What is the difference between scarring alopecia and secondary scarring alopecia, particularly in patients with a history of autoimmune disorders?

Scarring vs Secondary Scarring Alopecia

Primary scarring alopecia occurs when the hair follicle itself is the direct target of the disease process, while secondary scarring alopecia develops when the follicle is destroyed as an "innocent bystander" in a disease process primarily affecting the surrounding dermis or skin structures. 1

Key Distinguishing Features

Primary Scarring Alopecia

• The hair follicle is the primary target of the inflammatory or destructive process, with autoaggressive immunity specifically directed against follicular structures 1, 2
• Results from irreversible damage to epithelial stem cells located in the bulge region of the hair follicle, leading to permanent replacement by fibrous scar tissue 2
• Accounts for approximately 5% of all trichologic consultations 1
• Can be classified by the predominant inflammatory cell type infiltrating the follicle 1, 3:
  • Lymphocytic forms: discoid lupus erythematosus, lichen planopilaris, frontal fibrosing alopecia, central centrifugal alopecia, and pseudopelade 3
  • Neutrophilic forms: folliculitis decalvans, tufted folliculitis, and dissecting cellulitis of the scalp 3
  • Mixed infiltrate: folliculitis keloidalis 3

Secondary Scarring Alopecia

• The follicle acts as an "innocent bystander" when disease processes occurring outside the follicular unit secondarily destroy hair follicles 1
• The primary pathology targets the dermis or other skin structures, with follicular destruction being a collateral consequence 1
• Examples include scarring from burns, trauma, radiation, or infiltrative processes that secondarily affect follicles 1

Pathophysiologic Mechanism in Autoimmune Context

• In primary scarring alopecias, particularly relevant to patients with autoimmune disorders, the collapse of hair follicle immune privilege is a central mechanism 2
• Healthy hair follicle epithelial stem cells normally exist in an immunologically "privileged" niche that protects them from inflammatory assault 2
• When this immune privilege fails—as may occur in autoimmune conditions—the follicle becomes vulnerable to targeted autoimmune destruction 2
• Approximately 22% of patients with androgenic alopecia have coexisting autoimmune disease, and 42% have autoantibodies, highlighting the autoimmune connection 4

Clinical Implications for Diagnosis

• Histopathologic examination is essential for definitive diagnosis, as it reveals whether inflammation primarily targets the follicle (primary) or surrounding structures (secondary) 1, 5
• Dermoscopy can aid in early detection but cannot definitively distinguish primary from secondary scarring 6
• In patients with autoimmune disorders, consider that systemic conditions like lupus erythematosus can cause both scarring and non-scarring alopecia, making clinical-pathological correlation critical 6

Critical Pitfall to Avoid

• Do not assume all scarring alopecia in autoimmune patients is primary—systemic lupus, for example, can cause secondary scarring when dermal inflammation secondarily destroys follicles, or primary scarring when discoid lupus directly targets follicular structures 6, 3
• Early biopsy is warranted when scarring alopecia is suspected, as treatment aims to halt progression at the earliest phase before irreversible follicular destruction occurs 1