**4. Etiology and pathogenesis**

The cycle of hair follicle includes anagen, catagen, and telogen phases. In a normal scalp, about 90–95% of the hair follicles are in the anagen phase, 5–10% are in the telogen phase, and a loss of 100–150 hairs per day is accepted as normal. Telogen effluvium is an abnormality of hair cycling during which a higher percentage of the scalp hairs are in the telogen phase. In TE, the number and ratio of hair follicles in the telogen phase increase. The physiological daily shedding of about 100–150 telogen club hair from the scalp is a usual nature of the hair cycle. In addition, follicles normally retain telogen hair, until the reentry to the anagen phase. Eventually, a new anagen hair pushes the old telogen hair out. It is unlikely to result in visible alopecia and to alter the trichogram [13]. Temporary alopecia may develop, since the shorter telogon hair is replaced by the long telogen hair. When the new anagen hair grows within 3–6 months, alopecia resolves [7]. In addition, no genetic cause for TE has been proposed to date [2, 9].

According to the underlying etiology, TE can be physiologically and pathologically categorized. Physiological causes include neonatal and physiological TE. However, pathological causes of TE include inflammatory diseases, stress, drugs, endocrine disorders, organ dysfunctions, nutritional causes, exogenous factors, syphilis, and systemic lupus erythematous [2]. The main causes of TE are shown in **Table 1** [2, 10].


**Table 1.** Causes of telogen effluvium [2, 10].

Telogen effluvium is a noninflammatory disease characterized by diffuse loss of telogen hair, caused by any disruption of hair cycle which leads to increased and synchronized telogen shedding [4]. It is the leading cause of diffuse hair loss. However, the actual incidence of the disease remains unknown [2]. It has been suggested to result from an abrupt shift of large numbers of anagen hairs to telogen hairs on the scalp with altered ratio of anagen hair to telogen hair from the normal ratio of 90:10–70:30 [1]. The degree of telogen effluvium depends on

Although the effect of age also remains unclear; elderly women are reported to be more vulnerable to acute TE following high fever, surgical trauma, severe hemorrhage, or immense psychological stress. In children, TE is responsible for only a minority of cases with hair loss [6, 7]. Telogen effluvium can be classified into two groups, according to the duration of disease, as acute and chronic. The duration of disease is shorter than 6 months in acute TE, while it takes more than 6 months in chronic state. In acute TE, hair loss typically begins 2–3 months later than the triggering event, although the triggering factor is unknown in 33% of the cases. However, chronic TE is frequent in healthy females in the fourth to fifth decades [7]. In addition, telogen gravidarum is a variant of TE which follows childbirth. Significant telogen gravidarum can affect a third to a half of all women following childbirth [8]. In postpartum TE, follicles remain in a prolonged anagen phase, rather than cycling into the telogen phase. Once released from anagen, an increased shedding of telogen hair is one of the clinical signs of the disease [9].

The majority of TE is subclinical; however, its actual incidence or prevalence still remains unclear. It has no predilection for particular racial or ethnic groups. Although it affects both sexes, women are more likely to present for the evaluation of acute TE than men [10]. A higher number of women also suffer from chronic TE than men. Chronic TE, which is less common than the acute variant, mostly affects women between the ages of 30 and 60 years [11, 12].

The cycle of hair follicle includes anagen, catagen, and telogen phases. In a normal scalp, about 90–95% of the hair follicles are in the anagen phase, 5–10% are in the telogen phase, and a loss of 100–150 hairs per day is accepted as normal. Telogen effluvium is an abnormality of hair cycling during which a higher percentage of the scalp hairs are in the telogen phase. In TE, the number and ratio of hair follicles in the telogen phase increase. The physiological daily shedding of about 100–150 telogen club hair from the scalp is a usual nature of the hair cycle. In addition, follicles normally retain telogen hair, until the reentry to the anagen phase. Eventually, a new anagen hair pushes the old telogen hair out. It is unlikely to result in visible alopecia and to alter the trichogram [13]. Temporary alopecia may develop, since the shorter telogon hair is replaced by the long telogen hair. When the new anagen hair grows within 3–6 months, alopecia resolves

[7]. In addition, no genetic cause for TE has been proposed to date [2, 9].

the severity and duration of exposure, but not the type of the agent [5].

**3. Epidemiology**

126 Hair and Scalp Disorders

**4. Etiology and pathogenesis**
