Alopecia is defined as loss of hair from the body. Hair loss is often a cause of great concern to the patient for cosmetic and psychologic reasons, but it can also be an important sign of systemic disease.
(See also Alopecia Areata Alopecia Areata Alopecia areata is typically sudden patchy nonscarring hair loss in people with no obvious skin or systemic disorder. (See also Alopecia.) This photo shows patchy loss of scalp hair in alopecia... read more 
.)
Pathophysiology of Alopecia
Growth cycle
Hair grows in cycles. Each cycle consists of phases:
Anagen: A long (2- to 6-year) growing phase
Catagen: A brief (3-week) transitional apoptotic phase
Telogen: A short (2- to 3-month) resting phase
At the end of the resting phase, the hair falls out (exogen). Normally, about 50 to 100 scalp hairs reach the end of resting phase each day and fall out. When a new hair starts growing in the follicle, the cycle begins again.
Disorders of the growth cycle include
Anagen effluvium—a disruption of the growing phase causing abnormal loss of anagen hairs
Telogen effluvium—significantly more than 100 hairs/day going into resting phase
Classification
Alopecia can be classified as focal or diffuse and by the presence or absence of scarring.
Scarring alopecia is the result of active destruction of the hair follicle. The follicle is irreparably damaged and replaced by fibrotic tissue. Several hair disorders show a biphasic pattern in which nonscarring alopecia occurs early in the course of the disease, and then scarring alopecia and permanent hair loss occurs as the disease progresses. Scarring alopecias can be subdivided further into primary forms, where the target of inflammation is the follicle itself, and secondary forms, where the follicle is destroyed as a result of nonspecific inflammation (see table Some Causes of Alopecia Some Causes of Alopecia Alopecia is defined as loss of hair from the body. Hair loss is often a cause of great concern to the patient for cosmetic and psychologic reasons, but it can also be an important sign of systemic... read more 
).
Nonscarring alopecia results from processes that reduce or slow hair growth without irreparably damaging the hair follicle. Disorders that primarily affect the hair shaft (trichodystrophies) also are considered nonscarring alopecia.
Etiology of Alopecia
The alopecias comprise a large group of disorders with multiple and varying etiologies (see table Some Causes of Alopecia Some Causes of Alopecia Alopecia is defined as loss of hair from the body. Hair loss is often a cause of great concern to the patient for cosmetic and psychologic reasons, but it can also be an important sign of systemic... read more ).
The most common cause of alopecia is
Androgenetic alopecia (male-pattern or female-pattern hair loss)
Androgenetic alopecia is an androgen-dependent hereditary disorder in which dihydrotestosterone plays a major role. The prevalence of this form of alopecia increases with age, and it affects over 70% of men (male-pattern hair loss) and 50% of all women (female-pattern hair loss) over the age of 80 (1 Key Points Alopecia is defined as loss of hair from the body. Hair loss is often a cause of great concern to the patient for cosmetic and psychologic reasons, but it can also be an important sign of systemic... read more ). The prevalence is lower in Chinese, Asian, and black people than in white people.
Other common causes of hair loss are
Drugs (including chemotherapeutic agents)
Infection (eg, tinea capitis Tinea Capitis (Scalp Ringworm) Tinea capitis is a dermatophyte infection of the scalp. Diagnosis is by clinical appearance and by examination of plucked hairs or hairs and scale on potassium hydroxide wet mount. Treatment... read more
, kerion Kerion Tinea capitis is a dermatophyte infection of the scalp. Diagnosis is by clinical appearance and by examination of plucked hairs or hairs and scale on potassium hydroxide wet mount. Treatment... read more )Systemic disorders (disorders that cause high fever, systemic lupus erythematosus Systemic Lupus Erythematosus (SLE) Systemic lupus erythematosus is a chronic, multisystem, inflammatory disorder of autoimmune etiology, occurring predominantly in young women. Common manifestations may include arthralgias and... read more
, endocrine disorders, and nutritional deficiencies)Trauma
Traumatic causes include trichotillomania Trichotillomania Trichotillomania is characterized by recurrent pulling out of one's hair resulting in hair loss. Patients with trichotillomania repeatedly pull or pluck out their hair for noncosmetic reasons... read more 
, traction alopecia, central centrifugal cicatricial alopecia, burns, radiation, and pressure-induced (eg, postoperative) hair loss.
Less common causes are
Primary hair shaft abnormalities
Autoimmune diseases
Heavy metal poisoning
Rare dermatologic conditions (eg, dissecting cellulitis of the scalp, which most often affects young black men)
Etiology reference
1. Adil A, Godwin M: The effectiveness of treatments for androgenetic alopecia: A systematic review and meta-analysis. J Am Acad Dermatol 77(1):136–141.e5, 2017. doi: 10.1016/j.jaad.2017.02.054
Evaluation of Alopecia
History
History of present illness should cover the onset and duration of hair loss, whether hair shedding is increased, and whether hair loss is generalized or localized. Associated symptoms such as pruritus and scaling should be noted. Patients should be asked about typical hair care practices, including use of braids, rollers, and hair dryers, and whether they routinely pull or twist their hair.
