Scalp pain and discomfort are common complaints, often stemming from underlying dermatological conditions or imbalances in the scalp's microbial environment. Understanding these causes is crucial for accurate diagnosis and effective management.

One significant contributor to scalp pain is Allergic Contact Dermatitis (ACD) of the scalp, an eczematous condition resulting from exposure to allergens and a subsequent delayed type IV hypersensitivity reaction. ACD of the scalp is frequently overlooked and often misdiagnosed as other conditions, such as seborrheic dermatitis, due to overlapping symptoms. Unlike typical ACD presentations elsewhere on the body, scalp ACD may manifest with atypical symptoms like hair thinning, hair loss, burning sensations, and unrelenting pruritus. Erythematous lesions, while less common directly on the scalp, may appear on neighboring areas where scalp products run off, such as the periauricular areas, hairline, face, and neck, serving as strong indicators of scalp ACD.

Several types of allergens commonly cause scalp ACD:

Hair Dyes: Paraphenylenediamine (PPD), an ingredient frequently found in black, brown, and some blonde hair dyes, is a leading cause of ACD involving the scalp, face, and ears. The increasing use of hair dyes, particularly by women, contributes to the rise in scalp ACD.

Metals: Nickel, found in hair clasps, hairpins, and brushes, and Cobalt, often in light hair dyes, detergents, soaps, and antiperspirants, are significant allergens. Women are particularly susceptible to sensitization to these metals.

Preservatives and Fragrances: These are common culprits in personal care products. Methylisothiazolinone, for instance, is a widespread preservative that can cause contact dermatitis. Formaldehyde and formaldehyde-releasing preservatives (FRPs), used to prevent microbial growth and extend shelf life in products like hair straighteners, can also cause allergic contact dermatitis, with estimated allergy rates of 8% in the U.S.. Formaldehyde can be absorbed through the skin or inhaled during product use. Fragrances such as Balsam of Peru, Fragrance mix I, and Fragrance mix II are also frequently identified scalp allergens.

Rubber-based Items: Rubber and rubber accelerators (e.g., carba mix, black rubber mix, mercapto mix, thiuram mix) found in items like headphones, masks, hairbrushes, and hair glues can also trigger scalp ACD.

Topical Medications: Commonly used topical drugs, such as minoxidil solutions, can cause local scalp itching, increased dandruff, or dermatitis, with minoxidil itself and propylene glycol being frequent allergens.

The scalp's unique anatomy, including its greater skin thickness, generally increases resistance to allergen penetration. However, certain demographics, especially women (due to increased exposure to allergenic hair products), and older patients, are more susceptible. Accurate diagnosis of scalp ACD involves a thorough history of potential allergen exposure, trichoscopic examination to differentiate it from other conditions, and individualized patch testing using the patient's own products.

Another prevalent cause of scalp discomfort and itching is dandruff, or pityriasis capitis, which is considered a mild form of seborrheic dermatitis. Seborrheic dermatitis is a common disorder associated with scalp pruritus and presents as erythematous patches or plaques with scales. The pathogenesis of seborrheic dermatitis and dandruff involves complex interactions between the scalp skin, sebaceous secretions (sebum), Malassezia fungi, and the immune system.

Key microbiological insights into dandruff include:

Fungal Dysbiosis: The scalp normally harbors Malassezia species, primarily Malassezia restricta and Malassezia globosa. In dandruff, there's often a higher fungal metabolic diversity and an increased abundance of uncultured Malassezia species. While the abundance of M. restricta might not differ significantly, the ratio of M. restricta to M. globosa is notably higher in dandruff scalp. Malassezia species possess lipase activity, which hydrolyzes sebum triglycerides into free fatty acids, potentially causing scalp irritation and inflammation. Fungal pathways related to cell-host adhesion (N-glycan biosynthesis) are enriched in dandruff scalp, facilitating fungal growth.

Bacterial Imbalance: Prominent bacterial genera on the scalp include Propionibacterium and Staphylococcus. A healthy scalp is associated with Propionibacterium acnes, while dandruff scalp shows a significantly higher abundance of Staphylococcus epidermidis. A lower ratio of P. acnes to S. epidermidis is linked to dandruff.

Symptoms: Dandruff is associated with higher Malassezia and bacterial load counts, and a direct correlation exists between Malassezia spp. load and dandruff score, as well as higher pruritus and transepidermal water loss (TEWL).

Regular hair washing is often recommended for long-term management of dandruff and scalp itchiness, especially for individuals with afro-textured hair who may experience scalp discomfort due to irregular washing and frequent use of oil-based products.