Hair Disorders in Systemic Disease

Follicular Biology

In order to understand the behavior of the hair follicle in the setting of systemic disease, it is first important to appreciate normal follicular biology. At the time of birth, there are approximately 150,000 hair follicles on the scalp. Each undergoes a continuous cyclic pattern of changes (the follicular lifecycle) during the person’s lifetime. The first two cycles of hair growth occur synchronously in utero and then shortly after birth. After that, each follicle seems to have a growth cycle independent of those around it.

The duration of hair growth cycles is dependent on varying factors, including location of the hair, the individual’s age, nutritional habits, hormonal factors, and health status.

In healthy individuals, scalp hair grows approximately 0.35 mm/day (1 cm per month). It should be noted, however, that data suggest that African textured hair grows more slowly, at approximately 0.77 cm per month. Growth rates are also influenced by the type and anatomic location of hairs. Vellus hairs are widely distributed throughout the body, rooted in the upper dermis. They are nonpigmented, lacking melanin and a medulla, and are finely textured, measuring 0.03 mm or less in diameter and less than 1 cm in length. Growth rates for vellus hairs range from 0.03 to 0.13 mm/day. Hormonal influences at puberty induce vellus hairs to be replaced by terminal hairs in hormone-sensitive areas such as the axillae and pubis. Terminal hairs are medullated and contain melanin and thus are pigmented. They are anchored more deeply in the dermis.

Each hair follicle passes through three cycles: anagen (growth), catagen (involution), and telogen (rest) ( Fig. 43-1 ). A latency or exogen phase is noted in up to 80% of hair cycles. In exogen, the hair shaft is shed without regrowth, and a new anagen phase is turned on. Hair length is determined by the rate and duration of the anagen phase, which is terminated by an internal biologic “clock” built into every follicle. On average, anagen duration on the scalp varies from 2 to 6 years. The catagen phase marks the termination of the growing phase.

Catagen, a transitional phase of acute follicular regression, is irreversible and of relatively short duration, lasting only 2 to 3 weeks. The completion of catagen is marked by formation of the keratinized and depigmented club hair. Only 1% to 2% of scalp hairs are in catagen at any one time.

Telogen, the resting phase, begins when catagen is complete. With an average duration of 3 months, approximately 15% of scalp hairs are in telogen at any given time. Telogen hairs account for the 50 to 100 scalp hairs shed normally each day. The percentage of hairs in the telogen phase, called the telogen count, varies between individuals and even between parts of the scalp. The telogen hair (club hair) is shed during the exogen phase, which may or may not coincide with the new anagen phase. The next anagen phase begins anew from the reservoir of follicular stem cells residing in the bulge area, near where the arrector pili muscle inserts into the hair follicle. These stem cells proliferate rapidly downwards to form a new anagen hair.

Hair Disorders in Systemic Disease

The well-oiled machinery that drives the hair cycle can easily be disrupted by systemic disease. In many cases, hair loss (alopecia) is the result, although excess hair growth may also occur.

Alopecia

Alopecia has traditionally been subdivided into three main categories: scarring (cicatricial), nonscarring (noncicatricial), and hair shaft disorders. In addition, Solomon proposed the term “biphasic alopecia” to describe forms of alopecia that are nonscarring early in their course, yet result in scarring later in their course. Examples of biphasic alopecia include patterned hair loss and traction alopecia. Another proposed classification scheme divides hair loss into permanent and nonpermanent alopecia. For the purposes of our discussion, we will continue to use the terms scarring and nonscarring, as they are widely accepted and firmly entrenched in the literature.

Hair loss in the setting of systemic disease may occur through one of five mechanisms ( Table 43-1 ): telogen effluvium, anagen arrest, and hair miniaturization (all forms of nonscarring alopecia); scarring alopecia; and hair shaft disorders. An important concept in the evaluation and diagnosis of hair disorders is that more than one mechanism may be operating at a given time. This can amplify the degree of hair loss, modify its pattern, and complicate the clinical picture.