Pathogenesis of hair loss

The underlying abnormality in hair loss is the decrease in density of thick, pigmented terminal hair and increase in the density of short, fine, nonpigmented vellus hair (1).

This is marked by several key features

• alteration in hair cycle dynamics
  • the duration of anagen phase (growth phase of a hair follicle and controls hair length) is decreased and the duration of the telogen (resting) phase is increased
  • this results in a reduction in the hairs in the anagen phase to hairs in the telogen phase ratio from around 12:1 to 5:1
  • since the duration of the anagen phase which controls the length of hair is decreased, each hair cycle causes shorter new anagen hair than its predecessor and with time, new anagen hair ultimately fails to reach the skin surface
  • telogen hair is loosely attached to the follicle than anagen hairs which results in increased shedding
  • further more the latency period or the kenogen phase between telogen hair shedding and anagen regrowth is increased resulting in a reduction in the number of hairs present in the scalp (2)
• follicular miniaturization
  • reduction in the diameter of the follicle with each successive hair-growth cycle, thus the large (terminal) follicles become miniaturized (vellus) follicles
  • this makes hair more susceptible to falling out (2)

Reference:

• 1. Stough D et al. Psychological effect, pathophysiology, and management of androgenetic alopecia in men. Mayo Clin Proc. 2005;80(10):1316-22
• 2. Schweiger ES, Boychenko O, Bernstein RM.Update on the pathogenesis, genetics and medical treatment of patterned hair loss. J Drugs Dermatol. 2010;9(11):1412-9