Review of systems should include recent exposures to noxious stimuli (eg, drugs, toxins, radiation) and stressors (eg, surgery, chronic illness, fever, psychologic stressors). Symptoms of possible causes (eg, fatigue and cold intolerance [hypothyroidism] and, in women, hirsutism, deepening of the voice, and increased libido [virilization]) should be sought. Other features, including dramatic weight loss, dietary practices (including various restrictive diets), and obsessive-compulsive behavior Obsessive-Compulsive Disorder (OCD) Obsessive-compulsive disorder (OCD) is characterized by recurrent, persistent, unwanted, and intrusive thoughts, urges, or images (obsessions) and/or by repetitive behaviors or mental acts that... read more , should be noted. In women, a hormonal/gynecologic/obstetric history should be obtained.
Past medical history should note known possible causes of hair loss, including endocrine and skin disorders. Current and recent drug use should be reviewed for offending agents (see table Some Causes of Alopecia Some Causes of Alopecia Alopecia is defined as loss of hair from the body. Hair loss is often a cause of great concern to the patient for cosmetic and psychologic reasons, but it can also be an important sign of systemic... read more ). A family history of hair loss should be recorded.
Physical examination
Examination of the scalp should note the distribution of hair loss, the presence and characteristics of any skin lesions, and whether there is scarring. Part widths should be measured. Abnormalities of the hair shafts should be noted.
A full skin examination should be done to evaluate hair loss elsewhere on the body (eg, eyebrows, eyelashes, arms, legs), rashes that may be associated with certain types of alopecia (eg, discoid lupus lesions, signs of secondary syphilis or of other bacterial or fungal infections), and signs of virilization in women (eg, hirsutism, acne, deepening voice, clitoromegaly). Signs of potential underlying systemic disorders should be sought, and a thyroid examination should be done.
Red flags
The following findings are of particular concern:
Virilization in women
Signs of systemic illness or constellations of nonspecific findings possibly indicating poisoning
Interpretation of findings
Hair loss that begins at the temples and/or crown (vertex) and spreads to diffuse thinning or nearly complete hair loss is typical of male-pattern hair loss. Hair thinning in the frontal, parietal, and crown regions is typical of female-pattern hair loss (see figure Male- and female-pattern hair loss (androgenetic alopecia) Male- and female-pattern hair loss (androgenetic alopecia) Alopecia is defined as loss of hair from the body. Hair loss is often a cause of great concern to the patient for cosmetic and psychologic reasons, but it can also be an important sign of systemic... read more ). In androgenetic alopecia, the central part width is wider on the crown of the scalp than it is on the occipital scalp.
Male- and female-pattern hair loss (androgenetic alopecia)
 |
Hair loss that occurs 2 to 4 weeks after chemotherapy Systemic Cancer Therapy Systemic cancer therapy includes chemotherapy (ie, conventional or cytotoxic chemotherapy), hormone therapy, targeted therapy, and immune therapy (see also Overview of Cancer Therapy). The number... read more or radiation therapy Radiation Therapy for Cancer Radiation therapy can cure many cancers (see also Overview of Cancer Therapy), particularly those that are localized or that can be completely encompassed within a radiation field. Radiation... read more (anagen effluvium) can typically be ascribed to those causes. Hair loss that occurs 3 to 4 months after a major stressor (pregnancy, major febrile illness, surgery, medication change, or severe psychologic stressor) suggests a diagnosis of telogen effluvium.
Other findings help suggest alternative diagnoses (see table Interpreting Physical Findings in Alopecia Interpreting Physical Findings in Alopecia Alopecia is defined as loss of hair from the body. Hair loss is often a cause of great concern to the patient for cosmetic and psychologic reasons, but it can also be an important sign of systemic... read more ).
Anagen effluvium is a physiologic disruption of the anagen (growing) phase. It typically occurs several weeks after chemotherapy or radiation therapy. This photo shows an abrupt loss of hairs in the anagen phase along with sparse broken anagen hairs.
© Springer Science+Business Media
This photo shows typical acneiform lesions and deep keloidal scarring alopecia in a young male with acne keloidalis nuchae.
© Springer Science+Business Media
In this photo, the hair pulling is limited to the range of the person's right hand.
© Springer Science+Business Media
Other than hair loss, scalp symptoms (eg, itching, burning, tingling) are often absent and, when present, are not specific to any cause.
