Danielle E. Novack, BA

Atopic dermatitis (AD) and allergic contact dermatitis (ACD) are 2 common inflammatory skin conditions that may have similar clinical presentations. Historically, it was thought that these conditions could not be diagnosed simultaneously due to their differing immune mechanisms; however, this belief has been challenged by recent evidence suggesting a more nuanced relationship between the 2 disease processes. In this review, we examine the complex interplay between AD and ACD and explain how shifts in conventional understanding of the 2 conditions shaped our evolving recognition of their ability to coexist.

Epidemiology of AD and ACD

Atopic dermatitis is the most common inflammatory skin disease in children and adolescents, with an estimated prevalence reaching 21%.¹ In 60% of cases, onset of AD will occur within the first year of life, and 90% of cases begin within the first 5 years.² Resolution may occur by adulthood; however, AD may continue to impact up to 8% to 9% of adults, with an increased prevalence in those older than 75 years.¹ This may represent an underestimation of the burden of adult AD; one systematic review of 17 studies found that the pooled proportion of adult-onset AD was greater than 25%.³

In contrast, ACD previously was assumed to be a disease that more commonly impacted adults and only rarely children, primarily due to an early misconception that children were not frequently exposed to contact allergens and their immune systems were too immature to react to them even if exposed.^4,5 However, it is now known that children do have risk factors for development of ACD, including a thinner stratum corneum and potentially a more absorbent skin surface.⁴ In addition, a 2022 study by the North American Contact Dermatitis Group (NACDG) found similar rates of ACD in children (n=1871) and adults (n=41,699) referred for patch testing (55.2% and 57.3%, respectively) as well as similar rates of having at least 1 relevant positive patch test (49.2% and 52.2%).⁶

In opposition to traditional beliefs, these findings highlight that AD and ACD can occur across age groups.

Immune Mechanism

The pathogenesis of AD represents a multifactorial process involving the immune system, cutaneous flora, genetic predisposition, and surrounding environment. Immunologically, acute AD is driven by a predominantly T_H2 helper T-cell response with high levels of IL-4, IL-5, and IL-13⁷; T_H22, T_H17, and T_H1 also have been implicated.⁸ Notably, T_H17 is found in high levels during the acute eczema phase, while T_H1 and T_H22are associated with the chronic phase.⁷

The pathophysiology of ACD is not completely understood. The classic paradigm involves 2 phases: sensitization and elicitation. Sensitization involves antigen-presenting cells that take up allergens absorbed by the skin to present them in regional lymph nodes where antigen-specific T lymphocytes are generated. Elicitation occurs upon re-exposure to the allergen, at which time the primed T lymphocytes are recruited to the skin, causing inflammation.⁹ Allergic contact dermatitis initially was thought to be driven by T_H1 cytokines and IL-17 but now is understood to be more complex.¹⁰ Studies have revealed immune polarization of contact allergens, demonstrating that nickel primarily induces a T_H1/T_H17 response, whereas fragrance and rubber accelerators skew to T_H2; T_H9 and T_H22 also may be involved depending on the causative allergen.^11,12

Of note, the immunologic differences between AD and ACD led early investigators to believe that patients with AD were relatively protected from ACD.¹³ However, as previously described, there are several overlapping cytokines between AD and ACD. Furthermore, research has revealed that risk of contact sensitization might be increased in the chronic eczema phase due to the shared T_H1 pathway.¹⁴ Barrier-disrupted skin (such as that in AD) also may increase the cytokine response and the density of antigen-presenting cells, leading to a proallergic state.¹⁵ This suggests that the immunologic pathways of AD and ACD are more intertwined than was previously understood.

Underlying Risk Factors

Skin barrier dysfunction is a key step in the pathogenesis of AD. Patients with AD commonly have loss-of-function mutations in the filaggrin gene, a protein that is key to the function of the stratum corneum. Loss of this protein may not only impact the immune response as previously noted but also may lead to increased transepidermal water loss and bacterial colonization.¹⁶ Interestingly, a 2014 review examined how this mutation could lead to an increased risk of sensitization to bivalent metal ions via an impaired chelating ability of the skin.¹⁷ Furthermore, a 2016 study conducted in Dutch construction workers revealed an increased risk for contact dermatitis (irritant and allergic) for those with a loss-of-function filaggrin mutation.¹⁸

Importantly, this same mutation may explain why patients with AD tend to have increased skin colonization by Staphylococcus aureus. The abundance of S aureus and the relative decrease in the diversity of other microorganisms on the skin may be associated with increased AD severity.¹⁹ Likewise, S aureus may play a role in the pathogenesis of ACD via production of its exotoxin directed at the T-cell receptor V beta 17 region. In particular, this receptor has been associated with nickel sensitization.¹⁷

Another risk factor to consider is increased exposure to contact sensitizers when treating AD. For instance, management often includes use of over-the-counter emollients, natural or botanical remedies with purported benefits for AD, cleansers, and detergents. However, these products can contain some of the most prevalent contact allergens seen in those with AD, including methyl-isothiazolinone, formaldehyde releasers, and fragrance.²⁰ Topical corticosteroids also are frequently used, and ACD to steroid molecules can occur, particularly to tixocortol-21-pivalate (a marker for class A corticosteroids) and budesonide (a marker for class B corticosteroids).²¹ Other allergens (eg, benzyl alcohol, propylene glycol) also may be found as inactive ingredients of topical corticosteroids.²² These exposures may place AD patients at risk for ACD.

The Coexistence of AD and ACD

Given the overlapping epidemiology, immunology, and potentially increased risk for the development of ACD in patients with AD, it would be reasonable to assume that the 2 diagnoses could coexist; however, is there clinical data to support this idea? Based on recent database reviews, the answer appears to be yes.^20,23-26 An analysis from the Pediatric Contact Dermatitis Registry revealed that 30% of 1142 pediatric patch test cases analyzed were diagnosed as AD and ACD simultaneously.²⁴ The NACDG found similar results in its 2021 review, as 29.5% of children (n=1648) and 20.7% of adults (n=36,834) had a concurrent diagnosis of AD and ACD.²⁰ Notably, older results from these databases also demonstrated an association between the 2 conditions.^23,25,26

It remains unclear whether the prevalence of ACD is higher in those with or without AD. A comprehensive systematic review conducted in 2017 examined this topic through analysis of 74 studies. The results demonstrated a similar prevalence of contact sensitization in individuals with and without AD.²⁷ Another systematic review of 31 studies conducted in 2017 found a higher prevalence for ACD in children without AD; however, the authors noted that the included studies were too variable (eg, size, design, allergens tested) to draw definitive conclusions.²⁸

Even though there is no clear overall increased risk for ACD in patients with AD, research has suggested that certain allergens may be more prevalent in the setting of AD. An NACDG study found that adults with AD had increased odds of reacting to 10 of the top 25 NACDG screening allergens compared to those without AD.²⁰ Other studies have found that AD patients may be more likely to become sensitized to certain allergens, such as fragrance and lanolin.¹⁴

