Allergic Contact Dermatitis of the Scalp: A Review of an Underdiagnosed Entity

Summary: Objectives: This study reviews the potential under-diagnosis and misdiagnosis of scalp allergic contact dermatitis (ACD) and explores ways to improve diagnostic accuracy. We hypothesize that it is commonly mistaken for other disorders due to overlapping symptoms and unique clinical presentations. Methods: We conducted an extensive literature review to identify diagnostic challenges, common misdiagnoses, and diagnostic approaches for scalp ACD, focusing on standard versus targeted patch testing techniques. Results: Scalp ACD, often misdiagnosed as seborrheic dermatitis due to similar symptoms, has atypical presentations such as hair thinning, hair loss, and erythematous lesions affecting neighboring regions. Trichoscopy can help distinguish scalp ACD, identifying its patchy distributions of thin white scales, in contrast to seborrheic dermatitis’s even yellow scaling. Standardized patch testing further contributes to diagnostic errors, with a study reporting 83% of patients who tested negative with standardized patch tests were positive when using their personal products. Individualized patch testing is more effective in identifying causative allergens and accurately diagnosing scalp ACD. Conclusions: Several factors contribute to scalp ACD’s misdiagnosis for conditions like seborrheic dermatitis. The significant discrepancy in ACD detection rates between personalized and standardized patch tests emphasizes the importance of using patient-specific products in diagnostic testing. Incorporating scalp ACD more readily into one's differential, employing individualized patch testing with trichoscopy, and accounting for neighboring symptomatic areas are all crucial elements in increasing diagnostic accuracy for scalp ACD. The study's limitations include a reliance on secondary quantitative analysis, prompting the need for future investigation.