Disorders of the hair and nails

Embryology and anatomy

Hair follicles begin to form around 9–12 weeks of gestation on the face. They develop as a down-budding of the epidermis in association with proliferating mesenchyme, which eventually becomes the dermal papilla. The development of hair follicles continues cephalocaudally on the remainder of the body around 16–20 weeks of gestation. No new hair follicles develop after birth.

In the premature infant, the scalp, forehead, and trunk are covered by variable amounts of fine, soft, long, lightly pigmented lanugo hair ( Fig. 8.1 ). Lanugo is usually shed in utero at 7–8 months of gestation. The second covering of subtle lanugo is shed shortly after birth and replaced by short, fine, non-pigmented vellus hair. Terminal hair is thicker and more darkly pigmented and usually grows on the scalp, eyebrows, and eyelashes before puberty and the sites of sexual hair after puberty.

At term, most infants have full scalp coverage with normal terminal hair. However, shortly before or up to 4 months after birth, infants undergo a period of brisk physiologic hair shedding when actively growing hairs convert to the resting phase and are then synchronously shed ( Fig. 8.2 ). In blonds and redheads, the process is often complete before birth, and the hair is sparse at delivery. Shedding may be delayed in dark-haired individuals and may occur rapidly during early infancy. Parents should be reassured that this is a normal physiologic process.

Throughout life, each hair follicle undergoes normal hair cycling, consisting of three phases ( Fig. 8.3 ): anagen (growth phase), catagen (regression phase), and telogen (resting phase). The growth phase, anagen, typically lasts approximately 2–6 years. The length of anagen is genetically determined, varies by anatomic location, and dictates the final length of hair. After anagen, catagen (regression phase) occurs and typically lasts 2–3 weeks. During this phase, the lower two-thirds of the hair follicle involutes. Finally, the resting phase, telogen, occurs and typically lasts 3 months. During the telogen phase, the proximal end of the hair shaft forms a characteristic club shape as is seen in telogen effluvium ( Fig. 8.4 ). After the telogen phase, the hair shaft is shed and anagen occurs again. It is normal for a typical adult to shed approximately 100 scalp hairs per day.

In the newborn, hair growth is synchronous, with all scalp hairs existing in the same phase of the hair cycle described earlier. Conversely, in adults, hair growth is asynchronous, with individual scalp hairs existing in different phases of the hair cycle. At any given time, approximately 85–90% of adult scalp hairs exist in anagen, 10–15% in telogen, and less than 1% in catagen. The adult pattern of asynchronous hair growth generally becomes established anywhere between 4 months and 2 years of life. The normal ratio of hairs existing in anagen and telogen may be altered in certain hair disorders. For example, patients on chemotherapy may lose their hair in a process called anagen effluvium, where dividing cells in the anagen hair bulb arrest and hair is shed within 1–2 weeks. Such patients may show a greatly increased ratio of telogen hairs on their scalp. An increased ratio of telogen hairs on the scalp also occurs in alopecia areata and telogen effluvium.

Congenital and hereditary disorders

The normal pattern of hair growth and shaft morphology may be disturbed in a number of hereditary disorders and congenital syndromes ( Table 8.1 ).

Localized patches of alopecia may be associated with epidermal and connective tissue nevi, pigmented nevi, vascular tumors, and hamartomas, interrupting normal hair growth ( Fig. 8.5 a–e). Perinatal events such as vascular compromise produced by trauma to the scalp during passage through the birth canal can lead to halo scalp ring, an annular patch of non-scarring alopecia, with hair regrowth in the following 6 months. Scarring has been reported but is uncommon.

Triangular alopecia is a distinctive, non-scarring, congenital hair anomaly that may not become evident until later in childhood ( Fig. 8.5 f). This condition presents with a stable patch of triangular, round, or rectangular hair loss with the base of the lesion characteristically at the junctions of the frontal and temporal scalp. Lesions are usually unilateral but occasionally bilateral. They could be completely devoid of hair or contain vellus hairs and diminished terminal hairs.