Considerations for Management

Diagnosis of ACD in patients with AD can be challenging because these conditions may present similarly with chronic, pruritic, inflammatory patches and plaques. Chronic ACD may be misdiagnosed as AD if patch testing is not performed.²⁹ Given the prevalence of ACD in the setting of AD, there should be a low threshold to pursue patch testing, especially when dermatitis is recalcitrant to standard therapies or presents in an atypical distribution (ie, perioral, predominantly head/neck, hand and foot, isolated eyelid involvement, buttocks).^4,30 Various allergen series are available for patch testing adults and children including the NACDG Standard Series, American Contact Dermatitis Society Core Allergen Series, or the Pediatric Baseline Series.^31-33

If potentially relevant allergens are uncovered by patch testing, patients should be counseled on avoidance strategies. However, allergen avoidance may not always lead to complete symptom resolution, especially if AD is present concomitantly with ACD. Therefore, use of topical or systemic therapies still may be required. Topical corticosteroids can be used when dermatitis is acute and localized. Systemic corticosteroids are utilized for both diagnoses when cases are more severe or extensive, but their adverse-effect profile limits long-term use. Other systemic treatments, including conventional agents (ie, azathioprine, cyclosporine, methotrexate, mycophenolate mofetil), biologics, and small molecule inhibitors also may be considered for severe cases.^34,35 Dupilumab, a monoclonal antibody targeting IL-4/IL-13, is approved for use in moderate to severe AD in patients 6 months and older. Recent evidence has suggested that dupilumab also may be an effective off-label treatment choice for ACD when allergen avoidance alone is insufficient.³⁶ Studies have been conducted on secukinumab, a monoclonal antibody against IL-17; however, it has not been shown to be effective in either AD or ACD.^37,38 This indicates that targeted biologics may not always be successful in treating these diagnoses, likely due to their complex immune pathways. Finally, there is an emerging role for JAK inhibitors. Three are approved for AD: topical ruxolitinib, oral abrocitinib, and oral upadacitinib.³⁹ Further investigation is needed to determine the efficacy of JAK inhibitors in ACD.

Final Interpretation

Evolving evidence shows that AD and ACD can occur at the same time despite the historical perspective that their immune pathways were too polarized for this to happen. Atopic dermatitis may be an important risk factor for subsequent development of ACD. Management should include a low threshold to perform patch testing, while pharmacotherapies utilized in the treatment of both conditions should be considered.

Atopic dermatitis (AD) and allergic contact dermatitis (ACD) are 2 common inflammatory skin conditions that may have similar clinical presentations. Historically, it was thought that these conditions could not be diagnosed simultaneously due to their differing immune mechanisms; however, this belief has been challenged by recent evidence suggesting a more nuanced relationship between the 2 disease processes. In this review, we examine the complex interplay between AD and ACD and explain how shifts in conventional understanding of the 2 conditions shaped our evolving recognition of their ability to coexist.

Epidemiology of AD and ACD

Atopic dermatitis is the most common inflammatory skin disease in children and adolescents, with an estimated prevalence reaching 21%.¹ In 60% of cases, onset of AD will occur within the first year of life, and 90% of cases begin within the first 5 years.² Resolution may occur by adulthood; however, AD may continue to impact up to 8% to 9% of adults, with an increased prevalence in those older than 75 years.¹ This may represent an underestimation of the burden of adult AD; one systematic review of 17 studies found that the pooled proportion of adult-onset AD was greater than 25%.³

In contrast, ACD previously was assumed to be a disease that more commonly impacted adults and only rarely children, primarily due to an early misconception that children were not frequently exposed to contact allergens and their immune systems were too immature to react to them even if exposed.^4,5 However, it is now known that children do have risk factors for development of ACD, including a thinner stratum corneum and potentially a more absorbent skin surface.⁴ In addition, a 2022 study by the North American Contact Dermatitis Group (NACDG) found similar rates of ACD in children (n=1871) and adults (n=41,699) referred for patch testing (55.2% and 57.3%, respectively) as well as similar rates of having at least 1 relevant positive patch test (49.2% and 52.2%).⁶

In opposition to traditional beliefs, these findings highlight that AD and ACD can occur across age groups.

Immune Mechanism

The pathogenesis of AD represents a multifactorial process involving the immune system, cutaneous flora, genetic predisposition, and surrounding environment. Immunologically, acute AD is driven by a predominantly T_H2 helper T-cell response with high levels of IL-4, IL-5, and IL-13⁷; T_H22, T_H17, and T_H1 also have been implicated.⁸ Notably, T_H17 is found in high levels during the acute eczema phase, while T_H1 and T_H22are associated with the chronic phase.⁷

The pathophysiology of ACD is not completely understood. The classic paradigm involves 2 phases: sensitization and elicitation. Sensitization involves antigen-presenting cells that take up allergens absorbed by the skin to present them in regional lymph nodes where antigen-specific T lymphocytes are generated. Elicitation occurs upon re-exposure to the allergen, at which time the primed T lymphocytes are recruited to the skin, causing inflammation.⁹ Allergic contact dermatitis initially was thought to be driven by T_H1 cytokines and IL-17 but now is understood to be more complex.¹⁰ Studies have revealed immune polarization of contact allergens, demonstrating that nickel primarily induces a T_H1/T_H17 response, whereas fragrance and rubber accelerators skew to T_H2; T_H9 and T_H22 also may be involved depending on the causative allergen.^11,12

Of note, the immunologic differences between AD and ACD led early investigators to believe that patients with AD were relatively protected from ACD.¹³ However, as previously described, there are several overlapping cytokines between AD and ACD. Furthermore, research has revealed that risk of contact sensitization might be increased in the chronic eczema phase due to the shared T_H1 pathway.¹⁴ Barrier-disrupted skin (such as that in AD) also may increase the cytokine response and the density of antigen-presenting cells, leading to a proallergic state.¹⁵ This suggests that the immunologic pathways of AD and ACD are more intertwined than was previously understood.

Underlying Risk Factors

Skin barrier dysfunction is a key step in the pathogenesis of AD. Patients with AD commonly have loss-of-function mutations in the filaggrin gene, a protein that is key to the function of the stratum corneum. Loss of this protein may not only impact the immune response as previously noted but also may lead to increased transepidermal water loss and bacterial colonization.¹⁶ Interestingly, a 2014 review examined how this mutation could lead to an increased risk of sensitization to bivalent metal ions via an impaired chelating ability of the skin.¹⁷ Furthermore, a 2016 study conducted in Dutch construction workers revealed an increased risk for contact dermatitis (irritant and allergic) for those with a loss-of-function filaggrin mutation.¹⁸

Importantly, this same mutation may explain why patients with AD tend to have increased skin colonization by Staphylococcus aureus. The abundance of S aureus and the relative decrease in the diversity of other microorganisms on the skin may be associated with increased AD severity.¹⁹ Likewise, S aureus may play a role in the pathogenesis of ACD via production of its exotoxin directed at the T-cell receptor V beta 17 region. In particular, this receptor has been associated with nickel sensitization.¹⁷

Another risk factor to consider is increased exposure to contact sensitizers when treating AD. For instance, management often includes use of over-the-counter emollients, natural or botanical remedies with purported benefits for AD, cleansers, and detergents. However, these products can contain some of the most prevalent contact allergens seen in those with AD, including methyl-isothiazolinone, formaldehyde releasers, and fragrance.²⁰ Topical corticosteroids also are frequently used, and ACD to steroid molecules can occur, particularly to tixocortol-21-pivalate (a marker for class A corticosteroids) and budesonide (a marker for class B corticosteroids).²¹ Other allergens (eg, benzyl alcohol, propylene glycol) also may be found as inactive ingredients of topical corticosteroids.²² These exposures may place AD patients at risk for ACD.

