Anagen effluvium (chemotherapy hair loss)

Most scalp follicles are in anagen, the growing phase, at any one time, and the matrix cells at the base of a growing follicle divide faster than almost any other cell in the body. A drug that kills dividing cells therefore hits them hard: the hair shaft narrows where growth stopped, breaks at that point, and the hair falls as a crop rather than gradually. That is anagen effluvium, and the two to three week delay is the time it takes the weakened shaft to reach the surface. It is distinct from telogen effluvium, the slower, partial shedding that follows surgery, severe illness, anaesthesia or weight loss and which many people also have after a cancer diagnosis. Because the stem cells in the bulge of the follicle are usually spared, regrowth begins within weeks of the last dose in most people. Where those stem cells are damaged, by high cumulative taxane exposure or by transplant conditioning, regrowth is incomplete and the result is persistent chemotherapy-induced alopecia. Scalp cooling works on this mechanism by narrowing scalp vessels while drug levels in the blood are highest.