Aplasia cutis congenita presents on the scalp or elsewhere on the skin with an ulcer or depressed scar. Peripheral hypertrichosis surrounding the lesion (hair collar sign) could signify underlying neural tube defect ( Fig. 8.6 ). Incontinentia pigmenti is an X-linked disease of females (embryonic lethal in males) with four stages: vesicular, verrucous, hyperpigmentation, and hypopigmentation/atrophy. The fourth stage can manifest as atrophic Blaschkoid plaques with alopecia.

Generalized sparse or abnormal hair growth suggests an inherited hair shaft anomaly or genodermatosis ( Figs. 8.7 and 8.8 ). Monilethrix is a relatively common developmental hair defect that presents in infancy and sometimes after puberty. Affected individuals are born with normal hair, which is then replaced by the abnormal beaded hair in the neonatal period. It is characterized by brittle, beaded hair with nodes separated at relatively uniform intervals ( Fig. 8.9 ). The condition is inherited in autosomal-dominant or recessive patterns. It is mainly seen in the occipital scalp but can affect the entire scalp and other body areas. The condition persists throughout life but may improve in adolescence or adulthood.

Pili torti is another structural defect in which the hair shaft is twisted on its axis ( Fig. 8.10 ). Pili torti may be localized or generalized, and classic presentation is brittle hair in the first year of life in a blond-haired child, although a rarer variant presents around puberty in individuals with black hair. It can occur as an isolated hair defect or in association with a multisystem disorder such as Menkes kinky hair syndrome (an inherited disease of copper metabolism that also affects the central nervous, cardiovascular, and skeletal systems).

Trichorrhexis invaginata (bamboo hair) manifests as brittle, sparse hair in early infancy, which may improve with age. The hair shaft demonstrates a ball-in-socket morphology where the distal end intussuscepts in the proximal portion. It is seen in Netherton syndrome, an autosomal-recessive dermatosis consisting of severe atopic dermatitis, ichthyosis linearis circumflexa (serpiginous erythematous annular plaques with double-edge scale), and trichorrhexis invaginata ( Fig. 8.11 ).

In the ectodermal dysplasia syndromes, a heterogeneous group of genodermatoses, sparse hair is associated with dysmorphic facies and abnormalities of other structures, which include nails, sweat glands, and teeth ( Fig. 8.12 ). In the hypohidrotic variants, early diagnosis may be life-saving by preventing fatal hyperthermia, which may develop during otherwise self-limited childhood infections.

Trichothiodystrophy is a hair shaft defect of autosomal-recessive inheritance characterized by deficiency in sulfur-containing amino acids, which manifests in increased hair fragility, splitting of the hair, and alternating light and dark transverse bands on polarizing microscopy ( Fig. 8.13 ). Clinically, the hair is sparse, dull, and brittle. It presents either with isolated hair findings or with a constellation of findings summarized in the acronym PIBIDS: photosensitivity, intellectual impairment, brittle hair, ichthyosis, decreased fertility, and short stature.

Children with congenital hair disorders require a careful medical and neurodevelopmental evaluation. Family history aids in establishing a pattern of inheritance. Hairs are examined microscopically to detect specific anomalies.

Acquired alopecia

Acquired alopecias may be differentiated by the presence or absence of clinical scarring. In non-scarring processes, inflammation may be clinically evident, subtle, or absent. Non-scarring disorders may be further subdivided by the presence of localized or diffuse involvement. Importantly, chronic non-scarring alopecia can ultimately result in scarring.

Scarring (cicatricial) alopecia

Cicatricial alopecia is a broad umbrella encompassing different diseases. It presents with scarring patches of hair loss with absence of follicular ostia. A number of inflammatory disorders may involve the hair follicle primarily or by spread from contiguous skin. In this section we discuss the most common etiologies of scarring alopecia.