The Coexistence of AD and ACD

Given the overlapping epidemiology, immunology, and potentially increased risk for the development of ACD in patients with AD, it would be reasonable to assume that the 2 diagnoses could coexist; however, is there clinical data to support this idea? Based on recent database reviews, the answer appears to be yes.^20,23-26 An analysis from the Pediatric Contact Dermatitis Registry revealed that 30% of 1142 pediatric patch test cases analyzed were diagnosed as AD and ACD simultaneously.²⁴ The NACDG found similar results in its 2021 review, as 29.5% of children (n=1648) and 20.7% of adults (n=36,834) had a concurrent diagnosis of AD and ACD.²⁰ Notably, older results from these databases also demonstrated an association between the 2 conditions.^23,25,26

It remains unclear whether the prevalence of ACD is higher in those with or without AD. A comprehensive systematic review conducted in 2017 examined this topic through analysis of 74 studies. The results demonstrated a similar prevalence of contact sensitization in individuals with and without AD.²⁷ Another systematic review of 31 studies conducted in 2017 found a higher prevalence for ACD in children without AD; however, the authors noted that the included studies were too variable (eg, size, design, allergens tested) to draw definitive conclusions.²⁸

Even though there is no clear overall increased risk for ACD in patients with AD, research has suggested that certain allergens may be more prevalent in the setting of AD. An NACDG study found that adults with AD had increased odds of reacting to 10 of the top 25 NACDG screening allergens compared to those without AD.²⁰ Other studies have found that AD patients may be more likely to become sensitized to certain allergens, such as fragrance and lanolin.¹⁴

Considerations for Management

Diagnosis of ACD in patients with AD can be challenging because these conditions may present similarly with chronic, pruritic, inflammatory patches and plaques. Chronic ACD may be misdiagnosed as AD if patch testing is not performed.²⁹ Given the prevalence of ACD in the setting of AD, there should be a low threshold to pursue patch testing, especially when dermatitis is recalcitrant to standard therapies or presents in an atypical distribution (ie, perioral, predominantly head/neck, hand and foot, isolated eyelid involvement, buttocks).^4,30 Various allergen series are available for patch testing adults and children including the NACDG Standard Series, American Contact Dermatitis Society Core Allergen Series, or the Pediatric Baseline Series.^31-33

If potentially relevant allergens are uncovered by patch testing, patients should be counseled on avoidance strategies. However, allergen avoidance may not always lead to complete symptom resolution, especially if AD is present concomitantly with ACD. Therefore, use of topical or systemic therapies still may be required. Topical corticosteroids can be used when dermatitis is acute and localized. Systemic corticosteroids are utilized for both diagnoses when cases are more severe or extensive, but their adverse-effect profile limits long-term use. Other systemic treatments, including conventional agents (ie, azathioprine, cyclosporine, methotrexate, mycophenolate mofetil), biologics, and small molecule inhibitors also may be considered for severe cases.^34,35 Dupilumab, a monoclonal antibody targeting IL-4/IL-13, is approved for use in moderate to severe AD in patients 6 months and older. Recent evidence has suggested that dupilumab also may be an effective off-label treatment choice for ACD when allergen avoidance alone is insufficient.³⁶ Studies have been conducted on secukinumab, a monoclonal antibody against IL-17; however, it has not been shown to be effective in either AD or ACD.^37,38 This indicates that targeted biologics may not always be successful in treating these diagnoses, likely due to their complex immune pathways. Finally, there is an emerging role for JAK inhibitors. Three are approved for AD: topical ruxolitinib, oral abrocitinib, and oral upadacitinib.³⁹ Further investigation is needed to determine the efficacy of JAK inhibitors in ACD.

Final Interpretation

Evolving evidence shows that AD and ACD can occur at the same time despite the historical perspective that their immune pathways were too polarized for this to happen. Atopic dermatitis may be an important risk factor for subsequent development of ACD. Management should include a low threshold to perform patch testing, while pharmacotherapies utilized in the treatment of both conditions should be considered.

Atopic dermatitis (AD) and allergic contact dermatitis (ACD) are 2 common inflammatory skin conditions that may have similar clinical presentations. Historically, it was thought that these conditions could not be diagnosed simultaneously due to their differing immune mechanisms; however, this belief has been challenged by recent evidence suggesting a more nuanced relationship between the 2 disease processes. In this review, we examine the complex interplay between AD and ACD and explain how shifts in conventional understanding of the 2 conditions shaped our evolving recognition of their ability to coexist.

Epidemiology of AD and ACD

Atopic dermatitis is the most common inflammatory skin disease in children and adolescents, with an estimated prevalence reaching 21%.¹ In 60% of cases, onset of AD will occur within the first year of life, and 90% of cases begin within the first 5 years.² Resolution may occur by adulthood; however, AD may continue to impact up to 8% to 9% of adults, with an increased prevalence in those older than 75 years.¹ This may represent an underestimation of the burden of adult AD; one systematic review of 17 studies found that the pooled proportion of adult-onset AD was greater than 25%.³

In contrast, ACD previously was assumed to be a disease that more commonly impacted adults and only rarely children, primarily due to an early misconception that children were not frequently exposed to contact allergens and their immune systems were too immature to react to them even if exposed.^4,5 However, it is now known that children do have risk factors for development of ACD, including a thinner stratum corneum and potentially a more absorbent skin surface.⁴ In addition, a 2022 study by the North American Contact Dermatitis Group (NACDG) found similar rates of ACD in children (n=1871) and adults (n=41,699) referred for patch testing (55.2% and 57.3%, respectively) as well as similar rates of having at least 1 relevant positive patch test (49.2% and 52.2%).⁶

In opposition to traditional beliefs, these findings highlight that AD and ACD can occur across age groups.

