Androgenetic alopecia (AGA), also known as male pattern hair loss, is one of the most common conditions affecting men. In the United States, 35 million to 40 million men are affected by AGA. In some men AGA progresses to baldness over most of the scalp. Degrees of hair loss range from this most severe form of AGA to the least noticeable loss of hair in the front temporal area above the eyebrows. Loss of hair in the front temporal area is usually the first place where hair is lost in male AGA; in some men the loss stops there and never progresses while in other men hair loss continues into other areas of the scalp. Progression of hair loss is rapid in some men, slower in others. A physician hair restoration specialist can often predict the final appearance of hair loss based upon the rapidity of onset and progression.
Male AGA occurs in an array of patterns illustrated in the Norwood-Hamilton Scale. The Norwood-Hamilton Scale is used by physician hair restoration specialists in assessing hair loss and in planning hair loss treatment
The Norwood-Hamilton Scale illustrates a feature of AGA that makes hair transplantation possible: No matter how severe the hair loss, hair is never lost at the back or sides of the head or on the nape of the neck. These regions are under different genetic control from the gene(s) that affect hair follicles at the front and top of the head. This"preserved" hair at the back and sides of the head is a reservoir of healthy follicles that can be harvested and transplanted to scalp areas where hair has been lost.