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Dandruff
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Dandruff (also called scurf and historically termed Pityriasis capitis) is due to the excessive shedding of dead skin cells from the scalp. As it is normal for skin cells to die and flake off, a small amount of flaking is normal and in fact quite common. Some people, however, either chronically or as a result of certain triggers, experience an unusually large amount of flaking, which can also be accompanied by redness and irritation.
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Encyclopedia
Dandruff (also called scurf and historically termed Pityriasis capitis) is due to the excessive shedding of dead skin cells from the scalp. As it is normal for skin cells to die and flake off, a small amount of flaking is normal and in fact quite common. Some people, however, either chronically or as a result of certain triggers, experience an unusually large amount of flaking, which can also be accompanied by redness and irritation. Most cases of dandruff can be easily treated with specialized shampoos.
Excessive flaking can also be a symptom of seborrhoeic dermatitis, psoriasis, fungal infection or excoriation associated with infestation of head lice.
Those affected by dandruff find that it can cause social or self-esteem problems. Treatment may be important for both physiological and psychological reasons.
Causes
As the epidermal layer continually replaces itself, cells are pushed outward where they eventually die and flake off. In most people, these flakes of skin are too small to be visible. However, certain conditions cause cell turnover to be unusually rapid, especially in the scalp. For people with dandruff, skin cells may mature and be shed in 2 - 7 days, as opposed to around a month in people without dandruff. The result is that dead skin cells are shed in large, oily clumps, which appear as white or grayish patches on the scalp, skin and clothes.
Dandruff has been shown to be the result of three required factors:
- Skin oil commonly referred to as sebum or sebaceous secretions
- The metabolic by-products of skin micro-organisms (most specifically Malassezia yeasts)
- Individual susceptibility
Common older literature cites the fungus Malassezia furfur (previously known as Pityrosporum ovale) as the cause of dandruff. While this fungus is found naturally on the skin surface of both healthy people and those with dandruff, it was discovered that a scalp specific fungus, Malassezia globosa, is the responsible agent. This fungus metabolizes triglycerides present in sebum by the expression of lipase, resulting in a lipid byproduct oleic acid (OA). Penetration by OA of the top layer of the epidermis, the stratum corneum, results in an inflammatory response in susceptible persons which disturbs homeostasis and results in erratic cleavage of stratum corneum cells.
Rarely, dandruff can be a manifestation of an allergic reaction to chemicals in hair gels/sprays, hair oils, or sometimes even dandruff medications like ketoconazole.
There is convincing evidence that food (especially sugar and yeast), excessive perspiration, and climate have significant roles in the pathogenesis of dandruff.
Evolutionary origin
Dandruff is consistent with the aquatic ape hypothesis as the scales which form at the base of the hairs are angled in a way that would decrease water resistance by slicking down the hair. Also, the malassezia yeasts which are associated with dandruff provide secretions which would protect against ultra-violet radiation and so act as a sunscreen.
Seborrheic dermatitis
Flaking is a symptom of seborrheic dermatitis. Joseph Bark notes that "Redness and itching is actually seborrheic dermatitis, and it frequently occurs around the folds of the nose and the eyebrow areas, not just the scalp." Dry, thick, well-defined lesions consisting of large, silvery scales may be traced to the less common psoriasis of the scalp.
Seasonal changes, stress, and immuno-suppression seem to affect seborrheic dermatitis.
Treatment
Shampoos use a combination of ingredients to control dandruff. Salicylic acid (used in Sebulex) and coal-tar (used in Tegrin) removes dead skin cells from the scalp and decrease the rate at which these cells are created. Zinc pyrithione (used in Head and Shoulders) kills pityrospora. Selenium sulfide (used in Selsun Blue) achieves the results of both salicylic acid and zinc pyrithione.
Simply increasing usage with normal shampooing will remove flakes. However, elimination of the fungus results in dramatic improvement. Regular shampooing with an anti-fungal product can reduce recurrence.
Soothing preparations may contain Sodium bicarbonate (baking soda), and coal tar based products.
The most common antifungal agents used are Zinc pyrithione, Selenium sulfide and Ketoconazole Other products used include Tea tree oil and Piroctone olamine (Octopirox).
Anti-fungal/anti-dandruff shampoos (includes Head and Shoulders, and Selsun Blue) containing ketoconazole have been shown to be more effective than zinc pyrithione. Although a 1981 study reported selenium sulfide as being the most effective of the tested shampoos at treating dandruff, a 1999 comparative study concluded that ketoconazole was the most effective antifungal agent. (Although ketoconazole had been approved by F.D.A. in 1981, it was not approved for topical use in a shampoo until 1990, and was therefore not included in the 1981 study.)
See also
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