Androgenetic alopecia (AGA) is the most common cause of hair loss in men, presenting as loss of hair over the top (vertex) of the scalp. Although the term androgenetic alopecia denotes that both a genetic predisposition and the presence of androgens are necessary to cause expression, the specific mode of inheritance is unknown.
Male androgenetic alopecia induces an early hair loss phenomenon of varying incidence and extent, beginning any time after puberty. In this period of life, serum androgen levels in men are naturally higher than the low level seen in normal young boys. The progression of hair loss of male androgenetic alopecia occurs in an orderly pattern. The transition from large thick-pigmented terminal hairs to thinner, shorter, indeterminate hairs and finally to non-pigmented vellus (fine) hairs in the involved areas is gradual.
A thorough history and examination of the scalp by the clinician reveals the extent and trend of the hair loss process.
- The first change in men with androgenetic alopecia is usually a bitemporal recession, which is seen in 96 percent of sexually mature Caucasian males, including those men not destined to progress to further hair loss. There is a gradual thinning in the temporal areas followed by progressive thinning in the frontal and vertex areas of the scalp.
- Recession of the frontal hairline is common.
- Ultimately, the frontal and vertex thinning areas may merge, and all the hair over the crown is lost.
Hair loss tends to progress over the years, although the rate can vary dramatically from person to person and the rate of loss can vary significantly over time. The intermediate evolutive stages between minimal thinning and the stage when only a narrow horse-shoe shaped band remains exist due to a decrease in hair density. In a normal individual, hair density varies between 200 and 400 hairs/ cm sq. Alopecia becomes evident when the density is 50-150 terminal hairs/ cm sq.
Nevertheless, there is no way to predict what pattern of hair loss a young man with early androgenetic alopecia will eventually assume. In general, men who begin losing hair in the second decade are those in whom the alopecia will be the most progressive. In some men, initial male pattern hair loss may be delayed until the late third to fourth decade.
Knowledge of the dynamics of hair follicle cycling in normal as well as balding scalp is the key to understanding the clinical presentation of androgenetic alopecia. In normal scalp, the average duration of the anagen or growth phase of hair several years; and that of the telogen (resting) phase is 3 months. However, in men with pattern hair loss, the growth cycle is altered, resulting in a progressive reduction in the duration of the anagen phase. Consequently, there is production of short, thin, hypo-pigmented, insignificant wisps of hair, and the condition is perceived as loss of scalp hair.
Classification of male pattern hair loss
The clinical appearance of male androgenetic alopecia is universally and instantly recognizable in most cases. Several patterns are commonly recognized and classified according to the Hamilton and Norwood classifications. These classifications are based on the degree of hair thinning and the affected areas of the scalp.
Hamilton established a classification based on eight evolutive aspects and three sub-groups, and also made a comparison between the incidence of alopecia in Caucasians and the Chinese. In 1975, Norwood improved on the pictorial classification made by Hamilton and made a more detailed categorization, which is widely used today. He divided androgenetic hair loss in men into two common patterns: the Regular Type, characterized by hair loss that begins in two different areas (at the temples and in the crown) and that gradually merge into one; and the less common Type a, that is characterized by “front-to-back” hair loss. In 1976, Bouhana proposed a simplified classification of 5 evolutive phases with two sub-groups. This classification allowed a more detailed evaluation of the surgical indications of hair transplantations in treating androgenetic alopecia.
The scientific way of assessing the degree of baldness of the patient is to compare the patient’s balding pattern with the standard patterns described by Norwood, which depict the most common configurations of male pattern baldness. There are seven grades of hair loss in the main series and five grades of a variation called the “A” series. As a rule, the pattern of a person’s hair loss follows the specific type first presented. In other words, a person who begins as a Class 2 usually evolves into a Class 3, then a Class 4, etc. A Class 2a becomes a Class 3a and then a Class 4a, and so on.
Demographic and other data on male androgenetic alopecia
There are both racial as well as age-related differences in the incidence or pattern of hair loss in androgenetic alopecia. Studies show that sexually mature Oriental and Native American men are more likely to have preservation of the frontal hairline, later onset of baldness, and less extensive baldness. It has also been observed that African-American men may also have a lower incidence and extent of baldness with decreased frequency of fronto-parietal loss.
The vertex balding seen in men is rare in women. On the other hand, female pattern of balding is not uncommon in men, the frequency being higher in Asian men.