# Photodermatoses: Polymorphous Light Eruption and Beyond

## Introduction

Photodermatoses are a heterogeneous group of disorders in which skin disease is induced or exacerbated by ultraviolet (UV) or visible light exposure. Understanding the action spectra, immunopathogenesis, and clinical distinctions among these conditions is essential for accurate diagnosis and management.

## Classification of Photodermatoses

### Immunologically Mediated

The immunologically mediated photodermatoses include **polymorphous light eruption (PMLE)**, the most common photodermatosis, affecting up to 10 to 20% of the population in temperate climates. **Actinic prurigo** is a chronic variant with familial tendency, strongly associated with **HLA-DR4 (DRB1*0407)**. **Hydroa vacciniforme** is a rare, vesiculonecrotic eruption of childhood in which EBV-associated lymphoproliferative disorder must be excluded. **Solar urticaria** produces an immediate wheal-and-flare response to UV or visible light within minutes of exposure. **Chronic actinic dermatitis (CAD)** is a persistent eczematous photosensitivity that predominantly affects elderly men.

### Metabolic (Porphyrias)

The metabolic photodermatoses include **erythropoietic protoporphyria (EPP)**, which causes painful, non-blistering photosensitivity with elevated free erythrocyte protoporphyrin. **Porphyria cutanea tarda (PCT)** causes blistering on the dorsal hands due to reduced uroporphyrinogen decarboxylase activity. **Variegate porphyria** and **hereditary coproporphyria** produce combined acute neurovisceral and cutaneous features.

### DNA Repair Defects

DNA repair defect photodermatoses include **xeroderma pigmentosum (XP)**, an autosomal recessive nucleotide excision repair deficiency that dramatically increases skin cancer risk. **Cockayne syndrome** and **trichothiodystrophy** present with photosensitivity accompanied by developmental abnormalities.

| Photodermatosis | Category | Action Spectrum | Key Feature | Treatment |
|----------------|----------|----------------|-------------|-----------|
| PMLE | Immunologic | UVA > UVB | Polymorphous between patients, monomorphous within | Photoprotection; prophylactic NB-UVB; HCQ |
| Actinic prurigo | Immunologic | UVA/UVB | Chronic; HLA-DR4 associated | Thalidomide; photoprotection |
| Solar urticaria | Immunologic | Variable (UVA/UVB/visible) | Immediate wheal-and-flare | Antihistamines; omalizumab |
| Chronic actinic dermatitis | Immunologic | UVB ± UVA | Persistent eczema; may mimic CTCL | Strict photoprotection; immunosuppressants |
| EPP | Metabolic (porphyria) | Visible light (400–410 nm) | Painful, non-blistering; elevated FEP | Afamelanotide; photoprotection |
| PCT | Metabolic (porphyria) | UVA (Soret band ~400 nm) | Blistering dorsal hands; skin fragility | Phlebotomy; HCQ; iron reduction |
| Xeroderma pigmentosum | DNA repair defect | UVB/UVA | Skin cancers before age 10 | Strict UV avoidance |

## Polymorphous Light Eruption: Deep Dive

### Epidemiology and Pathogenesis

PMLE is the most common photodermatosis, with highest prevalence in young women and onset typically in the first three decades of life. A characteristic feature is the **hardening phenomenon**, in which lesions tend to improve as summer progresses due to UV-induced immunosuppression tolerance. The pathogenesis involves a failure of UV-induced immunosuppression, leading to a **delayed-type hypersensitivity reaction** against an as-yet-unidentified photoantigen. Langerhans cell persistence in UV-exposed skin is a hallmark finding, reflecting this failure of normal UV-mediated immune downregulation.

### Clinical Features

The eruption occurs **hours to days** after sun exposure, typically in spring or early summer. A critical teaching point is that the **morphology is polymorphous between patients but monomorphous within an individual**, meaning that each patient consistently develops the same lesion type. Variants include papular (the most common), papulovesicular, plaque-type, erythema multiforme-like, and pinpoint papular forms. The distribution favors **sun-exposed sites** that are spared by habitual exposure, such as the V of the chest and forearms, with the face less commonly affected.

