What is hair loss?

What is the definition of alopecia? Hair loss, or alopecia, is a frequent reason for consultation in dermatology. However, different types of hair loss (acute or chronic telogen effluvium, androgenetic alopecia or other types of hair loss) are due to different scalp problems at the hair root. So, they must be managed in an appropriate manner.



Hair loss and the hair life cycle are closely linked

The life of the hair follicle is cyclical in nature: hair cycle(or hair growth cycle). It is characterized by a succession of three well-defined phases during which the hair grows, regresses and falls out:

  • The anagen phase: the hair growth phase. During this phase, which lasts from 2 to 6 years, the cells of the hair root, or hair follicle, are actively multiplying. During this time, hair grows about one centimeter per month.
  • The catagen phase lasts 2 to 3 weeks. This is a short transition phase between the growth and shedding phases. The hair root is inactive and hair stops growing.
  • The telogen phase corresponds to the phase of hair loss or shedding of “dead” hair. This stage is not immediate, it takes about 2 to 3 months.

When hair falls out, a new cycle starts again with the growth of new hair in the anagen phase.
Over the course of a lifetime, a hair follicle can go through an average of 25 to 30 cycles, producing 25 to 30 hairs, each living for 2 to 6 years. The scalp has approximately 150,000 hair follicles, all at different stages of the cycle from neighboring follicles. That's why it's normal to lose 50 to 100 strands of hair per day. The number of strands of hair that grow is always much higher than the number that fall out.

What is alopecia?

When we talk about hair loss, we often hear the word "alopecia". But what is alopecia exactly? Is it different from "hair loss"?
The definition of alopecia is: a temporary, partial or total, loss of hair. So, alopecia and hair loss are synonyms.
The word "baldness" does not describe hair loss or alopecia, but its consequence in some people, i.e., the total or partial absence of hair on the scalp.

Reactional hair loss, chronic hair loss, androgenetic alopecia: different mechanisms

Hair loss can be caused by a number of things from one individual to another. There are three main families of hair loss: Reactional hair loss(acute telogen effluvium), chronic hair loss(chronic telogen effluvium, androgenetic alopecia), and anagen effluvium.

Acute telogen effluvium

Acute telogen effluvium, or occasional hair loss, is the most common form of hair loss in women. It is characterized by a sudden and sometimes very significant increase in hair loss 3 to 4 months after a triggering factor. This is why it is also called reactional hair loss. Seasonal hair loss is, for example, a form of acute telogen effluvium.
The hair cycle is disturbed, leading to abrupt and simultaneous hair loss. The proportion of hair in the anagen phase decreases to 70% (normally it is at 85%) while that in the telogen phase increases to 30% (instead of the usual 10%). Hair loss can reach 300 strands of hair per day instead of the 50 to 100 on a healthy scalp. This is diffuse alopecia that affects the entire scalp.
Stress, poorly balanced diet, childbirth, surgery, change of season: in these cases, telogen effluvium is considered acute, i.e., one-time. Once the cause of the hair loss has been identified and removed, it takes about 6 months for the hair to grow back.

Chronic telogen effluvium

When diffuse hair loss lasts for more than 6 months, it is called chronic telogen effluvium. With this form of hair loss, again, women are more affected than men. The factors that cause this form of alopecia are most often related to chronic fatigue or stress, an untreated hormonal imbalance, medication or a low-calorie diet over an extended period of time. If left untreated, it can lead to a progressive decrease in hair mass. Women often notice it when they do their hair: having to do an extra turn of the hair tie to put their hair up or needing to use a smaller barrette or clip than before.
Once the factor responsible for the hair loss has been identified and treated, hair regrowth and a return to the usual state of hair growth occurs in approximately 9 to 12 months. It may take longer for some people.

Anagen effluvium

Anagen effluvium corresponds to a sudden loss of hair during the growth phase as the multiplication of cells at the hair root is stopped. This causes the hair to break and fall out without going through the telogen phase. Its onset is therefore quite quick, on the order of a few days to a few weeks. The result is sudden hair loss with often severe, even total, baldness and can affect both men and women.
This type of hair loss is mainly observed following the administration of cancer treatments such as chemotherapy or exposure of the head and/or neck during radiotherapy sessions. Generally, the hair grows back after treatment is stopped.

Androgenetic alopecia

Behind the definition of androgenetic alopecia lies chronic hair loss (longer than 6 months). This type of hair loss in men is marked by a progressive decrease in hair density and finally, baldness.
As its name suggests, the mode of onset of androgenetic alopecia is both hormonal and genetic. It involves the male hormones, androgens, and more specifically testosterone. In people with androgenetic alopecia, testosterone overstimulates the hair life cycle and shortens it. This leads to the exhaustion of the hair follicle, which can no longer produce hair, and to the progressive miniaturization of the hair: hair becomes thinner and thinner until it stops growing and disappears.
This mechanism has the particularity of occurring only in certain very characteristic areas of the male scalp. In this case, we see localized alopecia on the top of the head.
This type of hair loss affects men more frequently and its frequency increases with age: about 15% of men at age 20, 30% at age 30 and one in two at age 50*. It can also affect women, albeit with more diffuse, less severe hair loss. The final stage of baldness is rarer in women.
*Severi G, et al. Androgenetic alopecia in men aged 40-69 years: prevalence and risk factors. Br J Dermatol. 2003;149(6):1207-1213.

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