alopecia areata

**Pathophysiology:** Loss of immune privilege of the anagen hair follicle. CD8+ cytotoxic T cells (NKG2D+) and IFN-γ/IL-15 signaling via the JAK-STAT pathway attack the peribulbar region of the follicle. The follicle is preserved (no scarring), so hair can regrow. Genetic predisposition (HLA-DR, family history) plus environmental triggers (stress, infection).

**Clinical features:** - Sudden onset of well-demarcated, round or oval patches of non-scarring alopecia with normal-appearing, smooth skin (no scale, erythema, or scarring). - Exclamation-point hairs (short broken hairs that taper toward the proximal end) at the patch margins. - Nail changes: pitting (most common), trachyonychia, ridging. - Positive hair-pull test at the margins of active disease. - Variants: patchy, alopecia totalis (entire scalp), alopecia universalis (entire body), ophiasis (band-like loss at the occipital/temporal periphery, a worse prognosis), and diffuse. - Poor prognostic factors: early onset, extensive disease, ophiasis pattern, nail involvement, atopy.

**Associations:** Atopy, autoimmune thyroid disease (Hashimoto, Graves), vitiligo, type 1 DM, celiac disease, Down syndrome, and psychiatric comorbidity (anxiety, depression).

**Diagnosis:** Usually clinical. Dermoscopy shows yellow dots, black dots, and exclamation-point hairs. Biopsy, if done, shows a "swarm of bees" peribulbar lymphocytic infiltrate around anagen follicles, with an increased proportion of catagen/telogen follicles. Consider checking TSH and other labs based on the history.

**Differential diagnosis:** Tinea capitis (scale, broken hairs, KOH+), trichotillomania (irregular shape, broken hairs of varying lengths, with a "hair-pull" history), telogen effluvium (diffuse), androgenetic alopecia, secondary syphilis ("moth-eaten" pattern), and scarring alopecias (discoid lupus, lichen planopilaris).

**Treatment:** - Limited patchy disease: intralesional triamcinolone (first-line), topical corticosteroids, and topical immunotherapy (diphencyprone). - Extensive disease: JAK inhibitors (baricitinib, ritlecitinib, and deuruxolitinib are FDA-approved for severe disease), plus systemic immunosuppressants in some cases. - Spontaneous regrowth occurs in many patients with limited disease, but relapse is common.

**Board pearls:** Non-scarring + exclamation-point hairs + nail pitting = alopecia areata. Pathology is a "swarm of bees." Think JAK inhibitors for severe disease. Don't confuse it with the tinea capitis scale or the irregular pattern of trichotillomania.