Signs of hair loss in patterns other than those described above are nondiagnostic and may require microscopic hair examination or scalp biopsy for definitive diagnosis.
Testing
Evaluation for causative disorders (eg, endocrinologic, autoimmune, toxic) should be done based on clinical suspicion.
Male-pattern or female-pattern hair loss usually requires no testing. When it occurs in young men with no family history, the physician should question the patient about use of anabolic steroids Anabolic Steroids Anabolic steroids are often used to enhance physical performance and promote muscle growth. When used inappropriately, chronically at high doses and without medical supervision, they can cause... read more and other drugs. In addition to questions regarding prescription drug and illicit drug use, women with significant hair loss and evidence of virilization should have levels of appropriate hormones (eg, testosterone and dehydroepiandrosterone sulfate [DHEAS]) measured (see Hirsutism Testing Hirsutism is the excessive growth of thick or dark hair in women in locations that are more typical of male hair growth patterns (eg, mustache, beard, central chest, shoulders, lower abdomen... read more 
).
The pull test helps evaluate diffuse scalp hair loss. Gentle traction is exerted on a bunch of hairs (about 40) on at least 3 different areas of the scalp, and the number of extracted hairs is then counted and examined microscopically. Normally, < 3 telogen-phase hairs should come out with each pull. If ˃ 4 to 6 hairs come out with each pull, the pull test is positive and is suggestive of telogen effluvium.
The pluck test involves sequentially pulling out about 50 individual hairs abruptly (“by the roots”). The roots of the plucked hairs are examined microscopically to determine the phase of growth and thus help diagnose a defect of telogen or anagen or an occult systemic disease. Anagen hairs have sheaths attached to their roots; telogen hairs have tiny bulbs without sheaths at their roots. Normally, 85 to 90% of hairs are in the anagen phase, about 10 to 15% are in telogen phase, and < 1% are in catagen phase. Telogen effluvium shows an increased percentage of telogen-phase hairs on microscopic examination (typically > 20%), whereas anagen effluvium shows a decrease in telogen-phase hairs and an increased number of broken hairs. Primary hair shaft abnormalities are usually obvious on microscopic examination of the hair shaft.
Scalp biopsy is indicated when alopecia persists and diagnosis is in doubt. Biopsy may differentiate scarring from nonscarring forms. Specimens should be taken from areas of active inflammation, ideally at the border of a bald patch. Fungal and bacterial cultures may be useful.
Daily hair counts can be done by the patient to quantify hair loss when the pull test is negative. Hairs lost during the first morning combing or during washing are collected in clear plastic bags daily for 14 days. The number of hairs in each bag is then recorded. Scalp hair counts of > 100/day are abnormal except after shampooing, when hair counts of up to 250 may be normal. Hairs may be brought in by the patient for microscopic examination.
Treatment of Alopecia
Drugs (including hormonal modulators)
Laser light therapy
Surgery
Androgenetic alopecia
Minoxidil (2% for women, 2% or 5% for men) works by mechanisms that are not completely understood. Topical minoxidil 1 mL 2 times a day applied to the scalp is most effective for vertex alopecia in male-pattern or female-pattern hair loss. However, usually only 30 to 40% of patients experience significant hair growth, and minoxidil is generally not effective or indicated for other causes of hair loss except possibly alopecia areata Alopecia Areata Alopecia areata is typically sudden patchy nonscarring hair loss in people with no obvious skin or systemic disorder. (See also Alopecia.) This photo shows patchy loss of scalp hair in alopecia... read more . Hair regrowth can take 8 to 12 months. Treatment is continued indefinitely because, once treatment is stopped, hair loss resumes. The most frequent adverse effects are mild scalp irritation, allergic contact dermatitis Allergic contact dermatitis (ACD) Contact dermatitis is inflammation of the skin caused by direct contact with irritants (irritant contact dermatitis) or allergens (allergic contact dermatitis). Symptoms include pruritus and... read more 
, and increased facial hair.
Finasteride inhibits the 5-alpha-reductase enzyme, blocking conversion of testosterone to dihydrotestosterone, and is useful for male-pattern hair loss. Finasteride 1 mg orally once/day can stop hair loss and can stimulate hair growth. Efficacy is usually evident within 6 to 8 months of treatment. Adverse effects include decreased libido; erectile and ejaculatory dysfunction, which may persist even after cessation of treatment (see Male Sexual Dysfunction Erectile Dysfunction Erectile dysfunction is the inability to attain or sustain an erection satisfactory for sexual intercourse. Most erectile dysfunction is related to vascular, neurologic, psychologic, and hormonal... read more ); hypersensitivity reactions; gynecomastia Gynecomastia This photo shows enlarged breast tissue in a male patient. Gynecomastia is hypertrophy of breast glandular tissue in males. It must be differentiated from pseudogynecomastia, which is increased... read more 
; and myopathy. There may be a decrease in prostate-specific antigen (PSA) Prostate-specific antigen (PSA) levels Benign prostatic hyperplasia (BPH) is nonmalignant adenomatous overgrowth of the periurethral prostate gland. Symptoms are those of bladder outlet obstruction—weak stream, hesitancy, urinary... read more levels in older men, which should be taken into account when this test is used for cancer screening. Common practice is to continue treatment for as long as positive results persist. Once treatment is stopped, hair loss returns to previous levels. Finasteride is sometimes used off-label in women of nonchildbearing potential; it is contraindicated in pregnant women because it has teratogenic effects in animals.
