What is the cause of seborrheic dermatitis?

Seborrheic dermatitis is a chronic inflammatory skin condition characterised by red patches topped withyellowish-white scales. The body regions affected are those that produce an excess of sebum, promoting the proliferation of a fungus responsible for the disease: Malassezia. The scalp is almost always affected; other common sites (in order of frequency) are the face, particularly the eyebrows, the chest, and intertriginous areas. Its histological appearance has strong similarities with psoriasis and constitutes one of the main challenges in differential diagnosis.

Oily or seborrheic skin is the primary risk factor. The symptoms of seborrheic dermatitis are as follows:

• Red scaly patches;

• Skin particles detaching from patches;

• Itching and pain at the site of the lesions.

Itching is typically moderate and primarily focused on the scalp. This issue can be a source of social embarrassment, especially due to scalp flaking, which can cause particular discomfort due to its perceived association with a lack of hygiene.

Seborrheic dermatitis is widely recognised as one of the most common skin conditions, although estimates of its prevalence are limited by the absence of validated criteria for diagnosis or classification of its severity. An infantile form is known and affects 70% of newborns within the first 3 months of life, but generally disappears by the age of one year.

The origin of seborrheic dermatitis is still unknown, but numerous factors have been cited as contributing to the development of this skin condition. Among them, we find:

• Hormones.

  The onset of seborrheic dermatitis is correlated with sebum levels. Apart from infantile seborrheic dermatitis, the disease is rare before puberty, and is more common in adolescence and young adulthood, when the sebaceous glands are at their most active. The disease begins to develop at puberty, but seborrheic dermatitis is more common in men than in women, suggesting an influence of androgens on the pilosebaceous unit.

• A proliferation of Malassezia.

  Patients suffering from seborrheic dermatitis have a higher level of Malassezia, a type of yeast also known as Pityrosporum, compared to healthy individuals. This yeast feeds on sebum. An excessive production of sebum can create an environment conducive to its proliferation. These yeasts are capable of converting sebum into fatty acids. When an excessive amount of these fatty acids is produced, the immune system may perceive this as a threat and react by triggering an inflammatory response that can result in redness, itching, and scalp irritation. Thus, a sebum-rich environment like the scalp promotes the excessive proliferation of this yeast, leading to the onset of seborrheic dermatitis.

• Existing diseases.

  Seborrheic dermatitis is more prevalent in patients suffering from Parkinson's disease, mood disorders, or those affected by HIV. Other observations have also shown that seborrheic dermatitis is associated with alcoholic pancreatitis, the Hepatitis C virus, and various cancers. However, there are not yet clear explanations to justify this link between the two diseases, and further studies are necessary to provide explanations for these observations.

• Other factors.

  Other factors have been suggested such as immunological, environmental or even lifestyle factors that could increase the predisposition to seborrheic dermatitis. However, it remains difficult to explain the link between these factors and the onset of this disease.

GUPTA A. K. & al. Seborrheic dermatitis. Journal of the European Academy of Dermatology and Venereology (2004).

NALDI L. & al. Seborrheic Dermatitis. The New England Journal of Medicine (2009).