### Diagnosis

The diagnosis is clinical in most cases. **Phototesting with UVA** (and sometimes UVB) can reproduce lesions in 50 to 70% of patients. Biopsy shows **dermal edema with a perivascular lymphocytic infiltrate**. It is essential to exclude lupus erythematosus by checking ANA, anti-Ro/La antibodies, and performing direct immunofluorescence.

<image>Clinical photograph showing papulovesicular PMLE lesions on sun-exposed forearms and V of chest with sparing of habitually exposed facial skin</image>

### Management

**Photoprotection** with broad-spectrum UVA/UVB sunscreen (SPF 50+), protective clothing, and behavioral avoidance is the foundation of management. **Prophylactic phototherapy** using a spring course of narrowband UVB (NB-UVB) or PUVA can induce hardening before the high-exposure season. **Topical corticosteroids** are used for acute flares, and **oral prednisolone** as a short course can manage severe episodes (for example, 25 mg tapering over 7 to 10 days). **Hydroxychloroquine** 200 mg twice daily serves as prophylaxis in refractory cases. Emerging options include **afamelanotide** (an alpha-MSH analogue) and oral polypodium leucotomos extract.

## Solar Urticaria

Solar urticaria has a variable **action spectrum** that may involve UVA, UVB, or visible light, and is classified by action spectrum and serum factor transferability. The mechanism involves **IgE-mediated hypersensitivity** to a photoallergen generated in the skin. Treatment centers on **non-sedating H1 antihistamines**, phototherapy desensitization, and omalizumab for refractory cases.

<image>Diagram illustrating the electromagnetic spectrum highlighting UVB (290-320nm), UVA (320-400nm), and visible light (400-700nm) ranges with corresponding photodermatoses mapped to their action spectra</image>

## Chronic Actinic Dermatitis

Chronic actinic dermatitis represents a spectrum formerly called **photosensitivity dermatitis/actinic reticuloid syndrome**. It presents as persistent eczematous dermatitis on photoexposed sites that may extend to covered areas. **Monochromator phototesting** shows reduced MED to UVB and often UVA. Histology may mimic **cutaneous T-cell lymphoma** in a pseudolymphoma pattern, making biopsy interpretation challenging. Treatment includes strict photoprotection, **potent topical corticosteroids**, tacrolimus, azathioprine, cyclosporine, and mycophenolate mofetil.

## Actinic Prurigo

Actinic prurigo typically presents in childhood, often accompanied by **cheilitis** and conjunctivitis. There is a strong association with **HLA-DR4 subtype DRB1*0407**, found in up to 90% of affected individuals in some populations, and the condition is particularly prevalent in Latin American indigenous populations. Treatment includes photoprotection and **thalidomide** (50 to 100 mg/day), which is the most effective agent, along with cyclosporine for refractory disease.

<image>Histopathology slide of PMLE showing prominent papillary dermal edema with dense perivascular lymphocytic infiltrate in the upper and mid-dermis</image>

## Diagnostic Approach to the Photosensitive Patient

The diagnostic approach begins with a thorough **history** assessing timing of eruption relative to sun exposure, action spectrum clues, seasonal pattern, and drug history. **Examination** should focus on the distribution pattern, comparing photoexposed versus photoprotected sites and noting sparing under the chin and behind the ears. **Investigations** include phototesting (MED to UVB/UVA), photoprovocation testing, ANA, anti-Ro/La, and porphyrin studies (urine, blood, stool). **Biopsy** is performed when needed to distinguish from lupus, dermatomyositis, or lymphoma.

## Key Clinical Pearls

PMLE is monomorphous within an individual patient despite the name "polymorphous," a fact that often trips up trainees. Lupus erythematosus must always be excluded in persistent photodermatoses by checking ANA, ENA, and considering DIF. The hardening phenomenon in PMLE, in which the condition improves with continued sun exposure, distinguishes it from lupus, which worsens with UV exposure. Actinic prurigo presenting with cheilitis in a child of Latin American descent should prompt consideration of HLA-DR4 and thalidomide therapy. Solar urticaria can cause anaphylaxis with extensive exposure, and patients should be counseled accordingly.

## References

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