Foot eczema is a common presenting complaint. The prevalence of allergic contact dermatitis (ACD) involving the feet varies considerably and is thought to affect between 1.5% and 24.2% of all patients undergoing patch testing.1,2 Concomitant involvement of both the hands and the feet is not uncommon. According to the literature, the prevalence of the condition when it affects both sites is 5.4% in the general population.3,4

Clinical diagnosis of foot eczema is relatively easy. However, classification is complex, since there are many morphological variations and it is difficult to distinguish between contact eczema (irritant or allergic) and atopic eczema, which can occur alone or in combination.1,2 Very often, it is difficult to differentiate between eczema and psoriasis. Therefore, in chronic disease, it is almost mandatory to perform patch tests to detect allergens that act as a cause or complication of foot eczema.5,6 The allergens most frequently involved in foot eczema are found in the material used for footwear and include potassium dichromate, p-tert butylphenol-formaldehyde resin, and mercaptobenzothiazole.7

Prevalence and patch test results have been widely analyzed in epidemiological studies in patients with hand eczema,3,4 although almost no studies provide data on foot eczema.8 Therefore, we performed a study to ascertain the demographic profile, final diagnosis, and allergens involved in patients with suspected foot eczema who come to our clinic for patch testing.

The main objectives of the study were as follows:

• –

  To determine the demographic characteristics of patients with foot eczema assessed by means of patch testing and to establish which allergens are most commonly involved in foot eczema.

The secondary objectives were as follows:

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  To determine whether there are differences between the final diagnosis and the results of patch testing between the sexes and age groups.

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  To compare epidemiological data, final diagnosis, and patch test results from patients referred for suspected eczema on the feet only with those from patients with suspected eczema on the hands and feet.

Ours was an observational, cross-sectional study performed between January 2004 and December 2016. We included all patients who had undergone patch testing with the standard series of the Spanish Contact Dermatitis and Skin Allergy Research Group (GEIDAC) at the skin allergy clinic of the dermatology department of a tertiary hospital. Patient information was obtained from the anonymized database used in clinical care. We recorded patient data, the allergen series applied, the results, and their relevance. The clinical data recorded for each patient were age, sex, location of the lesions (feet only, both hands and feet, and other sites, including patients with diffuse or generalized involvement who also had lesions on their feet), cause of eczema, and final diagnosis. The possible diagnoses were as follows:

• 1

  ACD.

• 2

  Irritant contact dermatitis (ICD).

• 3

  Atopic eczema (including dyshidrosis). Patients with dyshidrotic eczema were included in this section.

• 4

  Psoriasis.

• 5

  Other.

Based on the location of lesions, we established 2 study groups:

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  Total study group, other locations of eczema (including any site on the body where the lesion appeared).

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  Group with suspected foot eczema, which was in turn divided into:

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  Patients with suspected eczema on both the hands and the feet.

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  Patients with suspected eczema on the feet only. (Patients who in addition to presenting with foot eczema had involvement at sites other than the hands were not included in this group).

The groups were compared by sex (male/female) and age (<18 years/≥18 years).

Between January 2004 and December 2016, we performed patch testing with the standard GEIDAC series on 3265 patients, of whom 308 (9.4%) had foot eczema. Of these 308 patients, 176 (57.9%) had foot eczema only, whereas 132 (42.1%) had concomitant hand and foot eczema.

Suspected foot eczema is a frequent presenting complaint in our skin allergy unit and is observed in approximately 10% of patients who undergo patch testing. More than 40% of these patients also had hand lesions. This subgroup has unusual epidemiological characteristics, and the results of patch testing and the final diagnosis differ from those of the group with foot eczema only.

In epidemiological terms, the present series highlights a balance between the sexes. Other studies report a greater number of females.6,9,10 Of note, we performed patch testing in a high percentage of children and adolescents with foot eczema. While eczema is common in children, most are clinically diagnosed with atopic dermatitis and very few undergo patch testing despite current recommendations to perform patch testing in patients with treatment-refractory atopic dermatitis or atopic dermatitis with an “atypical” distribution.11–14 In the present series, we believe that the large number of children, especially in those with foot eczema only, is due to the fact that the feet are not considered a “typical” location for atopic dermatitis, with the result that the case is referred in order to rule out ACD. Furthermore, we agree with Isaksson et al.,15 who recommend patch testing in all children with atopic dermatitis who present with chronic eczema on the hands, feet, or both.

