Photo-dermatoses are a group of skin disorders that are precipitated by exposure to sunlight.  UVA is largely responsible for the photodermatoses and this penetrates through ordinary glass.  They can be broadly classified into two groups:

Photosensitive Dermatoses- these only happen as a result of light.  Most are immunologically mediated except those related to chemicals (drugs,porphyria) and DNA repair (Xeroderma pigmentosum).

Photo-aggravated Dermatoses:

These are often underlying skin disorders can also be exacerbated by sunlight in exposed area of skin.  Examples include:

Darier’s disease (A rare, autosomal dominantly inherited condition where dark, warty, papule appear on the skin), Herpes Simplex,  Systemic lupus erythematosus, Rosacea, Vitiligo.

The Photo-sensitive dermatoses can be divided by their aetiology as follows:

Idiopathic.-Polymorphic light eruption, Actinic Prurigo, Chronic actinic Dermatitis, solar urticaria,  Hdroa vacciniforme.

Genetic- Xeroderma Pigmentosum

Metabolic- Porphyria

Exogenous- drug-induced photosensitivity, photocontact allergic reactions, Phytophotodermatits.

Polymorphic Light eruption- this is a rash that itches or burns, which appears  1-2 days after exposure to light.  The rash can have small red bumps, larger red areas, and blisters. The rash comes up equally on both sides of the body, affecting mainly those parts of the skin that are kept covered over the winter.  It is more common in spring and improves as the summer progresses and affects 10% of the population.  There is no cure and the cause is unknown. The same rash will appear in the patient every year but may eventually disappear. Wearing a proper sunscreen and following the tips on sun protection- Protecting yourself for the sun will help matters. A topical steroid cream may help when the rash comes up.

Actinic Prurigo – This is a rare, less than 1:1000, type of skin sensitivity induced by sunlight. It is more common in women than men with a ratio of 2:1.  In actinic prurigo the skin has been itched and scratched for a long time and becomes firm, raised and itchy on the areas of the skin surface that are exposed to sun. This condition is genetically inherited in 60-70% of patients. The main symptoms of actinic prurigo are severe itching and painful inflammation of sun-exposed skin. Lips can become dry, sore and cracked and the eyes can develop conjunctivitis. The rash appears hours or days after exposure to sunlight and the association isn’t always recognised by the patient. This rash appears on sun-exposed areas of the body but also can affect non-exposed ares for example the buttocks. A Dermatologist will make the diagnosis of actinic prurigo by assessing the rash and the patient’s history. Special blood tests of the immune system and urine test and phototesting might also be suggested. Proper sun protection is essential.

Chronic Actinic Dermatits-Patients with CAD are oversensitive to sunlight and get an excematous skin reaction after very brief exposure to light, it is usually strikingly limited to sun-exposed areas. It is usually worse in summer where sunlight is at its strongest. The skin is red, hot, swollen,itchy and flakey.  These patients will usually be referred to the Dermatologist.  Usually patch testing and photopatch testing will be suggested to identify chemical allergies. There is no cure for CAD. The most important aspect of managing CAD is to protect the skin from sunlight. 

Solar urticaria- this describes a fairly rare type of urticarial rash (wheals, hives) which is triggered by exposing the skin of susceptible patients to sunlight and affects both exposed and non-exposed areas of skin. It can start at any age usually between 20-40 years old.  Staying out of the sun will help, also an anti-histamine will help alleviate symptoms. 

Hydroa Vacciniforme-HV is a very rare form of scarring photodermatoses that appears in childhood.

Xeroderma Pigmentosum-XP is a very rare disorder where the patient is highly sensitive to sunlight, has premature skin ageing and is prone to developing skin cancers. It is as a result of a defect in the DNA repair system. 

Porphyria – Is a group of disorders that cause nerve or skin problems. A porphyria that affects the skin is called cutaneous porphyria.

Exogenous forms of photodermatoses.

Drug induced- the most common drugs associated with phytotoxic reactions are tetracycline or doxycycline antibiotics- these are widely prescribed for the long-term treatment of acne vulgaris or rosacea. Ciprofloxacin or other quinolone antibiotics, Psoralens( PUVA- a light treatment in the Dermatology hospital or some plants), amiodarone,Nalidixic acid.

The most common drugs associated with a photo-allergic response are quinine, diuretics, carbamazepine, naproxen and other NSAIDs. The most important aspect of management is to try and find out which drug is responsible. However a patient can be on the drug for years without a problem and then develop a reaction (and they might not believe you!) and it can take many months for the drug to improve after cessation of the drug. 

Photocontact dermatitis:  A toxic or allergic reaction may occur when certain chemicals are applied to the skin and subsequently exposed to the sun. Photocontact dermatitis arise from an interaction between UV radiation and one or more of the products listed below: some sunscreens, oxybenone or cinnemates and others.  Coal tar products, (used in psoriasis),  Fragrances e.g. musk, Insecticides and disinfectants. These products contain drugs or chemicals that are photo-sensitising agents. The reaction can be photo-toxic or photo-allergic. Photo-toxic reactions result from direct damage to tissue caused by light activation of the photosensitising agent. Photo-allergic reactions are a cell mediated immune response in which the antigen is the light-activated photosensitising agent. 

Phyto-photodermatits.  This is a form of photo-contact dermatitis which arises form the interaction of UV radiation and photosensitising compounds found in various plants. The most common plant family to cause phytophotodermatitis is the Umbelliferae family.  This includes parsnips,celery,parsley,Cowparsnip,hogweed, Giant hogweed. The Rutacea family ( bergamot line, citron, zabon) , Morracea (figs) and Leguminosa (Beans) are other plant families.

The characteristic appearance of phyto-photodermatiits is blisters and brown streaks occurring from touching certain plants followed by sun exposure. These patterns can result from brushing against a plant’s stem or leaves or squeezing lime juice down your arm. Blisters or swelling may appear minutes to hours after exposure to the plant and light, but more usually erupt about 24 hours after exposure, peaking at 48-72 hours. Skin lesions may leave behind dark marking on the skin ( hpyer-pigmentation).

The main goal of treatment is identifying the offending agent and avoiding it.  Patch testing may be needed. Wearing sunscreen and sun-protective clothing will also help. If allergic to a sunscreen agent, choose one without the responsible chemical or, select a low irritant formula that relies on metal oxides such as zinc oxide or titanium dioxide.