Dutasteride, a drug used to treat benign prostatic hyperplasia, is a stronger inhibitor of 5-alpha-reductase than finasteride and is sometimes used to treat androgenetic alopecia.
Hormonal modulators such as oral contraceptives or spironolactone may be useful for female-pattern hair loss.
Low-level laser light therapy is an alternate or additional treatment for androgenetic alopecia that has been shown to promote hair growth. Physician-dispensed and over-the-counter devices are available.
Autologous platelet-rich plasma injected into the scalp is thought to contain growth factors that promote hair follicle growth and maintenance (1 Treatment references Alopecia is defined as loss of hair from the body. Hair loss is often a cause of great concern to the patient for cosmetic and psychologic reasons, but it can also be an important sign of systemic... read more ).
Surgical options include follicle transplant, scalp flaps, and alopecia reduction. Few procedures have been subjected to scientific scrutiny, but patients who are self-conscious about their hair loss may consider them (2 Treatment references Alopecia is defined as loss of hair from the body. Hair loss is often a cause of great concern to the patient for cosmetic and psychologic reasons, but it can also be an important sign of systemic... read more ).
Hair loss due to other causes
Underlying disorders are treated.
Treatment for alopecia areata includes topical, intralesional, or, in severe cases, systemic corticosteroids, topical minoxidil, topical anthralin, topical immunotherapy (diphenylcyclopropenone or squaric acid dibutylester), or methotrexate.
Treatment for traction alopecia is elimination of physical traction or stress to the scalp.
Treatment for tinea capitis Tinea Capitis (Scalp Ringworm) Tinea capitis is a dermatophyte infection of the scalp. Diagnosis is by clinical appearance and by examination of plucked hairs or hairs and scale on potassium hydroxide wet mount. Treatment... read more is oral antifungals.
Trichotillomania Trichotillomania Trichotillomania is characterized by recurrent pulling out of one's hair resulting in hair loss. Patients with trichotillomania repeatedly pull or pluck out their hair for noncosmetic reasons... read more is difficult to treat, but behavior modification, clomipramine, or a selective serotonin reuptake inhibitor Selective Serotonin Reuptake Inhibitors (SSRIs) Several drug classes and drugs can be used to treat depression: Selective serotonin reuptake inhibitors (SSRIs) Serotonin modulators (5-HT2 blockers) Serotonin-norepinephrine reuptake inhibitors... read more (SSRI—eg, fluoxetine, fluvoxamine, paroxetine, sertraline, citalopram) may be of benefit.
Scarring alopecia as in central centrifugal cicatricial alopecia or dissecting cellulitis of the scalp is best treated with an oral tetracycline plus a potent topical corticosteroid. Severe or chronic acne keloidalis nuchae can be treated similarly or with intralesional triamcinolone; if mild, topical retinoids, topical antibiotics, and/or topical benzoyl peroxide may suffice.
Lichen planopilaris; its variant, frontal fibrosing alopecia; and chronic cutaneous lupus lesions may be treated with drugs such as oral antimalarials, topical or intralesional corticosteroids, topical or oral retinoids, topical tacrolimus, or oral immunosuppressants.
Hair loss due to chemotherapy (anagen effluvium) is temporary and is best treated with a wig; when hair regrows, it may be different in color and texture from the original hair. Hair loss due to telogen effluvium is usually temporary as well and abates after the precipitating agent is eliminated.
Treatment references
1. Hesseler MJ, Shyam N: Platelet-rich plasma and its utilities in alopecia: A systematic review. Dermatol Surg 46(1):93–102, 2020. doi: 10.1097/DSS.0000000000001965
2. Adil A, Godwin M: The effectiveness of treatments for androgenetic alopecia: A systematic review and meta-analysis. J Am Acad Dermatol 77(1):136–141.e5, 2017. doi: 10.1016/j.jaad.2017.02.054
Key Points
Androgenetic alopecia (male-pattern and female-pattern hair loss) is the most common type of hair loss.
Concomitant virilization in women or scarring hair loss should prompt a thorough evaluation for an underlying disorder.
Microscopic hair examination or scalp biopsy may be required for definitive diagnosis.