Discoid lupus erythematosus (DLE) presents with scaly erythematous plaques in the early phases, then progresses to atrophic, hypopigmented scarred plaques, often with peripheral hyperpigmentation and complete loss of follicular ostia. It most commonly presents as an isolated skin disease but can also be associated with systemic lupus erythematosus. DLE mainly presents on the head and neck and can involve oral mucosa, especially the hard palate, with characteristic shallow erythematous ulcers and white borders. In hair-bearing areas, inflammation around the hair follicle is seen early in the disease and later results in irreversible scarring ( Fig. 8.14 ). Treatment involves topical steroids, topical calcineurin inhibitors, intralesional steroids, and systemic immunomodulators (e.g. hydroxychloroquine, methotrexate, mycophenolate mofetil).

Lichen planopilaris presents with perifollicular erythema and scale, cuffing the hair shaft at its exit from the scalp. Interfollicular erythema can also be observed. This later results in destruction of the hair follicle and scarred patches ( Fig. 8.14 ), which are often described as footprints in the snow. Treatment is largely similar to DLE.

Acne keloidalis is a scarring folliculitis that presents with inflamed papules and pustules at the nape of the neck and occipital scalp in post-pubescent African American men ( Fig. 8.15 ). These later heal with scarring, often resulting in hypertrophic scars and keloids. Treatment includes topical steroids and topical or systemic antibiotics.

Folliculitis decalvans is a severe variant of non-infectious folliculitis that produces slowly but relentlessly enlarging areas of cicatricial alopecia ( Fig. 8.16 ), often affecting the crown. Pili multigemini, multiple hairs arising from a single follicular opening, is often observed (hair tufting or doll’s hair). Some treatments include topical or intralesional steroids, vitamin D derivatives, antibiotics, isotretinoin, and biologics including tumor necrosis factor (TNF)-α inhibitors.

Dissecting cellulitis of the scalp may present with boggy, suppurative nodules; abscesses; and sinus tracts with patches of alopecia ( Fig. 8.17 ). It heals with disfiguring scars and/or keloids, most often involving the crown. It mainly affects young men of African ancestry but can affect children, too. Long-standing disease poses increased risk of squamous cell carcinoma. Treatment options include systemic antibiotics, isotretinoin, intralesional steroids, and biologics.

Physical agents including allergens, irritants (acids and alkalis), thermal burns, and blunt trauma may cause necrosis of the skin and non-specific scarring. Examples include alkali burns from hair-grooming products, surgical scars, and radiation-induced injuries. Scarring alopecia may also result from severe infection of the scalp. In addition, scarring alopecia may be associated with infiltrative disorders such as granulomatous infiltration in sarcoidosis or mucin infiltration in alopecia mucinosa ( Fig. 8.18 ).

Patients with scarring alopecia deserve a careful examination of the skin, nails, and mucous membranes to identify clues for diagnosis. When the cause is not readily apparent, a skin biopsy is considered. A delay in diagnosis and treatment may result in widespread patches of disfiguring, scarring alopecia.

Non-scarring alopecia

Most hair loss in children occurs without scarring. Clinically, hair loss can be localized or diffuse and may or may not be associated with scaling or inflammation (erythema, pustules, etc.).

Tinea capitis

Tinea capitis is the most common cause of hair loss in children between ages 2 and 10 years ( Fig. 8.19 ). It is mainly caused by Trichophyton and Microsporum species and is most common among African American children, but it can also be seen in adults and children of other races.

