Photodermatosis: what is it?

By Cristiano Antonino On May 6, 2023

The term ‘photodermatosis’ – also known as ‘photosensitivity’ or ‘sun allergy’ – refers to a rather abnormal reaction of the skin to exposure to sunlight

What is photodermatosis

The risks of excessive and unintended exposure to the sun’s rays have been known for years, which is why – to avoid the possible appearance of tumour formations such as melanomas – moderate exposure mediated through high UVA and UVB protection filters is always recommended.

These precautions, however, may not be sufficient for some subjects.

The subjects referred to are those suffering from photodermatosis: an excessive skin reactivity to exposure to ultraviolet rays (UVA and UVB), whether of natural origin (sun) or of artificial origin (sunlamps).

A photosensitive subject, after a minimal exposure to the action of the sun’s rays – certainly not enough exposure to cause damage in a healthy subject – experiences a deep-seated affection of the skin, which will present with urticaria, erythema or solar allergy.

This symptomatology is indicative of a true photoallergic or phototoxic reaction to the sun.

Phototoxic reaction

The phototoxic reaction to ultraviolet rays appears in the photosensitive individual within a 24-hour period from the time of exposure, manifesting itself through overt irritation or exaggerated sunburn in the portion of the skin that has been exposed to the sun.

This occurs because ultraviolet rays, on contact with the skin, react with a photosensitising substance that is activated and transformed into toxic compounds, which will irritate the affected portion of the skin.

The immune system is powerless against this reaction.

Photoallergic reaction

The photoallergic reaction to ultraviolet rays appears in the photosensitive individual within a longer period of time, generally 24 to 72 hours from the time of exposure of the skin to the sun’s rays.

In contrast to a phototoxic reaction, with a photoallergic reaction the immune system intervenes through a cell-mediated immunological response.

Exposure eruptions – with red, itchy, flaky patches and blisters – therefore tend to appear first in the areas affected by the action of ultraviolet rays, and then spread to areas that were not affected by the exposure.

The photoallergic reaction occurs – as with almost every allergic manifestation – in individuals previously sensitised by repeated exposure to the allergen over time or due to a topically applied drug that was then exposed to the action of the sun’s rays.

The causes of photodermatosis

According to their aetiology, the causes leading to photodermatosis can be classified into four macro-groups.

Causes of idiopathic origin

In this case, the actual causes of photodermatosis are unknown.

Exposure to ultraviolet rays may produce solar urticaria, polymorphous light rash, chronic actinic dermatitis, actinic prurigo.

Iatrogenic causes

When the causes are iatrogenic, it means that the photosensitisation is a consequence of local application or oral administration of photosensitising drugs, such as amiodarone, tetracyclines, antifungals, retinoids, diuretics.

The use of certain cosmetics, chemicals, perfumes, dyes or disinfectants may also cause photosensitivity.

Metabolic causes

One speaks of metabolic photodermatosis when it is a consequence of a metabolic defect or imbalance; these are the cases of pellagra or porphyria sufferers.

Genetic causes

On the other hand, we speak of genetic photodermatoses when the photodermatosis is the consequence or manifestation of a pre-existing genetic disease, such as albinism, Bloom syndrome or Rothmund-Thomson syndrome.

Photodermatosis: recognising the symptoms

Photodermatosis – depending on the characteristics of the photosensitive individual – may manifest itself with different symptoms and of different intensity, just as the degree of exposure to the action of ultraviolet rays necessary to trigger the allergic reaction is different.

As previously explained, the inflammatory response of the skin can be either phototoxic or allergic; in both cases the areas most susceptible to photosensitisation are the face, arms and upper chest.

These areas, when exposed to the action of the sun’s rays, may show reddening, swelling, pain, urticaria, eczema, itchy rashes, blisters in the photosensitive subject.

Hyperpigmented areas may also appear.

As a consequence of this symptomatology, some systemic complications – typical of excessive sun exposure – such as chills, headache, fever, nausea, fatigue and dizziness may occur.

If the subject has chronic photodermatosis, his skin will tend to scar after each exposure and thicken.

If the photodermatosis is of genetic origin, there is an increased likelihood of that person developing skin tumours.

Diagnosing and treating photodermatosis

From the moment one experiences exaggerated skin reactions after even minimal exposure to ultraviolet rays, it would be necessary to contact a dermatology specialist to try to identify the cause of the onset of photosensitivity.

The dermatologist, in order to be able to specifically and unequivocally identify the type of photo-induced reaction, will initially proceed with the performance of an objective test and with the collection of complete information for an accurate anamnesis.

To further define the patient’s clinical picture, some blood and urine tests may also be requested, useful for identifying any related diseases or to exclude the presence of metabolic or genetic causes.

An allergy test (photo-patch or photo-test) may also be requested by the dermatologist to try to identify more precisely what substances may have caused or worsened the patient’s photodermatosis.

Once the diagnosis has been made, the dermatologist will prescribe the most appropriate therapy for the patient to follow to try to resolve – or at least reduce – the photosensitivity.

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Some types of photodermatosis can be treated with phototherapy: a portion of the dermis is exposed to controlled light exposure to desensitise or try to control the symptoms.

Alternatively or concomitantly, the dermatologist may also prescribe drug therapy, usually based on antihistamines to reduce itching; steroids to reduce inflammation; glucocorticoids to control rashes; or immunodepressants to suppress the immune system’s action, especially in patients who are extremely sensitive to the sun.

Especially these particularly sensitive subjects may not be suitable for treatment with phototherapy, as an alternative to which hydroxychloroquine, thalidomide, beta-carotene or nicotinamide will be administered to make the skin more resistant to the harmful action of ultraviolet rays.

Photodermatosis: possible complications

Most cases of photodermatosis are only transient or easily resolved.

Once the causative agent of photosensitivity is identified, photodermatosis tends to regress and resolve completely spontaneously.

When photodermatosis is induced by genetic or metabolic causes, certain complications may arise that should not be underestimated.

Dark spots or hyperpigmented areas may appear on the skin, even after the irritation has disappeared; the skin may age prematurely; the formation of basal cell carcinoma of the skin, spinocellular carcinoma or melanoma may occur.

For this reason, anyone diagnosed with chronic photodermatosis is advised to take certain precautions at all times of the year to protect their skin from the harmful action of the sun’s rays, such as planning outdoor activities only during the cooler hours of the day, applying a broad-spectrum sunscreen at regular intervals, and always wearing protective clothing and sunglasses to protect their skin from the harmful action of the sun’s rays.

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