Photodermatoses and Photosensitivity
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Direct answer
Photosensitivity is an abnormal skin reaction to ultraviolet or visible light, and it may be inherited, idiopathic or drug induced. Polymorphic light eruption is the commonest photodermatosis, while drug reactions are classified as phototoxic or photoallergic. Porphyria cutanea tarda and xeroderma pigmentosum are classic examples that appear repeatedly in exam questions.
What you must remember
- Polymorphic light eruption: the commonest photodermatosis, with itchy papules and vesicles on sun-exposed skin that often improve with repeated exposure.
- Phototoxic drug reaction: an exaggerated sunburn-like response that needs no prior sensitisation; examples include doxycycline, amiodarone and fluoroquinolones.
- Photoallergic drug reaction: a delayed eczematous reaction that needs prior sensitisation; examples include sulfonamides and ketoprofen.
- Erythropoietic protoporphyria: painful burning photosensitivity starting in childhood, without blistering.
- Porphyria cutanea tarda: blistering on the sun-exposed dorsal hands, linked with alcohol, hepatitis C and iron overload.
- Acute intermittent porphyria: neurovisceral attacks with abdominal pain and raised urinary porphobilinogen, without photosensitivity; variegate porphyria and hereditary coproporphyria can combine attacks with skin lesions.
- Xeroderma pigmentosum: an autosomal recessive nucleotide excision repair defect causing severe sun sensitivity and early skin cancers.
A pathway from photosensitivity to its cause
A 40-year-old man develops itchy blisters on the backs of both hands after each summer outing, and he is taking doxycycline for acne. The steps below separate a drug effect from a porphyria or a primary photodermatosis.
- Confirm the light link: establish that the rash appears only on sun-exposed skin and follows exposure, which confirms a photosensitive process.
- Review medicines: look for phototoxic or photoallergic drugs, and consider photopatch testing when a photoallergic cause is suspected.
- Test for porphyria: urinary porphyrins and porphobilinogen, plasma and red cell protoporphyrin, and stool porphyrins distinguish the porphyria types.
- Consider polymorphic light eruption: ask about the season, the age of onset and the distribution, since it is typically itchy and appears after the first sun exposures of spring.
- Screen for lupus: antinuclear antibodies and related tests exclude cutaneous lupus, which is also photosensitive.
- Counsel protection: broad-spectrum sunscreen, protective clothing and sun avoidance prevent recurrence in every type.
Assumptions that mislead
Most exam errors come from a single assumption about the cause, and the points below test the alternatives.
- Trap: assuming every photosensitive rash is caused by a drug Correct: polymorphic light eruption and porphyrias are common alternatives, and they need different investigations.
- Trap: calling acute intermittent porphyria a photodermatosis Correct: acute intermittent porphyria causes neurovisceral attacks without photosensitivity, whereas cutaneous porphyrias cause blistering or painful light sensitivity, and variegate porphyria can cause both.
- Trap: treating phototoxic and photoallergic reactions the same way Correct: phototoxic reactions occur in anyone taking the drug and resolve when it is stopped, while photoallergic reactions need patch-photo testing to identify the culprit.
- Trap: expecting sunscreen alone to cure polymorphic light eruption Correct: sunscreen prevents flares, while topical steroids, antihistamines and gradual UV exposure help treat them.
Frequently asked questions
Which photodermatosis is the commonest?
Polymorphic light eruption is the commonest photodermatosis, presenting as intensely itchy papules and vesicles on sun-exposed skin. It typically appears after the first sun exposures of spring and often improves with repeated exposure. Prevention uses broad-spectrum sunscreen and protective clothing.
Why does erythropoietic protoporphyria cause pain without blistering?
Erythropoietic protoporphyria results from ferrochelatase deficiency, which leads to protoporphyrin accumulation in the skin. Light activates the accumulated protoporphyrin, causing intense burning pain rather than a blister. Onset is usually in childhood, and the pain is the dominant feature.
Which drugs cause phototoxic skin reactions?
Doxycycline, amiodarone and fluoroquinolones are classic phototoxic drugs, producing an exaggerated sunburn-like reaction. This reaction does not need prior sensitisation, so it can occur on the first exposure. It usually resolves once the drug is stopped and sun exposure is avoided.
What is the defect in xeroderma pigmentosum?
Xeroderma pigmentosum is an autosomal recessive defect in nucleotide excision repair, which prevents the repair of ultraviolet-induced DNA damage. Affected patients show severe sun sensitivity, freckling and early skin cancers. Strict sun protection is the main preventive measure.