We found patch testing to be very useful in patients with foot eczema, since the results were positive in a higher percentage than in the rest of the population studied, especially in those patients with foot involvement only. Present relevance was also more common in this population. Our data are similar to those of Febriana et al.16 and Priya et al.,8 who reported positive results in 66.7% and 88% of patients, respectively. Therefore, we recommend this approach in cases of chronic foot eczema. As for patients with concomitant hand and foot eczema, the results of patch testing were more often negative in patients aged <18 years. Negative results in patch testing should also be interpreted as being useful. In the subgroup with concomitant hand and foot disease in particular, we should consider the possibility that the patient has psoriasis or atopic dermatitis.

Consistent with results from other studies, we found the most common allergen to be potassium dichromate.17 This allergen is usually associated with footwear18; since it is used to tan leather, it is present in leather shoes. The association between foot eczema and allergy to potassium dichromate was so strong that we believe any positive result with chromium in patch tests should lead us to ask whether the patient has foot eczema, especially in summer. Hansen et al.19 concluded that patients with positive results for chromium (III) constitute a group with multiple allergies to footwear and that in patients with a positive result to chromium (VI), leather was the main suspected source of exposure. The European Union has been a pioneer in setting limitations for hexavalent chromium content in leather goods. The amount of chromium used in leather tanning in Europe is regulated by European standard EN 420 and the EU Ecolabel for footwear. As far back as 1994, the European Union included specifications on hexavalent chromium content in standard EN 420 on protective gloves, limiting the substance to a maximum of 2mg/kg. The EU Ecolabel for footwear specifies that the concentration of chromium (VI) in the finished product must not exceed 10 ppm.20 Nevertheless, we think that vegetable tanning, which scarcely uses chromium, should be incentivized in the manufacture of footwear. The majority of studies report that nickel sulfate is the most frequent allergen.21–25 In Indonesia, mercapto mix is the most prevalent allergen and is probably associated with the use of rubber slippers by some of the population.16 In a study on ACD of the feet performed in Turkey, Özkaya and Ekinci9 found nitrofurazone to be the most frequent allergen. Consequently, the main cause of ACD of the foot can differ from one country to another.

The most common final diagnosis in our study was ACD, and the most frequent cause was footwear. The high prevalence of ACD of the foot and its association with footwear can be seen in other studies,6,8,26 and the disease is thought to be more frequent in warm, humid climates.16 Foot sweating, together with friction and occlusion by the shoe, makes it easier for people to become sensitized to the materials used in the manufacture of footwear.

However, ACD was not the most frequent diagnosis in some subgroups in the present series. Psoriasis is more frequent in adults with suspected hand and foot eczema. This finding is rarely reported in the literature,27 although this is not surprising, since in clinical practice it is very difficult to distinguish between eczema and psoriasis, and patients often present eczematized psoriasis or the eczema acts as a Koebner phenomenon and triggers psoriasis. The refractoriness of palmoplantar psoriasis to currently available treatments leads the physician to refer the patient for examination in order to rule out overlapping ACD. In children and adolescents, however, the most common final diagnosis was atopic dermatitis. Of note, the feet are a common site for atopic dermatitis in childhood.28 In cases of psoriasis and atopic dermatitis with eczema on the hands and/or feet that do not respond to seemingly appropriate treatment, we believe that patch testing should be proposed in order to rule out overlapping ACD.

The suspicion of foot eczema is a common reason for visits to the skin allergy unit. Patch testing is very useful. Clinically relevant positive results are obtained in a larger percentage of patients than in the rest of the study population. ACD is the most common final diagnosis in patients with foot eczema only. Potassium dichromate is the most common allergen, and the components of footwear are the main cause.

In the case of patients who also present with hand lesions, the disease is more likely to be psoriasis (in adults) or atopic dermatitis (in children and adolescents). Patch testing is also very useful for ruling out overlapping ACD in this subgroup.

The authors declare that they have no conflicts of interest.