Immune Mechanism

The pathogenesis of AD represents a multifactorial process involving the immune system, cutaneous flora, genetic predisposition, and surrounding environment. Immunologically, acute AD is driven by a predominantly T_H2 helper T-cell response with high levels of IL-4, IL-5, and IL-13⁷; T_H22, T_H17, and T_H1 also have been implicated.⁸ Notably, T_H17 is found in high levels during the acute eczema phase, while T_H1 and T_H22are associated with the chronic phase.⁷

The pathophysiology of ACD is not completely understood. The classic paradigm involves 2 phases: sensitization and elicitation. Sensitization involves antigen-presenting cells that take up allergens absorbed by the skin to present them in regional lymph nodes where antigen-specific T lymphocytes are generated. Elicitation occurs upon re-exposure to the allergen, at which time the primed T lymphocytes are recruited to the skin, causing inflammation.⁹ Allergic contact dermatitis initially was thought to be driven by T_H1 cytokines and IL-17 but now is understood to be more complex.¹⁰ Studies have revealed immune polarization of contact allergens, demonstrating that nickel primarily induces a T_H1/T_H17 response, whereas fragrance and rubber accelerators skew to T_H2; T_H9 and T_H22 also may be involved depending on the causative allergen.^11,12

Of note, the immunologic differences between AD and ACD led early investigators to believe that patients with AD were relatively protected from ACD.¹³ However, as previously described, there are several overlapping cytokines between AD and ACD. Furthermore, research has revealed that risk of contact sensitization might be increased in the chronic eczema phase due to the shared T_H1 pathway.¹⁴ Barrier-disrupted skin (such as that in AD) also may increase the cytokine response and the density of antigen-presenting cells, leading to a proallergic state.¹⁵ This suggests that the immunologic pathways of AD and ACD are more intertwined than was previously understood.

Underlying Risk Factors

Skin barrier dysfunction is a key step in the pathogenesis of AD. Patients with AD commonly have loss-of-function mutations in the filaggrin gene, a protein that is key to the function of the stratum corneum. Loss of this protein may not only impact the immune response as previously noted but also may lead to increased transepidermal water loss and bacterial colonization.¹⁶ Interestingly, a 2014 review examined how this mutation could lead to an increased risk of sensitization to bivalent metal ions via an impaired chelating ability of the skin.¹⁷ Furthermore, a 2016 study conducted in Dutch construction workers revealed an increased risk for contact dermatitis (irritant and allergic) for those with a loss-of-function filaggrin mutation.¹⁸

Importantly, this same mutation may explain why patients with AD tend to have increased skin colonization by Staphylococcus aureus. The abundance of S aureus and the relative decrease in the diversity of other microorganisms on the skin may be associated with increased AD severity.¹⁹ Likewise, S aureus may play a role in the pathogenesis of ACD via production of its exotoxin directed at the T-cell receptor V beta 17 region. In particular, this receptor has been associated with nickel sensitization.¹⁷

Another risk factor to consider is increased exposure to contact sensitizers when treating AD. For instance, management often includes use of over-the-counter emollients, natural or botanical remedies with purported benefits for AD, cleansers, and detergents. However, these products can contain some of the most prevalent contact allergens seen in those with AD, including methyl-isothiazolinone, formaldehyde releasers, and fragrance.²⁰ Topical corticosteroids also are frequently used, and ACD to steroid molecules can occur, particularly to tixocortol-21-pivalate (a marker for class A corticosteroids) and budesonide (a marker for class B corticosteroids).²¹ Other allergens (eg, benzyl alcohol, propylene glycol) also may be found as inactive ingredients of topical corticosteroids.²² These exposures may place AD patients at risk for ACD.

The Coexistence of AD and ACD

Given the overlapping epidemiology, immunology, and potentially increased risk for the development of ACD in patients with AD, it would be reasonable to assume that the 2 diagnoses could coexist; however, is there clinical data to support this idea? Based on recent database reviews, the answer appears to be yes.^20,23-26 An analysis from the Pediatric Contact Dermatitis Registry revealed that 30% of 1142 pediatric patch test cases analyzed were diagnosed as AD and ACD simultaneously.²⁴ The NACDG found similar results in its 2021 review, as 29.5% of children (n=1648) and 20.7% of adults (n=36,834) had a concurrent diagnosis of AD and ACD.²⁰ Notably, older results from these databases also demonstrated an association between the 2 conditions.^23,25,26

It remains unclear whether the prevalence of ACD is higher in those with or without AD. A comprehensive systematic review conducted in 2017 examined this topic through analysis of 74 studies. The results demonstrated a similar prevalence of contact sensitization in individuals with and without AD.²⁷ Another systematic review of 31 studies conducted in 2017 found a higher prevalence for ACD in children without AD; however, the authors noted that the included studies were too variable (eg, size, design, allergens tested) to draw definitive conclusions.²⁸

Even though there is no clear overall increased risk for ACD in patients with AD, research has suggested that certain allergens may be more prevalent in the setting of AD. An NACDG study found that adults with AD had increased odds of reacting to 10 of the top 25 NACDG screening allergens compared to those without AD.²⁰ Other studies have found that AD patients may be more likely to become sensitized to certain allergens, such as fragrance and lanolin.¹⁴

Considerations for Management

Diagnosis of ACD in patients with AD can be challenging because these conditions may present similarly with chronic, pruritic, inflammatory patches and plaques. Chronic ACD may be misdiagnosed as AD if patch testing is not performed.²⁹ Given the prevalence of ACD in the setting of AD, there should be a low threshold to pursue patch testing, especially when dermatitis is recalcitrant to standard therapies or presents in an atypical distribution (ie, perioral, predominantly head/neck, hand and foot, isolated eyelid involvement, buttocks).^4,30 Various allergen series are available for patch testing adults and children including the NACDG Standard Series, American Contact Dermatitis Society Core Allergen Series, or the Pediatric Baseline Series.^31-33

If potentially relevant allergens are uncovered by patch testing, patients should be counseled on avoidance strategies. However, allergen avoidance may not always lead to complete symptom resolution, especially if AD is present concomitantly with ACD. Therefore, use of topical or systemic therapies still may be required. Topical corticosteroids can be used when dermatitis is acute and localized. Systemic corticosteroids are utilized for both diagnoses when cases are more severe or extensive, but their adverse-effect profile limits long-term use. Other systemic treatments, including conventional agents (ie, azathioprine, cyclosporine, methotrexate, mycophenolate mofetil), biologics, and small molecule inhibitors also may be considered for severe cases.^34,35 Dupilumab, a monoclonal antibody targeting IL-4/IL-13, is approved for use in moderate to severe AD in patients 6 months and older. Recent evidence has suggested that dupilumab also may be an effective off-label treatment choice for ACD when allergen avoidance alone is insufficient.³⁶ Studies have been conducted on secukinumab, a monoclonal antibody against IL-17; however, it has not been shown to be effective in either AD or ACD.^37,38 This indicates that targeted biologics may not always be successful in treating these diagnoses, likely due to their complex immune pathways. Finally, there is an emerging role for JAK inhibitors. Three are approved for AD: topical ruxolitinib, oral abrocitinib, and oral upadacitinib.³⁹ Further investigation is needed to determine the efficacy of JAK inhibitors in ACD.

