pattern.

Figure 9.

Clinical patterns of allergic contact dermatitis affecting the eyelids. A, Drip pattern; B, Contour pattern. C, Unilateral pattern.

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Drip pattern

Eczematous plaques on the lower eyelids and cheeks. Typical of eye drops (the upper eyelids may be involved) (Fig. 9A and Fig. 10).

Figure 10.

Allergic contact dermatitis caused by eye drops. A, Eczematous plaques mainly affecting the lower eyelids and cheeks. B, The upper eyelids may also be involved.

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Contour pattern

Eczematous plaques on the eyelids and periorbital region, more frequently affecting the upper eyelids and indicating direct application of cosmetic around the eyes. Typical of eye shadow, mascara, and moisturizing creams (Fig. 9B and Fig. 11).

Figure 11.

Contact dermatitis caused by eye shadow (contour pattern).

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The allergens responsible are many and varied and depend on the series studied.10,23,24 It is important to remember that the allergens can reach the eyelids via several routes (direct, indirect, airborne). Directly applied allergens are generally very similar to those found in ACD to cosmetics mentioned in the section on the face, that is, mainly preservatives and fragrances. However, in the case of eyelid ACD, we must remember that in some series, nickel is the most frequently involved allergen.23 Nickel can be found directly in cosmetic products containing colorants, for example, mascara, eye pencils, and eye shadow,25 or indirectly, for example, as a contaminant of a cosmetic or its container.26 Of note, many articles used to enhance the eyelashes (eg, eyelash curlers) contain nickel,27 which can also reach the eyelids ectopically, for example, through metallic nail files, contact with coins or keys,28,29 or after application of nail varnish or metallic nail polish containing nickel.30 As for the allergens in eye drops, the most relevant are preservatives and some medications.10,23,24,31 The main allergens in preservatives are benzalkonium chloride and thiomersal, and the main drugs involved are parasympathomimetic and sympathomimetic agents, carbonic anhydrase inhibitors, and adrenergic ß-blockers.10,23,32 The main indirectly applied allergens are those associated with nail enhancement products, such as the resin p-toluenesulfonamide formaldehyde (found in some polishes), acrylates (used as glue for artificial nails), and methacrylates (component of the paste of so-called porcelain nails).23,33 Lastly, it is worth mentioning gold, which is an uncommon allergen in ACD at other sites, although some series consider it the main allergen in isolated eyelid ACD.34 When gold in jewelry (eg, rings) comes into contact with certain particles, such as titanium dioxide (used in physical sunscreens or as an opacifier in some cosmetics), it releases ions that can cause ACD.35Table 3 summarizes the allergens that most commonly affect the eyelids, excluding those already mentioned in the section on the face.

The allergens involved vary widely depending on the series studied.43,46–49Table 4 summarizes the main ones. ACD, as is the case with ICD, is mainly associated with occupational and recreational activities in which the patient uses his/her hands. In addition, ICD is often complicated by ACD and vice versa. The professions that entail the greatest risk of ACD include food handlers, hairdressers, construction workers, and cleaning staff.46 The most commonly reported allergens are diallyl disulfide in patients who handle garlic,15,42 paraphenylenediamine in hairdressers,50,51 nickel in the case of people who handle coins,50,51 chrome in construction workers,51 cobalt in electronics workers, and specific preservatives (eg, quaternium-15 and formaldehyde) in the case of health care professionals and persons who work with textiles and wood.51,52 In some cases, the protective measures used to avoid contact with the substances involved (eg, gloves or protective creams) or treatment with corticosteroids may be the cause of ACD.48,53–56

Eczematous plaques affecting ≥3 areas of the body in a patchy and diffuse distribution, for example, the arms, trunk, and extremities59 (Fig. 18F and Fig. 24).

Figure 24.

Allergic contact dermatitis with a generalized patchy pattern. A, Patchy eczematous plaques on the forehead, cheeks, and chin. B, Patchy eczematous plaques on the anterior trunk. C, Patchy eczematous plaques on the back. D, Patchy eczematous plaques on both forearms.

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The list of allergens that can affect the trunk is extensive and varied, and an exhaustive clinical history is required before a potential culprit can be identified. The geometric, pocket, and nickel patterns are the easiest to identify and do not usually hamper diagnosis. The drip pattern is mainly associated with preservatives and fragrances in soaps and disinfectants, the main allergen being Kathon CG. The main allergens in the textile pattern are dyes, which have overtaken formaldehyde-releasing finishing products, the predominant allergen in the 1960s.60–62 Dyes are classified into 2 main groups: disperse dyes used in synthetic fabrics, and reactive dyes used in natural or mixed fibers. Disperse dyes bind more weakly to fabric than reactive dyes, with the result that they are released more easily and produce sensitization more often under specific conditions such as sweating, moisture, or repeated rubbing. Despite differences between the populations studied, the most commonly reported allergens are Disperse Blue 106, Disperse Blue 124, and Disperse Orange 3.60–62

In the generalized patchy pattern, the most common allergens are preservatives present in topical products, mainly imidazolidinyl urea, DMDM hydantoin, propylene glycol, and diazolidinyl urea.59Table 5 summarizes the most common allergens.

The major allergens are found mainly in footwear, followed by topical medications that are applied on the feet, such as antibiotics, antifungals, and corticosteroids.63–69

In the case of ACD caused by footwear, the most common allergens are the products used in the manufacture of rubber (especially black rubber), leather, and adhesives.63–70

Derivatives of paraphenylenediamine, mercaptobenzothiazole, and thiurams are the allergens most frequently associated with the use of rubber.70 Mercaptobenzothiazole may affect the hands and feet. The hands are affected by occupational ACD caused by the mercaptobenzothiazole present in anticorrosion liquids and, to a lesser extent, by rubber gloves. ACD on the feet is caused by work boots or sports shoes.70

Potassium dichromate is the allergen most frequently associated with leather shoes. Para-tertiary-butylphenol-formaldehyde resin71 and colophony are the allergens most frequently associated with adhesives.67,69,70

As for medications, the allergen is usually the active ingredient in antibiotics (more frequently neomycin); ACD to antifungals and corticosteroids is more frequently caused by the vehicle.60–67 There have also been occasional reports of photoallergy to topical anti-inflammatory drugs, with ketoprofen and etofenamate being the most common allergens.72,73 In these cases, symptoms are usually very acute, and tense blisters are a frequent finding (Fig. 29).

Figure 29.

Photoallergic contact dermatitis caused by topical anti-inflammatory drugs.

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Lastly, special attention should be given to a recently reported allergen, dimethyl fumarate. Dimethyl fumarate is used as a mold inhibitor during the storage and transport of various products, including shoes. Dimethyl fumarate–induced ACD was first associated with contact with furniture from China; however, in Spain, most cases were associated with shoes packaged with desiccant sachets containing dimethyl fumarate. The symptoms almost only affect women, and the most common pattern is the shoe pattern, although the clinical picture is usually very acute with marked edema, vesicles, and blisters.66Table 6 summarizes the main allergens.