How can I differentiate between alopecia incongenita and alopecia areata in clinical and pathological examinations?

Differentiating Alopecia Areata from Alopecia Congenita (Triangular Alopecia)

The key distinction is that alopecia areata shows preserved follicular ostia with potential for hair regrowth and characteristic exclamation mark hairs, while alopecia congenita (triangular alopecia) presents as a stable, congenital hairless patch with absent or miniaturized follicles and no disease activity markers. 1

Clinical Differentiation

Timing and History

• Age of onset: Alopecia congenita is present from birth or early childhood and remains stable, whereas alopecia areata typically develops after age 2 and shows episodic progression 1, 2
• Disease course: Alopecia areata demonstrates fluctuating activity with potential spontaneous remission (34-50% within 1 year), while triangular alopecia remains static throughout life 1
• Family history: Approximately 20% of alopecia areata patients have affected family members, whereas triangular alopecia shows no familial clustering 1

Physical Examination Features

• Exclamation mark hairs: These pathognomonic short broken hairs with fractured tips are present around expanding patches in active alopecia areata but completely absent in triangular alopecia 1, 2
• Pull test: Positive at the margins of alopecia areata patches during active disease, but negative in stable triangular alopecia 2
• Follicular ostia: Preserved and visible in alopecia areata (indicating potential for regrowth), but absent or significantly reduced in triangular alopecia 1, 2
• Scalp appearance: Normal skin texture in both conditions, but alopecia areata may show slight erythema during active phases 1

Dermoscopic Findings

• Yellow dots: Regular round yellow dots are commonly seen in alopecia areata and indicate active disease progression, but are absent in triangular alopecia 1
• Cadaverized hairs: Hairs fractured before emergence from the scalp are characteristic of alopecia areata but not present in triangular alopecia 1
• Black dots: May be visible in alopecia areata representing broken hairs within follicles, absent in triangular alopecia 1

Pathological Differentiation

Histopathological Features of Alopecia Areata

• Peribulbar lymphocytic infiltrate: Dense T-lymphocyte infiltration around anagen hair bulbs ("swarm of bees" pattern) is the hallmark of alopecia areata 1
• Increased telogen/catagen follicles: Shift toward resting phase follicles with preserved follicular structures 3
• Miniaturized follicles: Present but with intact follicular architecture and potential for recovery 3
• No scarring: Complete preservation of follicular units indicating reversibility 1

Histopathological Features of Triangular Alopecia

• Absent or markedly reduced terminal follicles: Replacement with vellus-like miniaturized follicles from birth 1
• No inflammatory infiltrate: Complete absence of lymphocytic inflammation distinguishes it from alopecia areata 1
• Normal follicular density: May show normal numbers but with miniaturized, non-functional follicles 1
• Permanent architectural changes: No potential for conversion to terminal follicles 1

Diagnostic Algorithm

Step 1: Clinical History

• Document age of onset, progression pattern, and family history of autoimmune disease 1, 4
• Inquire about associated conditions: atopy, thyroid disease, vitiligo (suggest alopecia areata) 1

Step 2: Physical Examination

• Search systematically for exclamation mark hairs at patch margins (present = alopecia areata) 1, 2
• Perform pull test at lesion edges (positive = active alopecia areata) 2
• Examine for preserved follicular ostia (present = alopecia areata) 1, 2

Step 3: Dermoscopy

• Look for yellow dots and cadaverized hairs (present = alopecia areata) 1
• Assess for signs of disease activity versus static appearance 1

Step 4: Biopsy When Diagnosis Uncertain

• Perform scalp biopsy when clinical and dermoscopic findings are equivocal, particularly in diffuse presentations 1, 3, 5
• Request two 4mm punch biopsies: one for vertical sectioning (to assess follicular architecture) and one for horizontal sectioning (to evaluate peribulbar inflammation) 3
• The presence of peribulbar lymphocytic infiltrate confirms alopecia areata, while its absence with miniaturized follicles suggests triangular alopecia 1, 3

Common Pitfalls to Avoid

• Assuming all localized hair loss is alopecia areata: Triangular alopecia is often misdiagnosed as alopecia areata, leading to unnecessary treatments 1
• Overlooking dermoscopy: This non-invasive tool provides critical diagnostic information and should be used routinely 1
• Failing to document disease activity markers: Exclamation mark hairs and positive pull test are essential for confirming active alopecia areata 1, 2
• Not considering biopsy early enough: When clinical diagnosis remains uncertain after careful examination, biopsy prevents prolonged diagnostic uncertainty 1, 3, 5