Final Interpretation

Evolving evidence shows that AD and ACD can occur at the same time despite the historical perspective that their immune pathways were too polarized for this to happen. Atopic dermatitis may be an important risk factor for subsequent development of ACD. Management should include a low threshold to perform patch testing, while pharmacotherapies utilized in the treatment of both conditions should be considered.

No time of the year is more exciting than the unveiling of the American Contact Dermatitis Society Allergen of the Year. Sometimes the selected allergen represents a completely novel cause of allergic contact dermatitis (ACD) with an unpronounceable chemical name. Not this time! The 2022 Allergen of the Year is likely to be lurking in your kitchen drawer at this very moment, as this year aluminum was chosen for this most prestigious honor.¹ But do not throw out your aluminum foil just yet—aluminum allergy tends to be confined to specific scenarios. In this article, we highlight the growing recognition of aluminum contact allergy, particularly in the pediatric population, focusing on distinct presentations of aluminum ACD, unique sources of exposure, and nuances of patch testing to this metal.

Aluminum Is All Around Us

As the third most common element in the Earth’s crust, aluminum can be found quite literally everywhere.¹ However, aluminum rarely is found in its pure elemental form; instead, it reacts with other elements around it, most commonly oxygen, to form aluminum-containing compounds. Known for their stability and safety, aluminum and its salts are incorporated in myriad products ranging from electronic equipment to foods and their packaging, medications, cosmetics, orthopedic and dental implants, and even tattoos. Aluminum also is found in the air and water supply and may even be encountered in certain workplaces, such as aircraft and machine industries. As such, contact with aluminum is all but certain in modern life.

The use of aluminum in consumer products is widely accepted as safe by public health agencies in the United States.² Although there has been public concern that aluminum could be linked to development of breast cancer or Alzheimer disease, there is no clear evidence that these conditions are associated with routine aluminum exposure through ingestion or consumer products.^3-5

Aluminum Contact Allergy

In part because of its ubiquity and in part because of the stability of aluminum-containing compounds, it was long thought that aluminum was nonallergenic. Contact allergy to elemental aluminum is rare; on the other hand, aluminum salts (the forms we are likely to encounter in daily life) are now recognized in the field of contact dermatitis as allergens of significance, particularly in the pediatric population.^1,6

First reported as a possible occupational allergen in 1944,⁷ aluminum allergy came to prominence in the 1990s in association with vaccines. Aluminum is included in some vaccines as an adjuvant that bolsters the immune response⁸; the eTable lists currently available aluminum-containing vaccines in the United States; of note, none of the COVID-19 vaccines approved in the United States or Europe contain aluminum.¹¹ Although the use of aluminum in vaccines is considered to be safe by the US Food and Drug Administration and Centers for Disease Control and Prevention,^12,13 a small number of children become sensitized to aluminum through vaccines and may develop persistent pruritic subcutaneous nodules (also known as vaccination granulomas) at the injection site; however, the incidence of this adverse effect was less than 1% in large studies including as many as 76,000 children, suggesting that it is relatively rare.^14,15 Upon patch testing, aluminum allergy has been detected in 77% to 95% of such cases.¹⁴ There is wide variation in the onset of the nodules ranging from weeks to years following vaccination.¹⁵ Due to pruritus, the examination may reveal accompanying excoriations, hyperpigmentation, and sometimes hypertrichosis at the injection site. Aluminum allergy related to vaccination also can manifest with widespread eruptions representing systemic contact dermatitis.¹⁶

Along with vaccines, the second major source of aluminum sensitization is allergen-specific immunotherapies administered by allergists/immunologists, many of which contain aluminum hydroxide.^17,18

On the consumer product front, antiperspirants are the most common source of cutaneous exposure to aluminum. Aluminum complexes react with electrolytes in sweat to form plugs in eccrine ducts, thereby preventing sweat excretion.⁶ Allergic contact dermatitis to these products presents with axillary-vault dermatitis. There also have been reports of ACD to aluminum in sunscreen and toothpaste, with the latter implicated in causing systemic ACD.^19,20

Prevalence of Sensitization to Aluminum

There have been a few large-scale studies evaluating rates of sensitization to aluminum in general patch-test patient populations; additionally, because of the complexities of testing this metal, investigators have utilized differing formulations for patch testing. A recent Swedish study found that 0.9% of 5448 adults and 5.1% of 196 children showed positive reactions to aluminum chloride hexahydrate (ACH) 10% in petrolatum and/or aluminum lactate 12% in petrolatum.²¹ Notably, there was a significant association between aluminum allergy and history of atopy for both adults (P=.0056) and children (P=.046), which remains to be further explored. A systematic review and meta-analysis found comparable rates of aluminum allergy in 0.4% of adults and 5.6% of children without vaccine granulomas who were tested.²² With this evidence in mind, it has been recommended by contact dermatitis experts that aluminum be included in pediatric baseline patch test series and also investigated for potential inclusion in baseline series for adults.¹

Differential Diagnosis of Aluminum ACD

The differential diagnosis for subcutaneous nodules following vaccination is broad and includes various forms of panniculitis, sarcoidosis, foreign body reactions, vascular malformations, infections, and malignancies.^23-25 The diagnosis may be obscured in cases with delayed onset. Biopsy is not mandatory to establish the diagnosis; although variable histopathologic findings have been reported, a common feature is histiocytes with abundant granular cytoplasm.²⁶ It may be possible to demonstrate the presence of aluminum particles in tissue using electron microscopy and X-ray microanalysis.

For those patients who present with axillary-vault dermatitis, the differential includes ACD to more common allergens in antiperspirants (eg, fragrance), as well as other axillary dermatoses including inverse psoriasis, erythrasma, Hailey-Hailey disease, and various forms of intertrigo. Dermatitis localized to the axillary rim suggests textile allergy.

Patch Testing to Aluminum

Due to its physicochemical properties, patch testing for aluminum allergy is complicated, and historically there has been a lack of consensus on the ideal test formulation.^1,27,28 At this time, it appears that the most sensitive formulation for patch testing to aluminum is ACH 10% in petrolatum.¹ Some contact dermatitis experts recommend that children younger than 8 years should be tested with ACH 2% in petrolatum to minimize the risk of extreme patch test reactions.^29,30 In some patients sensitized to aluminum, the use of aluminum patch test chambers has been noted to produce false-positive reactions, taking the form of multiple ring-shaped reactions to the chambers themselves or reactions to certain allergens whose chemical properties cause corrosion of the aluminum within the chambers.^31-33 Therefore, when testing for suspected aluminum allergy, plastic chambers should be used; given the higher prevalence of aluminum allergy in children, some clinics routinely use plastic chambers for all pediatric patch testing.³⁴ Importantly, elemental aluminum, including empty aluminum test chambers or aluminum foil, alone is not sufficient for patch testing as it lacks sensitivity.¹ Additionally, nearly 20% of positive tests will be missed if a day 7 reading is not performed, making delayed reading a must in cases with high suspicion for aluminum allergy.²¹

