ANDROGENIC ALOPECIA
Medicine has long recognized androgenetic alopecia as an inherited systemic disease associated with sexual development. Ancient Greek doctors realized that male pattern baldness can develop in men of any age after puberty. They recorded that young boys castrated before puberty did not develop androgenetic alopecia regardless of their genetic family history. However, boys castrated during or after puberty could develop androgenetic alopecia.
We now know that the reason for their observations is castration prepuberty stops hair follicles from being exposed to androgens made by the gonads during adolescence. Castration after puberty is too late. Once hair follicles have been exposed to androgens they are fated to become androgen sensitive and androgenetic alopecia can develop.
 Despite its standard name of “male pattern baldness”androgenetic alopecia is also the most common form of hair loss in women. Androgenetic alopecia develops as a gradual reduction of scalp hair follicle size, and reduced time in the anagen active growth phase, leading to more hair follicles in the telogen resting stage of the hair cycle. In men, the hair loss is limited to the top of the head and can involve thinning and/or receding hair lines.
The diagram below is a redrawing of Hamilton's original. Norwood's basic scale is described and illustrated below. Norwood's variant scale is described elsewhere.
Type I. Minimal or no recession of the hair line.
Type II. Triangular, usually symmetrical, areas of recession at the frontotemporal hair line.
Type III. This represents the minimal extent of hair loss sufficient to be considered as baldness according to Norwood. Most type III scalps have deep symmetrical recession at the temples that are bare or only sparsely covered by hair.
Type III vertex. In this presentation, the hair loss is primarily from the vertex with limited recession of the frontotemporal hair line that does not exceed the degree of recession seen in type III.
Type IV. The frontotemporal recession is more severe than in type III. There is sparse hair or no hair on the vertex. The two areas of hair loss are seperated by a band of moderately dense hair that extends across the top. This band connects with the fully haired fringe on the sides of the scalp.
Type IV is distinguished from type III vertex in which the loss is primarily from the vertex.
Type V. The vertex hair loss region is still seperated from the frontotemporal region but it is less distinct. The band of hair across the crown is narrower and sparser. The vertex and frontotemporal regions of hair loss are bigger. Viewed from above, types V, VI, and VII are all characterized by suviving hair on the sides and back of the scalp forming a distinct horseshoe shape.
Type VI. The bridge of hair that crossed the crown is now gone with only sprase hair remaining. The fronttemporal and vertex regions are now joined together and the extent of hair loss is greater.
Type VII. The most severe form of hair loss presents as extensive loss. A narrow band of of hair in a horseshoe shaps survives on the sides and back of the scalp. This hair is usually not dense and may be quite fine. The hair is alos sparse on the nape of the neck and in a semi circle over both ears.
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