Seborrhoeic dermatitis is caused by seborrhea, a pathologic overproduction of sebum, and subsequent infection and inflammation.

Fungal

Seborrhoeic dermatitis may involve an inflammatory reaction to a proliferation of a form of the yeast Malassezia,^[3][4] though this has not been proven.^[5]

The main species found in the scalp is Malassezia globosa, others being Malassezia furfur (formerly known as Pityrosporum ovale) and Malassezia restricta. The yeast produces toxic substances that irritate and inflame the skin. Patients with seborrhoeic dermatitis appear to have a reduced resistance to the yeast.^{[citation needed]} However, the colonization rate of affected skin may be lower than that of unaffected skin.^[6]

Only saturated fatty acids (FAs) have been shown to support Malassezia growth. It has also been shown that while number density of M. globosa and M. restricta do not directly correlate to dandruff presence or severity, removal correlates directly with amelioration of flaking. Furthermore, in dandruff-susceptible individuals pure oleic acid, an unsaturated FA and Malassezia metabolite, induces flaking in the absence of Malassezia by direct effects on the host skin barrier. These findings support the following hypothesis:

Malassezia hydrolyze human sebum releasing a mixture of saturated and unsaturated fatty acids. They take up the required saturated FAs, leaving behind unsaturated FAs. The unsaturated FAs penetrate the stratum corneum and because of their non-uniform structure breach the skin's barrier function. This barrier breach induces an irritation response, leading to dandruff and seborrheic dermatitis.^{[7][unreliable source?]}

Other

Genetic, environmental, hormonal, and immune-system factors have been shown to be involved in the manifestation of seborrhoeic dermatitis.^[8][9]

Seborrhoeic dermatitis may be aggravated by illness, psychological stress, fatigue, sleep deprivation, change of season and reduced general health.^[10]

In children, excessive vitamin A intake can cause seborrhoeic dermatitis.^[11] Lack of biotin,^[10] pyridoxine (vitamin B6)^[10][12] and riboflavin (vitamin B₂)^[10] may also be a cause.

Those with immunodeficiency (especially infection with HIV) and with neurological disorders such as Parkinson's disease (for which the condition is an autonomic sign) and stroke are particularly prone to it.

 

Seborrhoeic dermatitis' symptoms appear gradually and usually the first signs are flaky skin and scalp.^[14] Symptoms occur most commonly anywhere on the skin of the face, behind the ears and in areas where the skin folds. Flakes may be yellow, white or grayish.^[15] Redness and flaking may also occur on the skin near the eyelashes, on the forehead, around the sides of the nose, and the chest and upper back.

In more severe cases, yellowish to reddish scaly pimples appear along the hairline, behind the ears, in the ear canal, on the eyebrows, on the bridge of the nose, around the nose, on the chest, and on the upper back.^[16]

Commonly, patients experience mild redness, scaly skin lesions and in some cases hair loss.^[17] Other symptoms include patchy scaling or thick crusts on the scalp, red, greasy skin covered with flaky white or yellow scales, itching, soreness and yellow or white scales that may attach to the hair shaft.^[18]

Seborrheic dermatitis can occur in infants younger than three months and it causes a thick, oily, yellowish crust around the hairline and on the scalp. Itching is not common among infants. Frequently, a stubborn diaper rash accompanies the scalp rash.^[16] Usually, when it occurs in infants the condition resolves itself within days and with no treatment.

In adults, symptoms of seborrheic dermatitis may last from few weeks to years. Many patients experience alternating periods of inflammation. The condition is referred to a specialist when self-care has proven unsuccessful.