INTRODUCTION
Dermoscopy of hair and scalp or trichoscopy is a useful technique that can help clinicians to better detect hair and scalp disorders. With this technique, it is possible to measure and compare the hair shaft thickness in different scalp areas, to evaluate hair shaft abnormalities and to assess perifollicular and interfollicular patterns. Only few studies on normal scalp have been published, focusing mainly on hair shafts, hair follicle openings, and vascular patterns.
Normal hair shafts are uniform in thickness, shape, and color with continuous, interrupted, fragmented, or absent medulla. About 20% of human scalp hairs can be thin and up to 10% are vellus hairs. An increased proportion of thin hairs is characteristic of androgenetic alopecia.
For classification purposes, the hair can be divided into three main groups according to their racial origin: (1) Caucasian, (2) African, and (3) Asian. These groups differ mainly in their appearance, geometry, mechanical properties and water content.
NORMAL CAUCASIAN SCALP
On the scalp, there are evenly spaced groups of few hair shafts coming out of the same follicular ostium. In healthy individuals, two to three hairs emerge from one follicular unit (average: 2.6). Less than 5% of all follicular units consist of follicular units with four or more hairs. In the temporal area, up to 40% of the follicular units can have one hair (Figs. 1.1A to D). In the frontal region, single follicular units should be less than 35% and in the occipital area, less than 30%. The number of single follicular units increases with aging and some hair disorders.
There are marked differences in hair shaft structure among different ethnic groups. The shape of the hair shaft is partly formed by the shape of the hair follicle (Figs. 1.2 and 1.3). Caucasian hair is described as having intermediate thickness, between African and Asian hairs. The cross-sectional diameter ranges from 0.04 mm (in blond-haired people) to 0.09 mm (in dark-haired individuals). Some trichoscopic features, such as empty follicular openings and yellow dots are occasionally seen in healthy individuals.
The appearance of cutaneous microvessels may vary in type, arrangement, and number. Normal scalp vessels include thin arborizing vessels, found usually in the occipital and temporal areas, and interfollicular simple red loops, mainly observed in the frontal scalp (Figs. 1.4 and 1.5).
NORMAL AFRICAN SCALP
African descents have a large variability in their hair shaft diameter, ranging from 0.06 mm to 0.1 mm. Shafts are asymmetrical, with an elliptical or oval cross-sectional shape with a curved bulb, giving the African hair a curly and frizzy appearance.2
Figs. 1.1A to D: Normal Caucasian vertex scalp. (A) Normal hair shafts are uniform in thickness, shape, and color. Most follicular units are composed of two to three hairs. (B) Thin hairs may account for up to 20% of the total number of hairs. (C) Note the presence of less than 10% of vellus hairs. These are hairs that are nonmedullated, poorly pigmented, short (less than 3 cm), and fine (less than 0.03 mm)(*). (D) Follicular units with four or more hairs are rare and account for less than 5% of all follicular units.
Fig. 1.3: Normal Caucasian hair shaft. Note the interrupted and fragmented medulla. Interrupted medulla may be differentiated from pili annulati. In normal subjects, the medulla covers less than 50% of the interior of the hair shaft. In pili annulati, it covers 50–100%.
Fig. 1.4: Normal Caucasian vertex scalp. Interfollicular simple red loops. They are best viewed at 50X or higher magnifications with the camera probe angled tangentially.
Fig. 1.5: Normal Caucasian temporal scalp. Thin arborizing vessels are best viewed at 20X or higher magnifications.
Figs. 1.6A and B: Normal African vertex scalp. (A) Honeycomb-pigmented network (→) and pinpoint white dots(*). (B) Most follicular units consist of one or two vellus hairs and between two to four terminal hairs.
Fig. 1.7: Normal African scalp can show a mild erythema due to excessive traction(*). Also note few vellus and short regrowing hairs.
Because the hairs exit the epidermis at an oblique angle to the skin, this population has a higher prevalence of certain diseases, such as pseudofolliculitis barbae and acne keloidalis nuchae. African hairs have less tensile strength, less moisture, and tend to break more easily during combing or manipulation.
On trichoscopic examination, small pinpoint white dots regularly distributed on the scalp which correspond to follicular and eccrine sweat gland duct openings, and honeycomb-pigmented network can be observed (Figs. 1.6A and B). Scalp color varies from light brown to black. Usually vessels cannot be visualized. However, perifollicular and interfollicular erythema can sometimes be seen (Fig. 1.7). Hair density is significantly lower than in Caucasian patients but the hair diameter is larger. Each follicular unit contains one or two vellus hairs and two to four terminal hairs. Hair shafts show a coarser and curlier texture with irregular torsions (Fig. 1.8). The scalp frequently presents scales and residues of products used for hair styling and moisturizing the scalp.4
NORMAL ASIAN SCALP
Asians have round-shaped and straight hair shafts with a relatively large diameter, ranging from 0.08 mm to 0.12 mm and a higher thin-hair ratio (12.9%) (Fig. 1.9). Hair cuticle layers are more numerous, compact, and thicker compared to Caucasian hairs. These differences are important in determining increased resistance to chemical procedures, ultraviolet radiation, and mechanical trauma. Trichoscopic features are similar to those of Caucasians; however, the frequencies of the pigment patterns are different because of the contrast effect of the skin and hair color.
Fig. 1.9: Normal Asian vertex scalp. Hair shafts are uniform in thickness, shape, and color, similar to Caucasian hairs. Few vellus hairs can be present.
SUGGESTED READING
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