Tinea capitis has two major patterns: ectothrix, which has dermatophyte infection of the outside and inside of the hair shaft, and endothrix, which involves only the inside. A variety of clinical presentations of tinea capitis occur. Some present with mild redness and scaling in areas of partial alopecia. Others display widespread hair breakage caused by invasion of the hair shaft by the fungus, creating the appearance of black dots on the scalp with short, broken-off hairs poking up on the surface of the scalp. Occasionally, scalp lesions are annular, like patches of tinea corporis. In some children, there is associated edema and pustule formation. As the pustules rupture and the area weeps, thick, matted, yellow crusts form, which simulates impetigo. Less commonly, intense inflammation causes the formation of raised, tender, boggy plaques studded with pustules, known as kerions. Large, occasionally painful, occipital, postauricular, and preauricular adenopathy occurs with inflammatory tinea. Unless treated promptly and aggressively with oral antifungal agents and, in severe cases, oral corticosteroids, kerions may heal with scarring and permanent hair loss. Incision and drainage are not indicated.

Fungal infection of the scalp is readily confirmed by a potassium hydroxide (KOH) preparation of infected hairs. The best hairs for examination are those that are broken off at the surface. A good specimen may be obtained by scraping hairs and scale onto a glass slide with a #15 blade. A toothbrush may be used to brush hairs and scale from large areas of the scalp directly onto fungal medium for culture. Before 1970, most cases of tinea capitis in the United States were caused by Microsporum species, which fluoresce bright blue-green under Wood light. This screening tool is of little value today because the endothrix infection produced by Trichophyton tonsurans does not fluoresce. Favus is an uncommon, severe form of tinea capitis presenting with yellowish crust adherent to the hair shafts and in a honeycomb pattern. It is mainly caused by Trichophyton schoenleinii.

Tinea capitis is treated with systemic antifungals as a result of dermatophyte infection of the hair follicle, which cannot be targeted by topical therapies. Terbinafine has the highest efficacy against T. tonsurans, the most frequent pathogen, and is given for 4–8 weeks. It is also effective against Microsporum canis and other dermatophyte species. Griseofulvin, given for at least 8 weeks, has slightly better efficacy than terbinafine against M. canis; however, given the established safety, efficacy, and shorter treatment duration of terbinafine, the latter remains first-line therapy for M. canis as well. Fluconazole and itraconazole are possible alternatives, while systemic ketoconazole should be avoided given its risk of hepatotoxicity. Only fluconazole is available in a liquid formulation.

Although this usually eradicates infection, the risk of reinfection is high. Recurrent infection prompts a careful examination and possible culture of other family members to identify untreated cases. Concurrent use of antifungal shampoos such as selenium sulfide or ketoconazole may help minimize the risk of recurrence and spread.

Although most children who present with scalp pustules have tinea capitis, bacterial folliculitis should also be considered, particularly if pustules are small, are superficial, are not associated with hair loss, and appear at the base of hairs under tension, providing a breach for which bacteria invades. This disorder, termed impetigo of Bockhart, is easily treated by regular shampooing and loosening of the hair ( Fig. 8.20 ). Occasionally, an oral antibiotic is required. Seborrhea may be difficult to differentiate from tinea. However, seborrhea presents with diffuse patches of fine, flaky, or greasy scale. A child with patches of scalp “seborrhea” warrants a KOH preparation and fungal culture to exclude tinea.

Alopecia areata

Alopecia areata presents with patches of complete hair loss on the scalp, eyebrows, lashes, extremities, and/or trunk ( Fig. 8.21 ). Occasionally, hair loss progresses to involve the entire scalp (alopecia areata totalis) or the whole body (alopecia areata universalis) ( Fig. 8.22 ). The insult that triggers alopecia areata is unknown. Although clinical signs of inflammation are absent, skin biopsies from sites of active disease show perifollicular, lymphocytic inflammation, as well as deposition of antibody and immune complexes.

Clues to diagnosis include the absence of erythema and scaling in involved areas of the scalp and sometimes the presence of short (3–6 mm), easily epilated hairs at the margins of the patch. Under magnification, these hair stubs resemble exclamation points, as the hair shaft is narrower as they exit the scalp than at the distal end. Yellow dots, which represent enlarged sebaceous glands, are often seen in post-pubertal patients. Another finding in many patients with alopecia areata is Scotch-plaid pitting of the nails, which consists of rows of pits that cross in a transverse and longitudinal pattern. Alopecia areata is also a common cause of trachyonychia in children, which is nail dystrophy with sandpaper texture of the nail plates.