Management of Aluminum Allergy

The development of pruritic subcutaneous nodules is uncomfortable for children and their guardians alike and may be associated with prolonged symptoms that negatively impact quality of life^35,36; nonetheless, expert authorities have determined that the preventive benefits of childhood vaccination far outweigh any risk posed by the presence of aluminum in vaccines.^12,13,37 Because aluminum-free formulations may not be available for all vaccines, it is essential to educate patients and families who may be at risk for developing vaccine hesitancy or avoidance.^35,36,38 Given the hypothesis that epidermal dendritic cells mediate aluminum sensitization, it has been proposed that vaccine administration via deep intramuscular rather than subcutaneous injection may mitigate the risk, but more evidence is needed to support this approach.^39,40 The good news is that the nodules tend to fade with age, with a median time to resolution of 18 to 49 months.¹⁴ In addition, patients may experience loss of sensitization to aluminum over time⁴¹; in one study, 77% of 241 children lost patch test reactivity when retested 5 to 9 years later.⁴² The exact reason for this diminishment of reactivity is not well understood. Adjunctive treatments to relieve symptoms of vaccine granulomas include topical and intralesional corticosteroids and antihistamines.

For patients reacting to aluminum in antiperspirants, there are many aluminum-free formulations on the market as well as recipes for homemade antiperspirants.⁶ On a case-by-case basis, patients may need to avoid aluminum-containing medications, permanent tattoos, and orthopedic or dental implants. To the best of our knowledge, there is no evidence suggesting a need to avoid aluminum in foods and their containers in routine daily life; although some patients report exacerbations of their symptoms associated with food-related aluminum exposures (eg, canned food, dried fruit) and improvement with dietary modification, further investigation is needed to confirm the relevance of these sources of contact.^36,38 For patients who require allergen-specific immunotherapy, aluminum-free allergen extracts are available.⁶

Final Interpretation

Exposure to aluminum is ubiquitous; although relatively uncommon, awareness of the potential for ACD to aluminum is increasingly important, particularly in children. Given the prevalence of aluminum contact allergy, it has been recommended by contact dermatitis experts for inclusion in baseline pediatric patch test series.¹ Although it is a complex issue, the development of ACD in a small proportion of children exposed to aluminum in vaccines does not outweigh the benefit of vaccination for almost all children. When conducting patch testing to aluminum, studies support testing to ACH 10% in petrolatum for adults, and consider reducing the concentration to ACH 2% for children.

Acknowledgment—The authors thank Ian Fritz, MD (South Portland, Maine), for his critical input during preparation of this article.

No time of the year is more exciting than the unveiling of the American Contact Dermatitis Society Allergen of the Year. Sometimes the selected allergen represents a completely novel cause of allergic contact dermatitis (ACD) with an unpronounceable chemical name. Not this time! The 2022 Allergen of the Year is likely to be lurking in your kitchen drawer at this very moment, as this year aluminum was chosen for this most prestigious honor.¹ But do not throw out your aluminum foil just yet—aluminum allergy tends to be confined to specific scenarios. In this article, we highlight the growing recognition of aluminum contact allergy, particularly in the pediatric population, focusing on distinct presentations of aluminum ACD, unique sources of exposure, and nuances of patch testing to this metal.

Aluminum Is All Around Us

As the third most common element in the Earth’s crust, aluminum can be found quite literally everywhere.¹ However, aluminum rarely is found in its pure elemental form; instead, it reacts with other elements around it, most commonly oxygen, to form aluminum-containing compounds. Known for their stability and safety, aluminum and its salts are incorporated in myriad products ranging from electronic equipment to foods and their packaging, medications, cosmetics, orthopedic and dental implants, and even tattoos. Aluminum also is found in the air and water supply and may even be encountered in certain workplaces, such as aircraft and machine industries. As such, contact with aluminum is all but certain in modern life.

The use of aluminum in consumer products is widely accepted as safe by public health agencies in the United States.² Although there has been public concern that aluminum could be linked to development of breast cancer or Alzheimer disease, there is no clear evidence that these conditions are associated with routine aluminum exposure through ingestion or consumer products.^3-5

Aluminum Contact Allergy

In part because of its ubiquity and in part because of the stability of aluminum-containing compounds, it was long thought that aluminum was nonallergenic. Contact allergy to elemental aluminum is rare; on the other hand, aluminum salts (the forms we are likely to encounter in daily life) are now recognized in the field of contact dermatitis as allergens of significance, particularly in the pediatric population.^1,6

First reported as a possible occupational allergen in 1944,⁷ aluminum allergy came to prominence in the 1990s in association with vaccines. Aluminum is included in some vaccines as an adjuvant that bolsters the immune response⁸; the eTable lists currently available aluminum-containing vaccines in the United States; of note, none of the COVID-19 vaccines approved in the United States or Europe contain aluminum.¹¹ Although the use of aluminum in vaccines is considered to be safe by the US Food and Drug Administration and Centers for Disease Control and Prevention,^12,13 a small number of children become sensitized to aluminum through vaccines and may develop persistent pruritic subcutaneous nodules (also known as vaccination granulomas) at the injection site; however, the incidence of this adverse effect was less than 1% in large studies including as many as 76,000 children, suggesting that it is relatively rare.^14,15 Upon patch testing, aluminum allergy has been detected in 77% to 95% of such cases.¹⁴ There is wide variation in the onset of the nodules ranging from weeks to years following vaccination.¹⁵ Due to pruritus, the examination may reveal accompanying excoriations, hyperpigmentation, and sometimes hypertrichosis at the injection site. Aluminum allergy related to vaccination also can manifest with widespread eruptions representing systemic contact dermatitis.¹⁶

Along with vaccines, the second major source of aluminum sensitization is allergen-specific immunotherapies administered by allergists/immunologists, many of which contain aluminum hydroxide.^17,18

On the consumer product front, antiperspirants are the most common source of cutaneous exposure to aluminum. Aluminum complexes react with electrolytes in sweat to form plugs in eccrine ducts, thereby preventing sweat excretion.⁶ Allergic contact dermatitis to these products presents with axillary-vault dermatitis. There also have been reports of ACD to aluminum in sunscreen and toothpaste, with the latter implicated in causing systemic ACD.^19,20

Prevalence of Sensitization to Aluminum

There have been a few large-scale studies evaluating rates of sensitization to aluminum in general patch-test patient populations; additionally, because of the complexities of testing this metal, investigators have utilized differing formulations for patch testing. A recent Swedish study found that 0.9% of 5448 adults and 5.1% of 196 children showed positive reactions to aluminum chloride hexahydrate (ACH) 10% in petrolatum and/or aluminum lactate 12% in petrolatum.²¹ Notably, there was a significant association between aluminum allergy and history of atopy for both adults (P=.0056) and children (P=.046), which remains to be further explored. A systematic review and meta-analysis found comparable rates of aluminum allergy in 0.4% of adults and 5.6% of children without vaccine granulomas who were tested.²² With this evidence in mind, it has been recommended by contact dermatitis experts that aluminum be included in pediatric baseline patch test series and also investigated for potential inclusion in baseline series for adults.¹

Differential Diagnosis of Aluminum ACD

The differential diagnosis for subcutaneous nodules following vaccination is broad and includes various forms of panniculitis, sarcoidosis, foreign body reactions, vascular malformations, infections, and malignancies.^23-25 The diagnosis may be obscured in cases with delayed onset. Biopsy is not mandatory to establish the diagnosis; although variable histopathologic findings have been reported, a common feature is histiocytes with abundant granular cytoplasm.²⁶ It may be possible to demonstrate the presence of aluminum particles in tissue using electron microscopy and X-ray microanalysis.

