Red Patchy Skin: Common Causes, Diagnosis & When to See a Doctor

Red, patchy lesions on the arms, shins, chest, and neck have a broad differential. Distribution, morphology, symptoms, and history narrow the diagnosis.

Common Causes by Pattern

1. Eczema (Atopic Dermatitis)

• Distribution: Flexural (antecubital, popliteal), neck, wrists, ankles.
• Morphology: Erythematous, scaly, excoriated patches; lichenification if chronic.
• Symptoms: Intense pruritus. Personal/family atopy history.

2. Psoriasis

• Distribution: Extensor surfaces (elbows, knees), scalp, sacrum, nails.
• Morphology: Well-demarcated, erythematous plaques with silvery scale. Auspitz sign (pinpoint bleeding on scale removal).
• Variants: Guttate (post-strep), inverse (folds), palmoplantar.

3. Tinea Corporis (Ringworm)

• Morphology: Annular, scaly plaques with active advancing border, central clearing. KOH prep positive for hyphae.
• Spread: Centrifugal. Can affect any site.

4. Contact Dermatitis

• Allergic: Poison ivy, nickel, fragrance, preservatives. Geometric/linear patterns. Delayed (24–72h).
• Irritant: Soaps, solvents, friction. Immediate/burning. Hands common.

5. Pityriasis Rosea

• Herald patch: Single 2–5 cm oval plaque (trunk), precedes general eruption by 1–2 wks.
• Generalized: Smaller oval lesions along Langer's lines ("Christmas tree" distribution on back).
• Self-limited: 6–8 weeks. Viral etiology (HHV-6/7).

6. Drug Eruption

• Morbilliform (measles-like) most common – starts trunk, spreads. 7–14 days after new drug.
• Fixed drug eruption – recurrent same site, hyperpigmentation.
• DRESS/SJS/TEN – severe, mucosal involvement, systemic symptoms.

7. Insect Bites / Papular Urticaria

• Pruritic papules, often grouped/linear ("breakfast, lunch, dinner"). Exposed skin.
• Fleas, mosquitoes, bed bugs, mites.

8. Nummular Eczema

• Coin-shaped, vesicular/crusted plaques on extremities. Winter flare. Dry skin precursor.

9. Lichen Planus

• Violaceous, polygonal, flat-topped papules. Wickham striae. Wrists, ankles, oral mucosa.

10. Cutaneous Lupus (ACLE/SCLE)

• Photosensitive, annular/papulosquamous (SCLE) or malar rash (ACLE). Serology (ANA, anti-Ro).

Diagnostic Approach

1. History: Onset, progression, exposures, meds, atopy, systemic symptoms.
2. Exam: Morphology, distribution, blanching, scale, temperature, lymph nodes.
3. Tests: KOH prep, skin biopsy (H&E, DIF), patch testing, bloods (CBC, ESR/CRP, ANA, IgE).

When to See a Dermatologist

• Rapidly spreading, painful, or blistering rash
• Fever, joint pain, mucosal involvement
• Failure of OTC hydrocortisone × 2 weeks
• Diagnostic uncertainty
• Cosmetic concern / quality of life impact

Many red patches look similar. Context and targeted testing yield the correct diagnosis and effective treatment.