Photodermatitis

• Refers to symptoms, diseases, and conditions caused or aggravated by sunlight exposure.
• Rash due to photosensitivity is called photodermatosis.
  • Eczematous rash is photodermatitis.
• Chemical or drug causing photosensitivity is a photosensitiser.
  • Phototoxic reaction: exaggerated sunburn with no immune reaction.
  • Photoallergic reaction: photodermatitis due to delayed hypersensitivity.
• Photoexacerbated condition: flare of underlying skin disease due to sunlight.

Who gets photosensitivity?

• Occurs in males and females of all races and ages.
• Prevalence varies with age.
• Genetic and environmental factors involved.
• People with Fitzpatrick skin type 1 (very white skin, red hair, blue eyes) are often considered photosensitive relative to darker skin phototypes.

Classification of photosensitivity

1. Primary photodermatoses
   • Unknown causes, distinct disease entity on sun exposure:
     • Polymorphic light eruption
     • Juvenile spring eruption
     • Actinic folliculitis
     • Actinic prurigo
     • Solar urticaria
     • Chronic actinic/photosensitivity dermatitis
     • Hydroa vacciniforme
2. Exogenous photodermatoses
   • Phototoxic/photoallergic reaction due to external photosensitiser:
     • Drug-induced photosensitivity (e.g., thiazides, tetracyclines)
     • Photocontact dermatitis (e.g., psoralens, fragrances)
     • Pseudoporphyria (drugs, renal insufficiency)
3. Photoexacerbated dermatoses
   • Lupus erythematosus
   • Dermatomyositis
   • Darier disease
   • Rosacea
   • Pemphigus vulgaris
   • Pemphigus foliaceus
   • Atopic dermatitis
   • Psoriasis
4. Metabolic photodermatoses
   • Caused by metabolic defect:
     • Porphyrias (e.g., porphyria cutanea tarda)
     • Pellagra (vitamin B3 deficiency)
5. Genetic photodermatoses
   • Associated with genetic disorders:
     • Xeroderma pigmentosum
     • Bloom syndrome
     • Rothmund-Thomson syndrome
     • Cockayne syndrome
     • Hartnup disease

Causes

• Abnormal reaction to sunlight component and skin chromophore.
• Electromagnetic spectrum:
  • UVC: 200–290 nm
  • UVB: 290–320 nm
  • UVA: 320–400 nm (most common photosensitivity)

Clinical features

• Affects sun-exposed areas (face, neck, hands).
• May spare habitually exposed areas (e.g., face in polymorphic light eruption).
• Can follow indoor UVR exposure.
• Seasonal exacerbation, sharp cut-off between affected and covered skin, sparing of certain areas (e.g., upper eyelids).

Complications

• Severe photosensitivity can lead to inability to go outdoors, social isolation, and depression.
• Some photodermatoses cause permanent scarring.

Diagnosis

• Based on history and examination.
• Phototests: artificial light exposure to reproduce rash.
• Photopatch tests: test for contact photosensitivity.
• Investigations:
  • Full blood count
  • Connective tissue antibodies
  • Porphyrins in blood, urine, faeces
  • Liver function and iron tests for porphyria cutanea tarda

Treatment

• Sun protection and treatment of underlying disorder.

Prevention

• Careful sun protection and avoidance of artificial UVR.
• Protection measures:
  • Be mindful of UVR intensity and timing.
  • Use high SPF (50+) broad-spectrum sunscreens.
  • Wear protective clothing and UVR-absorbing film on windows.
  • Take vitamin D supplements if necessary.
  • Use antioxidants and nicotinamide.

Outlook

• Prognosis depends on specific disorder, treatment, location, and sun protection measures.
• Severely light-sensitive patients may need to curtail normal activities. Some opt for night work and sleep during the day.