UVA and UVB Phototherapy - Ray Reaction
- Erythema (sunburn) is the principal acute cutaneous reaction to UV radiation, dose-dependent.
- UVB Erythema:
- Onset: 2-6 hours post-exposure.
- Peak: 12-24 hours.
- Duration: 24-72 hours.
- Appearance: Bright red, tender; severe reactions may blister.
- Primarily prostaglandin-mediated.
- UVA Erythema (PUVA):
- Requires psoralen sensitization.
- Onset: More delayed, 24-36 hours.
- Peak: 48-72 hours (can be up to 96 hours).
- Duration: Longer, days to weeks.
- Appearance: Deeper, dusky erythema.
- Minimal Erythema Dose (MED): Lowest UV dose producing minimal, perceptible erythema with sharp borders at 24 hours (for UVB).
⭐ UVB-induced erythema typically peaks at 12-24 hours, while PUVA erythema (UVA + psoralen) peaks much later at 48-72 hours.
UVA and UVB Phototherapy - Spectrum Specifics
| Feature | NB-UVB | BB-UVB | PUVA (Psoralen + UVA) |
|---|---|---|---|
| Wavelength (nm) | 311-313 | 290-320 | 320-400 (UVA-I) |
| Psoralen | No | No | Yes (8-MOP, 5-MOP) |
| Efficacy | High | Moderate | Very High |
| Key Uses | Psoriasis, Vitiligo, Atopic Dermatitis | Psoriasis (less common) | Severe Psoriasis, Vitiligo, Mycosis Fungoides (MF), Eczema |
⭐ PUVA therapy (especially oral) carries the highest long-term risk of non-melanoma skin cancer (NMSC), particularly squamous cell carcinoma, among these phototherapy modalities.
- NB-UVB: Peak emission at 311 nm; optimal for psoriasis with fewer side effects than BB-UVB.
- BB-UVB: Contains more erythemogenic wavelengths (290-300 nm); largely replaced by NB-UVB.
- PUVA: Combines psoralen (oral/topical) with UVA light; potent for recalcitrant cases. 📌 Psoralen Used with UVA.
UVA and UVB Phototherapy - Disease Defeaters
Phototherapy uses specific wavelengths of UV light to treat various skin conditions.
| Condition | Preferred Phototherapy | Rationale/Notes |
|---|---|---|
| Psoriasis | NB-UVB (311-313 nm), PUVA | Antiproliferative, immunomodulatory. PUVA for severe/resistant cases. |
| Vitiligo | NB-UVB (311-313 nm), PUVA | Stimulates melanocyte function/migration. NB-UVB often 1st line. |
| Atopic Dermatitis | NB-UVB | Immunomodulatory, reduces S. aureus. PUVA for severe/recalcitrant. |
| Mycosis Fungoides (CTCL) | NB-UVB (early stage), PUVA | Induces apoptosis of malignant T-cells. PUVA for thicker plaques. |
| Polymorphous Light Eruption (PMLE) | NB-UVB, PUVA (prophylaxis) | Hardening effect, induces immune tolerance. Start weeks before sun exposure. |
| Uremic Pruritus | NB-UVB | Anti-inflammatory, alters nerve fiber function. |
| Lichen Planus | PUVA, NB-UVB | Immunomodulatory, antiproliferative. Oral LP may use UVA-1. |
UVA and UVB Phototherapy - Guarding Glow
- Mechanism: Induces T-cell apoptosis, alters cytokines, immunomodulation.
- NB-UVB (311-313 nm):
- Indications: Psoriasis, vitiligo, atopic dermatitis, Mycosis Fungoides (MF) (early), Polymorphic Light Eruption (PLE).
- Dosimetry: Minimal Erythema Dose (MED). Start 50-70% MED. Escalate 10-20% per session, 2-5x/week.
- Advantages: Effective, less erythemogenic than Broadband UVB (BB-UVB), pregnancy-safe.
- PUVA (Psoralen + UVA): Oral/topical psoralen + UVA (320-400 nm).
- Indications: Severe psoriasis, vitiligo, MF, Graft-versus-host disease (GVHD).
- Dosimetry: Minimal Phototoxic Dose (MPD).
- Side Effects: Acute: Nausea (oral), phototoxic erythema. Chronic: ↑ Squamous Cell Carcinoma (SCC), melanoma risk, photoaging. 📌 "P"soralen "P"otentiates "P"hotocarcinogenesis.
- General Side Effects (UVB/UVA): Acute: Erythema, pruritus, xerosis, blisters (if dose high). Chronic: Photoaging, Non-Melanoma Skin Cancer (NMSC).
- Contraindications: Absolute: Xeroderma Pigmentosum (XP), Systemic Lupus Erythematosus (SLE), Dermatomyositis (DM). Relative: Melanoma Hx, photosensitizing drugs.
⭐ NB-UVB (311-313 nm) is the most common phototherapy for psoriasis, offering an optimal balance of efficacy and safety, and is considered safe during pregnancy.
High‑Yield Points - ⚡ Biggest Takeaways
- NB-UVB (311 nm): Preferred for psoriasis, vitiligo; good efficacy/safety profile.
- UVB mechanism: Primarily immunosuppression and antiproliferative effects.
- PUVA (Psoralen + UVA): Forms DNA adducts; potent for psoriasis, mycosis fungoides; ↑ SCC risk.
- UVA1 (340-400 nm): Deepest penetration; for scleroderma, severe atopic dermatitis.
- Chronic risks: Photoaging, skin cancer. Eye protection crucial with PUVA.
- Dosing: MED (Minimal Erythema Dose) for UVB, MPD (Minimal Phototoxic Dose) for PUVA.
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