This new series will describe characteristic patterns of allergic contact dermatitis and provide practical schematics to illustrate these patterns by region. The first focus in this series is the scalp. Other articles will focus on eyelids; neck; feet; face; anogenital region; hands; mouth, lips and perioral region; and extremeties.
Contact dermatitis is a common skin condition frequently seen by dermatologists. It has been reported to affect approximately 20% of people in the United States. It is responsible for nearly 95% of all reported occupational skin diseases. Irritant contact dermatitis (ICD) is generally far more frequent than allergic contact dermatitis (ACD). It is helpful to keep in mind that when the term “contact dermatitis” is used, it embodies both irritant and allergic etiologies.
The list of allergens that cause ACD continues to grow. Currently, it is reported that there are more than 3,500 environmental contact allergens.1 Through this short series of articles, it is our goal to provide a regional approach to contact dermatitis with the hope of making this vast subject area more approachable and clinically useful.
Although the scalp is commonly exposed to many articles and products containing known allergens, isolated scalp dermatitis due to contact dermatitis is relatively uncommon. This appears to be primarily a topographical property innate to the scalp. The thicker scalp skin, with abundant pilosebaceous and a relative absence of rhytids or crevices, is the ideal barrier against contact dermatitis. As we will see later in this series, the eyelids are on the other end of the spectrum, with very thin skin and many folds that retain substances, increasing exposure. For these reasons, contact dermatitis is unlikely to be at the top of the differential diagnosis for isolated scalp dermatitis. Even in cases where an aggressive allergen is present, the scalp is often not affected or only minimally affected, despite significant involvement of the face, ears and/or neck.2 It is often more useful to talk about “scalp-applied” irritants and allergens rather than isolated scalp contact dermatitis.
Potential allergens involved in scalp dermatitis have been reviewed. Patients with documented scalp dermatitis who underwent patch testing showed that hair dyes, hair-cleansing products and medicaments combined for nearly 2/3 of the positive patch test reactions.3 Unfortunately, the study was not designed to assess the relevance of these positive patch tests.
Regional consideration of the scalp in contact dermatitis requires the clinician to ask two important questions:
First, “Is there a primary dermatitis involving the scalp?” As with any anatomical region, geometric areas of dermatitis are nearly pathognomonic for contact dermatitis. On the scalp, this may take the form of jewelry, such as nickel hairpins, clasps or other decorative items.4 Curling irons and straighteners may also be a source of allergen exposure. These products most often cause problems in nickel-sensitive patients.4 Bands of dermatitis that span the forehead, encircle the head and/or affect the helices of the ears is suggestive of head accessories with leather or rubber parts, such as in hat bands or hat linings5 (see Figure 1, below). With such distribution, exposure to adhesive tapes used to fix wigs to the scalp should also be considered.6 Table 1 (above) highlights some of the more useful patterns suggestive of scalp contact dermatitis.
Figure 2 (right): Rinse-off pattern due to shampoo, conditioner and other rinse-off products.
Secondly, “Is there a primary dermatitis suggestive of a scalp applied allergen?” Allergic reactions to hair products are not largely restricted to the scalp and often involve the face, eyelids, ears and neck; a high degree of suspicion is critical to the diagnosis. The rinse-off or drip pattern sign is a clinically useful clue to suggest a scalp-applied allergen (see Figure 2, above). This appears as a well-demarcated and relatively linear streaking dermatitis involving the pre-auricular face and lateral neck. In patients with classic rinse-off pattern of dermatitis, personal hair care products should be considered.3 The most important potential allergens in shampoos and conditioners are fragrances, cocamidopropyl betaine (CAPB) and preservatives.7 CAPB is of particular interest and is contained in many shampoos, including those marketed as “no tears” products. Two somewhat unique patterns have been observed with CAPB sensitivity: chronic scalp pruritus and flaking, and a chronic dermatitis with episodic flares.3
Hair dye is a scalp-applied allergen that needs to be considered. In one study, hair dye was the most common cause of scalp dermatitis.3 Paraphenylenediamine (PPD) is a frequently used oxidative colorant. In 2006 and 2007, it was reported that PPD contact allergy had increased significantly in the general population and, in 2006, PPD was named Contact Allergen of the Year by the American Contact Dermatitis Society.8 In PPD-sensitive patients, there is often a robust acute dermatitis involving the face, eyelids and neck with only minimal scalp involvement (see Figure 3, above).
An emerging allergen frequently applied to the scalp is Melaleuca alternifolia, commonly known as tea tree oil. Recent popularity is due, in part, to reports showing efficacy in the treatment of seborrheic dermatitis.9 As with any potential contact allergen, melaleuca sensitization and irritation is increased when exposure to inflamed and damaged skin occurs. Clinicians should consider this allergen in patients with recalcitrant, worsening or flaring seborrheic dermatitis or sebopsoriasis. In this setting, asking the patient about the use of “natural” or over-the-counter remedies may lead to the discovery of melaleuca exposure.
Minoxidil may be the most frequent cause of scalp dermatitis medicamentosa.2 Although irritant contact dermatitis is the most frequent reported outcome of topical use of minoxidil, there are reports of allergic contact dermatitis on the scalp. A pustular eruption of the scalp has also been reported.10,11
Management of suspected contact dermatitis of the scalp should include patch testing. However, an empiric trial of hypoallergenic products can be performed. Table 2 highlights some useful scalp products that are minimally or hypoallergenic.
Ms. Huynh is with the Center for Dermatology Research and the department of dermatology at Wake Forest University School of Medicine in Winston-Salem, NC.
Drs. Sheehan and Zirwas and Mr. Chung are with the Center for Dermatology Research at Wake Forest University School of Medicine.
Dr. Feldman is with the Center for Dermatology Research and the departments of dermatology, pathology and public health sciences at Wake Forest School of Medicine.
Disclosure: The Center for Dermatology Research is supported by an unrestricted educational grant from Galderma Laboratories, L.P. Dr. Feldman has received research, speaking and/or consulting support from Galderma, Abbott Labs, Warner Chilcott, Aventis Pharmaceuticals, 3M, Connetics, Roche, Amgen, Biogen, Stiefel, GlaxoSmithKline and Genentech. Dr. Zirwas receives consulting support from Coria Labs, Taro Pharma and SmartPractice. Dr. Sheehan, Ms. Huynh and Mr. Chung have no conflicts to disclose.