White : Diseases of the Skin - Photodermatology and Photodermatoses

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Gary M. White & Neil H. Cox

Diseases of the Skin

17	Photodermatology and Photodermotosess

APPROACH TO A PATIENT WHO HAS APPARENT PHOTOSENSITIVITY

In addition to a number of standard questions, which are applicable for any dermatosis, there are specific questions that have diagnostic importance in photodermatoses (summarized in Table 17.4). Some of these are aimed at distinguishing between specific photosensitivity disorders, but others aim to identify eruptions that may be confused with photosensitivity. For example, airborne contact dermatitis (i.e. contact dermatitis to an airborne allergen) may affect exposed skin and therefore mimic photosensitivity; in chronic actinic dermatitis, patients may have both photosensitivity and contact allergies.

Table 17.4 QUESTIONS AND CLINICAL SIGNS OF PARTICULAR RELEVANCE IN PHOTODERMATOSES AND ABNORMAL LIGHT SENSITIVITY

Age and sex of patient

How soon does the eruption occur after exposure to sun?

What time of year does the problem occur? (Mainly spring, mainly summer, all year?)

Type of and severity of symptoms (burning, itch, etc.)

Normal response to light (skin type) and relative amount required to produce the problem compared with ‘ordinary' sunburn

Type of light that produces the problem (sunlight, sunbed, both)

Effect of window glass: does the eruption still occur?

Medications (oral and topical) and alcohol intake (relevant in porphyria)

Other topical agents that may be implicated as photosensitizers

Family history of a similar problem

Other systemic symptoms

Sites affected by the eruption, and areas of sparing

Morphology of the eruption (erythema, urticarial, papular, blisters, etc.)

History and examination

Specific attention should be paid to the following.

•	Age and sex of the patient. The genodermatoses with photosensitivity are usually apparent in children; polymorphic light eruption usually has an onset in later childhood and is predominantly a disorder that occurs in women; actinic reticuloid is mainly (but not exclusively) a disorder of older men.
•	Timing of the eruption in relation to exposure. Solar urticaria occurs within seconds or minutes, whereas polymorphic light eruption typically occurs many hours after exposure.
•	Timing of the eruption in relation to the season. Polymorphic light eruption is typically prominent in the spring and improves later in the summer, recurring from year to year.
•	Type and severity of symptoms. Burning pain is typical of erythropoietic protoporphyria, while itch is more typical of most photosensitivity eruptions. This also needs to be put into context in terms of the previous ‘normal' sunlight response for the individual.
•	Type and amount of exposure required to provoke symptoms and response to photoprotection. This gives some indication of severity and may also be useful for determining the causative wavelengths (e.g. triggering by tanning beds implies that UVA is responsible).
•	Effect of window glass. This blocks UVB but not UVA, so development of the eruption when protected by window glass implies that UVA is responsible.
•	Medications. This should include a specific enquiry about systemic medications, topical agents (including sunscreens), and anything that the patient has purchased without specific medical advice. Some drugs, such as quinine for nocturnal cramps, are important photosensitizers but are often not viewed as medicines by patients.
•	External agents that may be applied. Several external agents may provoke photosensitivity, including some topical medications, sunscreens, perfumed agents (e.g. bath oils, perfumes, and perfumed moisturizers), and plants. It may also be important to know what type of sunscreen the patient has tried, as it may or may not be appropriate to prevent the problem (sunscreens are discussed in more detail at the end of this chapter).
•	Family history. This may be important for porphyrias and rarer genodermatoses, and is often positive in polymorphic light eruption.
•	Other symptoms and previous history. These may reveal preexisting dermatoses or symptoms suggestive of, for example, lupus erythematosus or variegate porphyria.
•	Pattern of involvement and sites of sparing. In the differential diagnosis between photosensitivity and airborne contact dermatitis, sparing of shielded areas (lower eyelids, beneath the nose, behind ears, and neck and face creases) is a useful indicator of photosensitivity (Figs 17.19 – 17.21). However, these areas of sparing are lost in more chronic photosensitivity, and lack of sparing does not exclude photosensitivity. Similarly, acute photosensitivity is usually confined to exposed skin, but extension of the rash to shielded areas occurs with chronicity.
•	Morphology of the eruption. Sheet-like erythema is suggestive of drug-induced phototoxicity, pure wheals occur in solar urticaria, polymorphic light eruption may have early urticarial features but eczematous morphology later, photoallergic eruptions tend to be more papular than those with phototoxic mechanisms, and blisters may occur in any severe photosensitivity but are a typical feature of porphyria cutanea tarda and reactions to plants (Fig. 17.22).

Figure 17.19 Photodermatitis showing a typical distribution pattern. There is a cut-off from short sleeves, and relative sparing of the ulnar border of the hand, finger webs, and distal fingers.

Figure 17.20 Photosensitivity due to a thiazide diuretic, showing sparing under clothing.