The course of alopecia areata is unpredictable. In adolescents and young adults, hair loss usually resolves without permanent alopecia over months to years. In infants and young children, particularly when alopecia is diffuse, the prognosis is more guarded. Atopic dermatitis is associated with earlier onset and worse prognosis of alopecia areata.

Treatment includes safe measures that do not carry the risk of systemic toxicity. Topical and intralesional corticosteroids, local irritants (e.g. tar preparations, dithranol), topical minoxidil, topical sensitizers (e.g. squaric acid dibutyl ester), calcipotriene, and ultraviolet light therapy have been used with some success. Although oral corticosteroids may induce hair regrowth, these medications do not change prognosis. Consequently, their use should be restricted to short courses in selected patients with widespread, rapidly progressive disease. Studies until 2020 have demonstrated promising results with JAK inhibitors (ruxolitinib and tofacitinib), but controlled clinical trials are still lacking.

Traumatic hair loss

A common form of benign hair loss in infants is observed on the occipital scalp because of rubbing in infants who commonly sleep on their back. Parents should be reassured that this is a benign process and hair will grow back.

Traction alopecia is a form of traumatic alopecia common in young girls and women whose hairstyles, such as ponytails, plait, and braids, maintain a tight pull on the hair shafts ( Fig. 8.23 ). This traction causes shaft fractures, as well as follicular damage. If prolonged, permanent scarring can result. When this affects the frontal or temporal scalp, an important clue is the fringe sign, where shorter hairs along the frontotemporal hairline are preserved as they are not included and pulled with the rest of the hair.

Trichotillomania, or hair pulling, is a common disorder seen in toddlers, children of school age, and adolescents resulting from repetitive pulling and/or twisting of hair ( Fig. 8.24 ). It often presents with oddly shaped patches of hair loss, often in broad, linear bands on the vertex or sides of the scalp, where the hair is easily twisted and pulled out. Rarely, the entire scalp, eyebrows, and eyelashes are involved. Hair pulling is most often confused with alopecia areata and can be distinguished based on the bizarre pattern of alopecia and presence of short, broken-off hairs of various lengths. Skin biopsy of the scalp from an area of recent hair pulling may demonstrate large numbers of catagen hairs, perifollicular hemorrhage, and trichomalacia. Shaving the hair in a given area results in normal hair regrowth as the hairs cannot be grabbed and pulled, thus supporting the diagnosis.

Although hair pulling may occur in children with severe obsessive-compulsive disorder or other psychiatric diseases, many cases, particularly in preschool children, are associated with habitual behavior or situational stress ( Fig. 8.25 ). In young, habitual hair twirlers, positive reinforcement or replacement of hair twirling with other socially acceptable behaviors usually succeeds in extinguishing hair pulling. Adolescents and their parents often deny adamantly that hair loss could be caused by the child; thus, the diagnosis rests on a high index of suspicion and recognition of the clinical findings.

Traumatic alopecia may also be caused by breakage of the hair shaft resulting from acquired structural defects of the hair occurring by traumatic hair care practices. The most common defect is acquired trichorrhexis nodosa, which can develop at any age as brittle, short hairs that are easily broken. Microscopically, the distal ends of the hairs are frayed, resembling a broomstick ( Fig. 8.26 ). Other hairs may have nodules that resemble two broomsticks stuck together on microscopy. Acquired trichorrhexis nodosa most commonly occurs in black patients arising from trauma of combing tightly curled hairs or from excessive use of hot combs, hairdryers, hair straighteners, or other chemicals. This disorder is self-limited and normal hairs regrow when the source of damage is eliminated.

Other forms of deliberate and accidental physical, thermal, and chemical injury to the hair shaft and scalp can also cause alopecia. Examples include child abuse and pressure necrosis of the skin ( Fig. 8.27 ).