Differential Diagnoses for Single Rounded Redness 1 cm on Scalp
A single 1 cm rounded red lesion on the hair-bearing scalp most likely represents tinea capitis (scalp ringworm), folliculitis, or early alopecia areata, with tinea capitis being the primary concern requiring fungal culture for definitive diagnosis. 1, 2
Primary Differential Diagnoses
Tinea Capitis (Scalp Ringworm)
- Presents as patchy hair loss with scalp inflammation, erythema, and scaling 3, 2
- May show subtle signs making diagnosis challenging 3
- Requires fungal culture for definitive diagnosis 3, 4
- Must be treated systemically with oral antifungal therapy 2
- More common in patients with skin of color and highly textured hair 1
Folliculitis (Infected Hair Follicle)
- Furunculosis presents as localized infection on outer third of ear canal but can occur on scalp 5
- Clinical findings include focal swelling, pustular lesions, and localized tenderness 5
- Most commonly caused by Staphylococcus aureus 5
- Treatment may include local heat, incision and drainage, or systemic antibiotics covering S. aureus 5
Early Alopecia Areata
- Autoimmune condition mediated by T lymphocytes targeting hair follicles 3, 4
- Characterized by patchy, non-scarring hair loss 3
- Look for exclamation mark hairs (short broken hairs) around expanding patches 3, 4
- Dermoscopy shows yellow dots, exclamation mark hairs, and cadaverized hairs 3
- Approximately 20% have family history 3, 4
Secondary Considerations
Contact Dermatitis
- Allergic or irritant contact dermatitis can present as localized erythema 5
- Characterized by yellow exudate and polymorphic vessels on trichoscopy 6
- May result from hair care products, shampoos, or hair sprays 5
- Management involves removing sensitizing agent and applying topical steroid 5
Seborrheic Dermatitis
- Can present with erythema and scaling on scalp 6, 1
- Trichoscopy shows yellowish-white scaling and thin arborizing vessels 6
- More common in patients with skin of color 1
Scalp Psoriasis
- May present as localized red patch with scaling 6
- Trichoscopy reveals silver-white scaling, regularly distributed dotted vessels, and punctate hemorrhages 6
Diagnostic Approach
Clinical Examination
- Examine for scaling, pustules, or hair loss within the lesion 4, 6
- Assess for exclamation mark hairs suggesting alopecia areata 3, 4
- Check if hair can be easily pulled out (pull test) 4
- Look for signs of inflammation or infection 4
Trichoscopy (Dermoscopy)
- Non-invasive diagnostic tool providing valuable information 3, 6
- Can differentiate between conditions based on vessel patterns and scaling characteristics 6
- Particularly useful for diagnosing alopecia areata, psoriasis, and seborrheic dermatitis 6
Laboratory Testing
- Fungal culture is essential when tinea capitis is suspected 3, 4
- Skin biopsy recommended for difficult cases or when diagnosis is uncertain 3, 4
- Most cases of alopecia areata can be diagnosed clinically without laboratory workup 3, 4
Critical Pitfalls to Avoid
- Do not assume all scalp erythema is benign dermatitis—tinea capitis requires systemic treatment 2
- Failing to perform fungal culture when tinea capitis is suspected delays appropriate treatment 3, 4
- Missing early alopecia areata when exclamation mark hairs are present 3, 4
- Overlooking trichoscopy as a diagnostic tool that can avoid unnecessary biopsy 3, 6