Figure 17.21 Photosensitivity due to a thiazide diuretic, showing sparing of skin creases on the neck.

Figure 17.22 Papules and blisters on the wrist due to contact and subsequent phototoxic reaction to plant sap; note the streaky pattern.

Phototests, patch tests, and photopatch tests

Provocation testing

Although this would appear to be a simple test, most photodermatoses cannot be easily provoked using small test areas under artificial conditions. One reason for this is that most photodermatoses are UVA-triggered, and routinely available UVA lamps require long exposures to achieve the required stimulus. Additionally, under test conditions, irradiation of relatively wide areas of skin is required on 2 or 3 consecutive days. Provocation tests:

•	are almost always positive in solar urticaria using small doses of UVR (usually UVA) and readings after a few minutes;
•	are variably positive in polymorphic light eruption (up to 90% positive using consecutive daily exposures with a solar simulator source) and systemic drug-induced phototoxicity (single exposure read at 24h, repeated on and off the drug); and
•	may be positive in lupus erythematosus using a UVB source.

Phototesting

Phototesting involves exposure of the skin to a series of doses of UVR to determine the MED. By comparison with the range of results for the normal population, it can be determined whether the patient has abnormal photosensitivity (i.e. an MED below the lower limit of normal).

The readily available fluorescent lamps are of limited value for testing, however: UVB-producing sources can achieve sufficient doses in a short time, but most photosensitivity is in the UVA range and standard UVA lamps require long exposure times to administer appropriate doses. Monochromator testing is the most accurate technique, using a series of narrow bands centered at specific wavelengths. This can be important, as some individuals are abnormally sensitive to a relatively narrow range of wavelengths. Monochromator testing also allows accurate repeat testing to monitor progression. The pattern of abnormal results in any individual may also be important in determining the likely cause (e.g. the wavelengths producing abnormal results may correlate with the known pattern for a photosensitizing drug).

Other test techniques include the use of metal halide lamps, solar simulators (optically filtered xenon arc lamps, which emit a spectrum of UVR similar to that of natural sunlight), phototherapy equipment (which is available in most dermatology departments), and other fluorescent tube lamps.

Patch testing

Patch testing is potentially helpful in patients for whom there is diagnostic uncertainty between photosensitivity and an airborne contact dermatitis (Fig.17.23). Allergies to balsams and perfume agents are particularly relevant in this differential diagnosis. Some allergens, notably the Compositae mix (an extract of plants in the daisy family), are of importance because positive results are common in chronic actinic dermatitis. Allergy to sunscreens may cause confusion in a small proportion of patients, and a sunscreen series is often tested in photosensitive patients. Most sunscreen allergy is due to fragrances or antimicrobials; allergy (or photoallergy, see later) to the sunscreen ingredient itself is much less common.

Figure 17.23 Airborne contact dermatitis (to balsams, probably in sawdust) affecting the face, V of the neck, and exposed areas of the arms. This can mimic photosensitivity.

Photopatch testing

Photopatch tests are used in investigation because some topically applied agents cause a contact dermatitis only when they are on the skin and are also exposed to light (i.e. they cause a photocontact allergic reaction). Sunscreens, balsams, and perfumes (Figs 17.24 – 17.26), and some reactions to plants, are particularly likely to do this. The technique of investigation involves applying a duplicate set of relevant patch tests. After 24h, one set of tests is removed, irradiated with UVA at a dose that is insufficient to cause erythema in itself (determined by previous phototesting), and reapplied for a further 24h before reading. Comparison of corresponding tests may indicate:

•	a standard contact reaction (both tests positive and equivalent);
•	a pure photocontact reaction (irradiated test positive, non-irradiated test negative); or
•	both contact and photocontact reactions (both tests positive, but greater reaction in the irradiated series).

Figure 17.24 Photocontact reaction to a fragranced bath additive. This demonstrates the typical cut-off at the neck where the light exposure is shielded, although the whole body was exposed to the fragrance material.

Figure 17.25 Photocontact reaction to a fragranced bath additive. This demonstrates the severity of the blistering in the patient shown in Figure 17.24.

Figure 17.26 Photocontact reaction to a fragranced bath additive, showing a positive photopatch test (labeled ‘Radox’). This was in the same patient as shown in Figures 17.24 and 17.25. The other (negative) tests were to a topical evening primrose oil preparation (‘EPO’) and a plant leaf with which the patient had been in contact.

PRACTICE POINTS

•	All effects of sunlight exposure, including freckling and tanning, indicate that some solar damage has occurred.
•	There are a number of specific questions that may help to elucidate the likely diagnosis or cause of photosensitivity, or that may help to advise about appropriate sun protection, that would not be asked in the dermatologic history for other types of disorder.
•	It may be very difficult to reliably differentiate between airborne contact allergy and photosensitivity without additional investigations.