What is the differential diagnosis for a single bald spot on the scalp?

Differential Diagnosis for a Single Bald Spot on the Scalp

The most likely diagnosis for a single bald spot on the scalp is alopecia areata, but you must systematically rule out tinea capitis (especially in children), trichotillomania, early scarring alopecias, and less commonly secondary syphilis or lupus erythematosus. 1

Primary Differential Diagnoses

According to the British Association of Dermatologists guidelines, the key conditions that cause diagnostic difficulty with a single patch of hair loss include 1:

1. Alopecia Areata (Most Common)

• Presents as well-demarcated patches with normal-appearing skin (may be slightly reddened)
• Look for exclamation mark hairs at the margins (short broken hairs with tapered roots)
• Dermoscopy findings: Regular round yellow dots, dystrophic hairs with fractured tips, cadaverized hairs (fractured before emergence)
• No scarring, hair follicles preserved
• 80% spontaneous remission within 1 year for small patches 1

2. Tinea Capitis (Critical to Rule Out)

• The scalp will show inflammation, though signs may be subtle 1
• Look for scaling, broken hairs at different lengths (unlike exclamation marks)
• In endothrix infections (T. tonsurans): "black dot" appearance with broken-off hair stubs 2
• In ectothrix infections (Microsporum): fine scaling with dull grey patches
• Key distinction: Requires fungal culture for confirmation and systemic antifungal treatment
• More common in children

3. Trichotillomania (Most Confusing Mimic)

• Incomplete hair loss pattern (not completely bald)
• Broken hairs are firmly anchored and remain in anagen phase (growing phase) 1
• This contrasts with exclamation mark hairs in alopecia areata
• May coexist with alopecia areata in some cases
• Irregular, bizarre-shaped patches

4. Early Scarring Alopecia

• Includes lichen planopilaris, discoid lupus erythematosus, frontal fibrosing alopecia
• Look for perifollicular erythema or scale early on
• Follicular plugging on examination
• Permanent hair loss if not caught early
• Biopsy often needed for definitive diagnosis 1

5. Secondary Syphilis

• "Moth-eaten" alopecia pattern
• Associated systemic symptoms
• Serology required for diagnosis 1

6. Systemic Lupus Erythematosus

• May present as patchy nonscarring alopecia 3
• Look for other signs of lupus
• Serology for lupus erythematosus when suspected 1

7. Temporal Triangular Alopecia

• Lancet-shaped patch in temporal region
• Contains normal numbers of follicles but they are vellus (miniaturized) hairs 4
• Usually develops in first few years of life
• Benign, non-progressive

Diagnostic Approach

When examining a single bald spot, systematically assess:

1. Scalp surface: Normal vs. inflamed vs. scarred
2. Hair pull test: Positive in alopecia areata and telogen effluvium
3. Examine extracted hairs: Look for exclamation mark morphology (tapered roots) vs. anagen hairs (firm anchoring)
4. Dermoscopy/Trichoscopy: Yellow dots and dystrophic hairs suggest alopecia areata; comma hairs and broken hairs suggest tinea 5
5. Pattern and borders: Well-demarcated (alopecia areata) vs. irregular (trichotillomania) vs. scaling (tinea)

Investigations When Diagnosis is Uncertain

Investigations are unnecessary in most cases of alopecia areata when the clinical diagnosis is clear 1. However, when in doubt:

• Fungal culture: Essential if any suspicion of tinea capitis
• Skin biopsy: For suspected scarring alopecia or atypical presentations
• Serology for syphilis: If moth-eaten pattern or systemic symptoms
• Lupus serology: If other features suggest SLE
• Iron studies are NOT recommended routinely 1

Critical Pitfalls to Avoid

1. Missing tinea capitis: Always consider fungal infection, especially in children, as it requires systemic antifungal therapy and is contagious
2. Delaying diagnosis of scarring alopecia: Permanent hair loss occurs if treatment is delayed
3. Assuming all single patches are alopecia areata: The differential is broader than commonly appreciated
4. Misdiagnosing kerion as bacterial abscess: This is a severe inflammatory tinea capitis variant 2
5. Over-investigating typical alopecia areata: Most cases are clinically obvious and don't require testing 1