What is the most likely cause and appropriate management for a sudden, well‑defined patch of hair loss on the shin?

Sudden Well-Defined Hair Loss on the Shin

The most likely diagnosis is alopecia areata, an autoimmune condition that can affect any hair-bearing area including the shin, presenting as well-demarcated patches of non-scarring hair loss on otherwise normal-appearing skin. 1, 2, 3

Clinical Diagnosis

This diagnosis can be made clinically without laboratory testing in most cases. 2 Look for these specific features:

• Exclamation mark hairs (short broken hairs) around the margins of the patch—these are pathognomonic for alopecia areata 1, 2, 4
• Well-demarcated round or oval patch with complete hair loss 3, 5
• Skin appears normal or slightly reddened with no scaling or inflammation 1
• Pull test may be positive at the margins if the patch is actively expanding 2

Dermoscopy Findings (If Available)

If you have access to dermoscopy, look for:

• Yellow dots (present in 6-100% of cases, indicate active disease) 2, 4
• Exclamation mark hairs 2, 4
• Black dots (present in 0-84% of cases) 4

These findings confirm alopecia areata and eliminate the need for biopsy. 2, 4

Key Differential Diagnoses to Exclude

• Trichotillomania: Incomplete hair loss with firmly anchored broken hairs that remain in growing phase, unlike the exclamation mark hairs of alopecia areata 1, 2, 4
• Tinea capitis: Would show scalp inflammation and scaling; requires fungal culture for diagnosis 2, 4
• Secondary syphilis: Presents with "moth-eaten" patchy hair loss; obtain serology if risk factors present 2

When to Order Laboratory Tests

Laboratory testing is unnecessary when the clinical presentation is classic. 2 However, consider targeted testing if:

• Diagnosis remains uncertain after clinical examination 2
• Multiple patches or atypical presentation 2
• Suspicion for systemic disease (check TSH for thyroid disease, lupus serology if systemic features present) 2
• Fungal culture only if inflammation or scaling present 2

Management Approach

For a single well-defined patch on the shin, intralesional corticosteroid injections (triamcinolone acetonide 5-10 mg/mL) are first-line treatment with the strongest evidence. 1, 6, 7

Alternative approach: Observation is reasonable since 34-50% of patients with limited patchy alopecia areata recover spontaneously within one year. 1, 2 This "watch-and-wait" strategy is particularly appropriate for recent-onset, small patches. 5

Prognostic Factors

• Single small patch has favorable prognosis (68% disease-free at follow-up if <25% hair loss initially) 4
• Childhood onset and ophiasis pattern (scalp margin involvement) carry poorer prognosis 1
• Nail involvement (pitting, ridging) occurs in 10% and predicts worse outcomes 1, 2

Common Pitfalls to Avoid

• Do not order excessive laboratory tests when clinical diagnosis is evident 2
• Do not overlook the psychological impact—assess for anxiety and depression as alopecia areata causes considerable psychological disability 2, 6
• Do not assume treatment is always necessary—many cases are self-limited 1, 2
• Do not diagnose tinea capitis without fungal culture—incorrect diagnosis is the most common cause of treatment failure 2