Allergic coNtAct DermAtitis of the foot - The Dermatologist

sic “shoe pattern dermatitis” on the footis suggestive of allergic contact dermatitis(Figure 1). This pattern shows aneczematous process involving the dorsalfoot and ankle with notable sparing ofthe inter-digital spaces. There is oftena relative symmetry, but one side maybe more heavily affected than the other.Spreading of the process up the anklesover time is also a clue to ACD. Thisis particularly true in the setting of achronic, stable, well-localized dermatitisthat then shifts to become poorly localizedand spreading.Endogenous dermatitis is often themost difficult to separate from allergiccontact dermatitis. This category includesatopic foot dermatitis, nummularfoot dermatitis, juvenile plantar dermatosisand the pompholyx-dyshidrosisspectrum. In many cases, the diagnosisof endogenous dermatitis is one of exclusion,made only after a negative patchtest has been performed to help rule outallergic contact dermatitis.Acral papulosquamous entities maybe easily diagnosed if typical stigmataare seen elsewhere on the patient. However,if involvement is limited to thefeet, there can be significant morphologicoverlap with chronic eczematousprocesses. Acral psoriasis, palmoplantarpustulosis and pityriasis rubra pilarisfall into this category. In a study inthe International Journal of Dermatology,70 patients suspected of having allergiccontact dermatitis of the foot underwentpatch testing; 30 were given a finaldiagnosis of psoriasis. 20Infectious entities of the feet aretypically less likely to be misdiagnosedas contact dermatitis, with one majorexception. Dermatophyte infection caneasily be missed and should always beconsidered in the differential diagnosisof a patient with a possible contact dermatitisof the foot.Hybrid dermatitis is a clinically usefulterm that takes into account the factthat many patients have multifactorialdisease. It is often the case that a patientmay be atopic with a component of irritantdermatitis and superimposed allergiccontact dermatitis. Utilizing theterm “hybrid dermatitis” brings intofocus the need to address all underlyingfactors to ensure clinical success. Similarto the endogenous group, this diagnosisis typically only made after the completionof a full work-up, including comprehensivepatch testing.Practical Aspects of Patch TestingAs discussed above, the differential diagnosisof foot dermatitis can be a challenge.Patch testing is the gold standardfor making the diagnosis of allergic contactdermatitis. Most patch test expertsadvocate an approach that utilizes anexpanded series, including testing componentsof the patient’s own footwearand socks. The most common allergenshave been listed in Table 1.While management of ACD of thefoot can be challenging, the majorityof patients should be able to attainsustained improvement or resolutionof their symptoms if proper patch testingis carried out and relevant allergensare avoided. 12 To maximize the yield ofpatch testing, it is important to recognizewhich patterns of foot dermatitisare most likely to show relevant positivereactions. For example, the previouslymentioned “classic shoe dermatitis pattern”with dorsal foot involvement is thefoot pattern most likely to show relevantpositive reactions on patch testing. 21Conversely, some patterns are importantto recognize because of their relativelylow yield on patch testing. One suchpattern is symmetric involvement ofboth the central palms and plantar feet.This pattern is more likely to be an endogenouseczema or psoriasis and has alower yield on patch testing for relevantpositive reactions.It is also important to know how tointerpret positive results. For example,nickel is a commonly tested and frequentlypositive allergen, but the relevanceof nickel allergy is somewhatunclear when considering foot dermatitis.Dimethylglyoxime-positive metalsnaps, fasteners, buckles or islets may berelevant in a patient with foot dermatitisand a positive patch test to nickel.Some reports also suggest that systemicnickel exposure in an allergic patientcan present as a dyshidrotic hand andfoot eczema. 22,23 Outside of these twosettings, a positive nickel patch test mayonly represent past sensitization withoutcurrent relevance. Similarly, a positivereaction to fragrance, neomycin,bacitracin or corticosteroid may or mayTable 3. ExPanded Shoe Series 15Allergen FocusAdhesivesp-tert-butylphenol-formaldehyde resinColophonyEpoxy ResinBenzoyl PeroxideDyesParaphenylendiamineDisperse Blue 106 or 124Rubber ComponentsBlack Rubber MixCarba MixMercaptobenzothiazole or Mercapto MixThiourea MixThiuram MixDithiodimorpholineLeather Tanning AgentsPotassium DichromateGlutaraldehydeFormaldehydenot currently be relevant. Evaluationof relevance for allergens that are notcore components of footwear like theabove allergens requires that the presenceof retained allergens be excluded.For example, odor-eliminating powdersand sprays may be a source of fragranceretention in footwear. Similarly, topicalmedicaments are frequently applied tothe feet and it is possible for these medicamentsto become embedded into thesock and footwear. 24,25When determining what allergensto test, a standard series to screen forcommon shoe allergens and occult allergens,such as those described in theprevious paragraph, is useful. However,it is also important to expand testingbeyond this standard series. It has beenshown that, in 12.7% of patients withfoot dermatitis, the NACDG standardseries failed to identify any relevantallergens. 14 This number of non-reactorsis reduced when testing includessamples taken directly from the patient’ssocks and shoes. 12,14 Samplescan be taken from footwear with an8-mm punch, thereby yielding piecesequal in size to a standard 8-mm Finnchamber. In addition, it is often helpfulto utilize an expanded shoe series.A good series will adequately screenfor sensitivity to adhesives, dyes, rubbercomponents and leather tanningagents, which may not be included onsome standard series. Table 3 high-November 2012 | The Dermatologist ® | www.the-dermatologist.com 23