For those patients who present with axillary-vault dermatitis, the differential includes ACD to more common allergens in antiperspirants (eg, fragrance), as well as other axillary dermatoses including inverse psoriasis, erythrasma, Hailey-Hailey disease, and various forms of intertrigo. Dermatitis localized to the axillary rim suggests textile allergy.

Patch Testing to Aluminum

Due to its physicochemical properties, patch testing for aluminum allergy is complicated, and historically there has been a lack of consensus on the ideal test formulation.^1,27,28 At this time, it appears that the most sensitive formulation for patch testing to aluminum is ACH 10% in petrolatum.¹ Some contact dermatitis experts recommend that children younger than 8 years should be tested with ACH 2% in petrolatum to minimize the risk of extreme patch test reactions.^29,30 In some patients sensitized to aluminum, the use of aluminum patch test chambers has been noted to produce false-positive reactions, taking the form of multiple ring-shaped reactions to the chambers themselves or reactions to certain allergens whose chemical properties cause corrosion of the aluminum within the chambers.^31-33 Therefore, when testing for suspected aluminum allergy, plastic chambers should be used; given the higher prevalence of aluminum allergy in children, some clinics routinely use plastic chambers for all pediatric patch testing.³⁴ Importantly, elemental aluminum, including empty aluminum test chambers or aluminum foil, alone is not sufficient for patch testing as it lacks sensitivity.¹ Additionally, nearly 20% of positive tests will be missed if a day 7 reading is not performed, making delayed reading a must in cases with high suspicion for aluminum allergy.²¹

Management of Aluminum Allergy

The development of pruritic subcutaneous nodules is uncomfortable for children and their guardians alike and may be associated with prolonged symptoms that negatively impact quality of life^35,36; nonetheless, expert authorities have determined that the preventive benefits of childhood vaccination far outweigh any risk posed by the presence of aluminum in vaccines.^12,13,37 Because aluminum-free formulations may not be available for all vaccines, it is essential to educate patients and families who may be at risk for developing vaccine hesitancy or avoidance.^35,36,38 Given the hypothesis that epidermal dendritic cells mediate aluminum sensitization, it has been proposed that vaccine administration via deep intramuscular rather than subcutaneous injection may mitigate the risk, but more evidence is needed to support this approach.^39,40 The good news is that the nodules tend to fade with age, with a median time to resolution of 18 to 49 months.¹⁴ In addition, patients may experience loss of sensitization to aluminum over time⁴¹; in one study, 77% of 241 children lost patch test reactivity when retested 5 to 9 years later.⁴² The exact reason for this diminishment of reactivity is not well understood. Adjunctive treatments to relieve symptoms of vaccine granulomas include topical and intralesional corticosteroids and antihistamines.

For patients reacting to aluminum in antiperspirants, there are many aluminum-free formulations on the market as well as recipes for homemade antiperspirants.⁶ On a case-by-case basis, patients may need to avoid aluminum-containing medications, permanent tattoos, and orthopedic or dental implants. To the best of our knowledge, there is no evidence suggesting a need to avoid aluminum in foods and their containers in routine daily life; although some patients report exacerbations of their symptoms associated with food-related aluminum exposures (eg, canned food, dried fruit) and improvement with dietary modification, further investigation is needed to confirm the relevance of these sources of contact.^36,38 For patients who require allergen-specific immunotherapy, aluminum-free allergen extracts are available.⁶

Final Interpretation

Exposure to aluminum is ubiquitous; although relatively uncommon, awareness of the potential for ACD to aluminum is increasingly important, particularly in children. Given the prevalence of aluminum contact allergy, it has been recommended by contact dermatitis experts for inclusion in baseline pediatric patch test series.¹ Although it is a complex issue, the development of ACD in a small proportion of children exposed to aluminum in vaccines does not outweigh the benefit of vaccination for almost all children. When conducting patch testing to aluminum, studies support testing to ACH 10% in petrolatum for adults, and consider reducing the concentration to ACH 2% for children.

Acknowledgment—The authors thank Ian Fritz, MD (South Portland, Maine), for his critical input during preparation of this article.

No time of the year is more exciting than the unveiling of the American Contact Dermatitis Society Allergen of the Year. Sometimes the selected allergen represents a completely novel cause of allergic contact dermatitis (ACD) with an unpronounceable chemical name. Not this time! The 2022 Allergen of the Year is likely to be lurking in your kitchen drawer at this very moment, as this year aluminum was chosen for this most prestigious honor.¹ But do not throw out your aluminum foil just yet—aluminum allergy tends to be confined to specific scenarios. In this article, we highlight the growing recognition of aluminum contact allergy, particularly in the pediatric population, focusing on distinct presentations of aluminum ACD, unique sources of exposure, and nuances of patch testing to this metal.

Aluminum Is All Around Us

As the third most common element in the Earth’s crust, aluminum can be found quite literally everywhere.¹ However, aluminum rarely is found in its pure elemental form; instead, it reacts with other elements around it, most commonly oxygen, to form aluminum-containing compounds. Known for their stability and safety, aluminum and its salts are incorporated in myriad products ranging from electronic equipment to foods and their packaging, medications, cosmetics, orthopedic and dental implants, and even tattoos. Aluminum also is found in the air and water supply and may even be encountered in certain workplaces, such as aircraft and machine industries. As such, contact with aluminum is all but certain in modern life.

