Background
Telogen effluvium is a form of nonscarring alopecia characterized by diffuse hair shedding, often with an acute onset. [1, 2] A chronic form with a more insidious onset and a longer duration also exists, [3] though it has been suggested that this may not be a truly distinct condition. [4] Telogen effluvium is a reactive process caused by a metabolic or hormonal stress or by medications. Generally, recovery is spontaneous and occurs within 6 months, unless a background of pattern alopecia is present. [5]
Pathophysiology
Telogen effluvium can affect hair on all parts of the body, but in general, only loss of scalp hair is symptomatic.
Understanding the pathophysiology of telogen effluvium requires knowledge of the hair growth cycle. All hair has a growth phase, termed anagen, and a resting phase, telogen. On the scalp, anagen lasts approximately 3 years and telogen roughly 3 months, though these times can vary widely from one individual to another. During telogen, the resting hair remains in the follicle until it is pushed out by growth of a new anagen hair.
In most people, 5-15% of the hair on the scalp is in telogen at any given time. Telogen effluvium is triggered when a physiologic stress or hormonal change causes a large number of hairs to enter telogen at one time. Shedding does not occur until the new anagen hairs begin to grow. The emerging hairs help to force the resting hairs out of the follicle. Evidence has suggested that the mechanism by which a telogen hair is shed is an active process that may occur independent of the emerging anagen hair. The interval between the inciting event in telogen effluvium and the onset of shedding corresponds to the length of the telogen phase, 1-6 months (average, 3 mo).
In 1993, Headington described the following five functional subtypes of telogen effluvium, defined according to which portion of the hair cycle is abnormally shortened or lengthened [6] :
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Immediate anagen release
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Delayed anagen release
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Immediate telogen release
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Delayed telogen release
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Short anagen phase
These subtypes represent variations on the principles discussed above. It is rarely possible to distinguish these subtypes clinically.
Etiology
Physiologic stress is the cause of telogen effluvium. These inciting factors can be organized into several categories (see below). Evidence from murine studies has indicated that psychological stress can induce catagen (the transition phase from anagen to telogen), mainly by exerting effects on neurotransmitters and hormones. [7] In humans, however, the role these effects play in hair loss remains to be determined. Although substance P has been extensively studied in human hair follicles in vitro, in-vivo studies have not been performed. [8] In HIV disease, [9] apoptosis may be related to HIV-1 viral protein R. [10] The following have been identified as causes:
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Chronic illnesses, such as malignancy (particularly lymphoproliferative malignancy), as well as any chronic debilitating illness, such as systemic lupus erythematosus, end-stage renal disease, or liver disease
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Hormonal changes, such as pregnancy and delivery (can affect both mother and child), thyroid dysfunction [13] (particularly hypothyroidism; see the image below), and discontinuance of estrogen-containing medications (see the image below)
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Changes in diet, such as crash dieting, anorexia, low protein intake, and chronic iron deficiency [14, 15, 16] ; one study examining the possible role of iron deficiency in female-pattern hair loss found that although iron deficiency was common in women, it was not significantly greater in patients with female-pattern hair loss or chronic telogen effluvium than in control subjects [17]
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Allergic contact dermatitis of the scalp [21]
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Psychological stress [16]
Telogen effluvium secondary to hypothyroidism.
Several studies have reported increased rates of telogen effluvium associated with COVID-19 infection. [22, 23] A multicenter study from South Korea supported this association but did not find a significant association between COVID-19 vaccination and telogen effluvium. [24]
Epidemiology
Telogen effluvium is quite common, but its exact prevalence remains to be defined. A large percentage of adults have experienced an episode of telogen effluvium at some point in their lives.
Telogen effluvium can occur at any age. It is not uncommon for infants in the first months of life to experience an episode of telogen effluvium.
Acute telogen effluvium can occur in either sex if the proper inciting conditions occur. Because hormonal changes in the postpartum period are a common cause of telogen effluvium, women may be more likely to experience this condition. In addition, women tend to find the hair shedding more troublesome than men do and thus are more likely to seek medical attention for it. Chronic telogen effluvium has been reported mainly in women. [25]
No racial predilection has been recognized.
Prognosis
Mortality has not been reported. Morbidity is limited to mild cosmetic changes. However, telogen effluvium can have substantial impact on those affected. [26, 27] In acute telogen effluvium, the prognosis is good for recovery of normal hair density. In chronic telogen effluvium, a good cosmetic outcome can be expected, even if hair shedding continues.
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Telogen effluvium secondary to hypothyroidism.
