Photodiagnostic Service

These conditions can be broadly classified as idiopathic (the commonest being polymorphic light eruption), drug-induced, porphyrias, genophotodermatoses and photoaggravated skin diseases. Severe photosensitivity can have a major adverse impact on quality of life and can cause very serious morbidity. It is essential that a correct diagnosis and management plan is made in order to minimise the adverse consequences of photosensitivity on a patient’s health and lifestyle and the only way that this is possible is if assessment is undertaken in a specialist Unit, such as the Scottish Photobiology Service, based in the Photobiology Unit (PBU) in Dundee, where the appropriate equipment and skills are in place to undertake detailed investigation and interpretation of findings in the clinical setting.

If a patient is suspected to be abnormally sensitive to light, then a range of diagnostic investigations can be carried out so that a correct diagnosis and appropriate management plan can be made. Within the electromagnetic spectrum, it is the ultraviolet (UVB & UVA) and, to a lesser extent, visible light that produces most of the abnormal responses elicited and it may be that the skin is affected by any or all of these spectra.

To establish which spectra are responsible, monochromator phototesting is carried out. A monochromator is a device that disperses light into its constituent wavelengths, which the patient is then exposed to. Bentham irradiation monochromators are the devices used in the Photobiology Unit to measure skin sensitivity to ultraviolet and visible radiation. The equipment comprises of a 450W xenon arc lamp with elliptical mirror coupled to a monochromator. Order-sorting filters are mounted on a filter wheel and the light is transmitted to the patient by way of a liquid light guide. The irradiation monochromator is driven by a precision motor under computer control. Irradiance levels are measured using a calibrated photodiode and integrating sphere. Calibration is traceable to the National Physical Laboratory.

This procedure is painless and involves the patient sitting on a chair whilst the light guide is positioned against the back for various lengths of time equating to different wavelengths and doses of UV and visible light. At each of the wavelengths a minimal erythema dose (MED) is defined, that is the minimum amount of light required to induce a just perceptible redness on the skin. This value is compared to normal ranges that have been developed over a number of years and from this we can determine whether or not a patient has abnormal photosensitivity.

Provocation testing which is designed to try to provoke the abnormal response that the patient experiences when exposed to sunlight. Normally a 4cm2 area is exposed to a broadband light source (UVA and visible light), therefore the patient is exposed to a wide range of wavelengths rather than monochromatic wavelengths. One to three repeat exposures to this light source may be necessary to induce the abnormal response that some patients experience on exposure to natural light. This response may then be biopsied to give the clinicians more diagnostic information. Some patients will not develop an abnormal response on provocation testing, although that can also be helpful in making the diagnosis.

Patch testing is a technique for detecting an allergic response to substances that come into contact with the skin. Photopatch testing is a technique for detecting photoallergic responses to substances, usually sunscreen chemicals, in the presence of light. For both methods, small amounts of the test substances are applied to aluminum chambers, which adhere to hypoallergenic tape strips. In patch testing, the strips of chambers are applied to the skin (normally the upper back) and kept in place for 48 hours. However, for photopatch testing duplicate sets are applied to the skin, kept in place for 24 hours only and then one set is irradiated with UVA light. The amount of UVA light is small and unlikely to cause any skin reaction on its own. The test sites for both patch and photopatch tests are kept under observation for 48 hours after removing the strips of chambers and results recorded accordingly.

Solar simulator phototesting is a technique used to measure skin sensitivity to a range of ultraviolet and visible radiation wavelengths. The Oriel® Sol UVTM Solar Simulator comprises of a filtered 1600 W Xenon Arc lamp together with a power supply suitable for the proper operation of high power Arc lamps. The system also includes an illuminator with a built-in Arc lamp ignitor, a shutter switch and a heat sink. The spectral output represents the solar spectrum and therefore may be used to mimic and measure skin sensitivity to solar radiation. Three filters are installed in the system ensuring appropriate output radiation within a defined wavelength region of the UV/visible spectrum: the UG11 filter reduces the visible radiation output; the UVA filter set blocks the UVB radiation output; and Filter #2 allows for both UVA and UVB radiation output (whole spectrum). A partial sun attenuator within the system could be adjusted to control the output irradiance level. This was pre-set to allow for maximum output irradiance. At a 10 cm working distance, the beam size is approximately 12 cm circular.

The output from the solar simulator complies with the required Relative Cumulative Erythemal Effectiveness (%RCEE) values of the acceptance limits, which are stipulated by the Cosmetic, Toiletry and Perfumery Association (COLIPA).

Other investigations include: minimal erythema dose (MED) testing, phototesting to compact fluorescent lamps (CFLs), infrared testing and investigations for physical urticarias, lupus and the porphyrias.