The use of aluminum in consumer products is widely accepted as safe by public health agencies in the United States.² Although there has been public concern that aluminum could be linked to development of breast cancer or Alzheimer disease, there is no clear evidence that these conditions are associated with routine aluminum exposure through ingestion or consumer products.^3-5

Aluminum Contact Allergy

In part because of its ubiquity and in part because of the stability of aluminum-containing compounds, it was long thought that aluminum was nonallergenic. Contact allergy to elemental aluminum is rare; on the other hand, aluminum salts (the forms we are likely to encounter in daily life) are now recognized in the field of contact dermatitis as allergens of significance, particularly in the pediatric population.^1,6

First reported as a possible occupational allergen in 1944,⁷ aluminum allergy came to prominence in the 1990s in association with vaccines. Aluminum is included in some vaccines as an adjuvant that bolsters the immune response⁸; the eTable lists currently available aluminum-containing vaccines in the United States; of note, none of the COVID-19 vaccines approved in the United States or Europe contain aluminum.¹¹ Although the use of aluminum in vaccines is considered to be safe by the US Food and Drug Administration and Centers for Disease Control and Prevention,^12,13 a small number of children become sensitized to aluminum through vaccines and may develop persistent pruritic subcutaneous nodules (also known as vaccination granulomas) at the injection site; however, the incidence of this adverse effect was less than 1% in large studies including as many as 76,000 children, suggesting that it is relatively rare.^14,15 Upon patch testing, aluminum allergy has been detected in 77% to 95% of such cases.¹⁴ There is wide variation in the onset of the nodules ranging from weeks to years following vaccination.¹⁵ Due to pruritus, the examination may reveal accompanying excoriations, hyperpigmentation, and sometimes hypertrichosis at the injection site. Aluminum allergy related to vaccination also can manifest with widespread eruptions representing systemic contact dermatitis.¹⁶

Along with vaccines, the second major source of aluminum sensitization is allergen-specific immunotherapies administered by allergists/immunologists, many of which contain aluminum hydroxide.^17,18

On the consumer product front, antiperspirants are the most common source of cutaneous exposure to aluminum. Aluminum complexes react with electrolytes in sweat to form plugs in eccrine ducts, thereby preventing sweat excretion.⁶ Allergic contact dermatitis to these products presents with axillary-vault dermatitis. There also have been reports of ACD to aluminum in sunscreen and toothpaste, with the latter implicated in causing systemic ACD.^19,20

Prevalence of Sensitization to Aluminum

There have been a few large-scale studies evaluating rates of sensitization to aluminum in general patch-test patient populations; additionally, because of the complexities of testing this metal, investigators have utilized differing formulations for patch testing. A recent Swedish study found that 0.9% of 5448 adults and 5.1% of 196 children showed positive reactions to aluminum chloride hexahydrate (ACH) 10% in petrolatum and/or aluminum lactate 12% in petrolatum.²¹ Notably, there was a significant association between aluminum allergy and history of atopy for both adults (P=.0056) and children (P=.046), which remains to be further explored. A systematic review and meta-analysis found comparable rates of aluminum allergy in 0.4% of adults and 5.6% of children without vaccine granulomas who were tested.²² With this evidence in mind, it has been recommended by contact dermatitis experts that aluminum be included in pediatric baseline patch test series and also investigated for potential inclusion in baseline series for adults.¹

Differential Diagnosis of Aluminum ACD

The differential diagnosis for subcutaneous nodules following vaccination is broad and includes various forms of panniculitis, sarcoidosis, foreign body reactions, vascular malformations, infections, and malignancies.^23-25 The diagnosis may be obscured in cases with delayed onset. Biopsy is not mandatory to establish the diagnosis; although variable histopathologic findings have been reported, a common feature is histiocytes with abundant granular cytoplasm.²⁶ It may be possible to demonstrate the presence of aluminum particles in tissue using electron microscopy and X-ray microanalysis.

For those patients who present with axillary-vault dermatitis, the differential includes ACD to more common allergens in antiperspirants (eg, fragrance), as well as other axillary dermatoses including inverse psoriasis, erythrasma, Hailey-Hailey disease, and various forms of intertrigo. Dermatitis localized to the axillary rim suggests textile allergy.

Patch Testing to Aluminum

Due to its physicochemical properties, patch testing for aluminum allergy is complicated, and historically there has been a lack of consensus on the ideal test formulation.^1,27,28 At this time, it appears that the most sensitive formulation for patch testing to aluminum is ACH 10% in petrolatum.¹ Some contact dermatitis experts recommend that children younger than 8 years should be tested with ACH 2% in petrolatum to minimize the risk of extreme patch test reactions.^29,30 In some patients sensitized to aluminum, the use of aluminum patch test chambers has been noted to produce false-positive reactions, taking the form of multiple ring-shaped reactions to the chambers themselves or reactions to certain allergens whose chemical properties cause corrosion of the aluminum within the chambers.^31-33 Therefore, when testing for suspected aluminum allergy, plastic chambers should be used; given the higher prevalence of aluminum allergy in children, some clinics routinely use plastic chambers for all pediatric patch testing.³⁴ Importantly, elemental aluminum, including empty aluminum test chambers or aluminum foil, alone is not sufficient for patch testing as it lacks sensitivity.¹ Additionally, nearly 20% of positive tests will be missed if a day 7 reading is not performed, making delayed reading a must in cases with high suspicion for aluminum allergy.²¹

Management of Aluminum Allergy

The development of pruritic subcutaneous nodules is uncomfortable for children and their guardians alike and may be associated with prolonged symptoms that negatively impact quality of life^35,36; nonetheless, expert authorities have determined that the preventive benefits of childhood vaccination far outweigh any risk posed by the presence of aluminum in vaccines.^12,13,37 Because aluminum-free formulations may not be available for all vaccines, it is essential to educate patients and families who may be at risk for developing vaccine hesitancy or avoidance.^35,36,38 Given the hypothesis that epidermal dendritic cells mediate aluminum sensitization, it has been proposed that vaccine administration via deep intramuscular rather than subcutaneous injection may mitigate the risk, but more evidence is needed to support this approach.^39,40 The good news is that the nodules tend to fade with age, with a median time to resolution of 18 to 49 months.¹⁴ In addition, patients may experience loss of sensitization to aluminum over time⁴¹; in one study, 77% of 241 children lost patch test reactivity when retested 5 to 9 years later.⁴² The exact reason for this diminishment of reactivity is not well understood. Adjunctive treatments to relieve symptoms of vaccine granulomas include topical and intralesional corticosteroids and antihistamines.

For patients reacting to aluminum in antiperspirants, there are many aluminum-free formulations on the market as well as recipes for homemade antiperspirants.⁶ On a case-by-case basis, patients may need to avoid aluminum-containing medications, permanent tattoos, and orthopedic or dental implants. To the best of our knowledge, there is no evidence suggesting a need to avoid aluminum in foods and their containers in routine daily life; although some patients report exacerbations of their symptoms associated with food-related aluminum exposures (eg, canned food, dried fruit) and improvement with dietary modification, further investigation is needed to confirm the relevance of these sources of contact.^36,38 For patients who require allergen-specific immunotherapy, aluminum-free allergen extracts are available.⁶

Final Interpretation

Exposure to aluminum is ubiquitous; although relatively uncommon, awareness of the potential for ACD to aluminum is increasingly important, particularly in children. Given the prevalence of aluminum contact allergy, it has been recommended by contact dermatitis experts for inclusion in baseline pediatric patch test series.¹ Although it is a complex issue, the development of ACD in a small proportion of children exposed to aluminum in vaccines does not outweigh the benefit of vaccination for almost all children. When conducting patch testing to aluminum, studies support testing to ACH 10% in petrolatum for adults, and consider reducing the concentration to ACH 2% for children.

Acknowledgment—The authors thank Ian Fritz, MD (South Portland, Maine), for his critical input during preparation of this article.