Solar urticaria is a rare condition in which exposure to ultraviolet or UV
radiation, or sometimes even visible light, induces a case of urticaria or
hives that can appear in both covered and uncovered areas of the skin. It is
classified as a type of physical urticaria. The classification of disease
types is somewhat controversial. One classification system distinguished
various types of SU based on the wavelength of the radiation that causes
the breakout; another classification system is based on the type of allergen
that initiates a breakout. The agent in the human body responsible
for the reaction to radiation, known as the photoallergen, has not yet been
identified. The disease itself can be difficult to diagnose properly because
it is so similar to other dermatological disorders, such as polymorphic light
eruption or PMLE. The most helpful test is a diagnostic phototest, a specialized
test which confirms the presence of an abnormal sunburn reaction. Once
recognized, treatment of the disease commonly involves the administration of
antihistamines, and desensitization treatments such as phototherapy. In more
extreme cases, the use of immunosuppressive drugs and even
plasmapheresis may be considered. The initial discovery of the disease is
credited to P. Merklen in 1904, but it did not have a name until the suggestion
of "solar urticaria" was given by Duke in 1923. However, their research
contributed to the study of this uncommon disease. More than one hundred
cases have been reported in the past century.
Signs and symptoms Generally, the areas affected are
exposed skin not usually protected by clothing; however it can also occur in
areas covered by clothing. Areas constantly subjected to the sun's rays
may only be slightly affected if at all. People with extreme cases will also have
reactions to light bulbs that emit a UV wavelength. Parts of the body only
thinly covered can also potentially be subjected to an outbreak.
Life with SU can be difficult. Patients are subject to constant itching and
pain, as within minutes of the initial exposure to UV radiation a rash will
appear. The urticarial reaction begins in the form of pruritus, later
progressing to erythema and edema in the exposed areas of the skin. If vast areas
of the body are affected, the loss of fluid into the skin could lead to
light-headedness, headache, nausea, and vomiting. Extremely rarely, patients
have been reported to experience an increase in heart rate that can cause a
stroke or heart attack due to the body cavity swelling. Other rare side effects
can be bronchospasm and glucose instability issues. Also, if a large
area of the body is suddenly exposed the person may be subject to an anaphylactic
reaction. Once free of exposure, the rash will usually fade away within
several hours; rare and extreme cases can take a day or two to normalize
depending on severity of the reaction. Causes
Solar urticaria is an immunoglobulin E-mediated hypersensitivity that can be
introduced through primary or secondary factors, or induced by exogenous
photosensitization. Primary SU is believed to be a type I hypersensitivity
in which an antigen, or substance provoking an immune response, is
"induced by UV or visible radiation." Secondary SU can occur when a person
comes into contact with chemicals such as tar, pitch, and dyes. People who use
drugs such as benoxaprofen or patients with erythropoietic protoporphyria may
also contract this secondary form. These items that cause this photosensitivity
are exogenous photosensitizers because they are outside of the body and cause
it to have a greater sensitivity to light.
Also, there have been a few unorthodox causes of solar urticaria. For those
susceptible to visible light, white T-shirts may increase the chances of
experiencing an outbreak. In one case, doctors found that the white T-shirt
absorbed UVA radiation from the sun and transformed it into visible light which
caused the reaction. Another patient was being treated with the antibiotic
tetracycline for a separate dermatological disorder and broke out in
hives when exposed to the sun, the first case to implicate tetracycline as a
solar urticaria inducing agent. It is not yet known what specific agent
in the body brings about the allergic reaction to the radiation. When patients
with SU were injected with an irradiated autologous serum, many developed
urticaria within the area of injection. When people who did not have SU were
injected, they did not demonstrate similar symptoms. This indicates that
the reaction is only a characteristic of the patients with solar urticaria and
that it is not phototoxic. It is possible that this photoallergen is
located on the binding sites of IgE that are found on the surface of mast cells.
The photoallergen is believed to begin its configuration through the absorption
of radiation by a chromophore. The molecule, because of the radiation, is
transformed resulting in the formation of a new photoallergen.
Diagnosis Solar urticaria can be difficult to
diagnose, but its presence can be confirmed by the process of
phototesting. There are several forms of these tests including photopatch tests,
phototests, photoprovocation tests, and laboratory tests. All of these are
necessary to determine the exact infliction that the patient is suffering
from. Photopatch tests are patch tests conducted when it is believed that a
patient is experiencing certain symptoms due to an allergy that will only occur
when in contact with sunlight. After the procedure, the patient is given a low
dosage of UVA radiation. Another test known as a phototest is the
most useful in identifying solar urticaria. In this test, one centimeter
segments of skin are subject to varying amounts of UVA and UVB radiation in
order to determine the specific dosage of the certain form of radiation that
causes the urticaria to form. When testing for its less intense form,
phototesting should be conducted only in the areas where the hives have appeared
to avoid the possibility of getting false-negative results.
A third form of testing is the photoprovocation test which is used to
identify disorders instigated by sun burns. The process of this test involves
exposing one area of a patient's arm to certain dosage of UVB radiation and one
area on the other arm to a certain dosage of UVA radiation. The amount of
radiation that the patient is exposed to is equal to that "received in an hour of
midday summer sun." If the procedure produces a rash, then the patient will
undergo a biopsy. Finally, there are laboratory tests which generally involve
procedures such as blood, urine, and fecal biochemical tests. In some
situations, a skin biopsy may be performed.
= Classification = Solar urticaria, due to its particular
features, is considered to be a type of physical urticaria or light sensitivity.
Physical urticaria arises from physical factors in the environment, which in the
case of solar urticaria is UV radiation or light. SU may be classified based on
the wavelength of the radiative energy that causes the allergic reaction; known
as Harber's classification, six types have been identified in this system.
Type I solar urticaria is caused by UVB radiation, with wavelengths ranging from
290–320 nm. Type II is induced by UVA radiation with wavelengths that can
range from 320–400 nm. The wavelength range of type III and IV spans from 400
to 500 nm, while type V can be caused by UVB radiation to visible light. Type VI
has only been known to occur at 400 nm. Another classification distinguishes two
types. The first is a hypersensitivity caused by a reaction to photoallergens
located only in people with SU; while the second is caused by photoallergens
that can be found in both people with SU and people without it.
A subgroup of solar urticaria, fixed solar urticaria, has also been
identified. It is a rare, less intense form of the disease with wheals that
affect certain, fixed areas of the body. Fixed solar urticaria is induced by a
broad spectrum of radiative energy with wavelengths ranging from 300–700 nm.
= Differential diagnosis = Polymorphous light eruption is the
easiest disease to mistake for solar urticaria because the locations of the
lesions are similar. However, patients with SU are more likely to develop
lesions on the face. Also, a reaction with PMLE will take a greater amount of
time to appear than with solar urticaria. Lupus erythematosus has been
mistaken for SU; however, lesions from lupus erythematosus will take a longer
amount of time to go away. Furthermore, when being tested for the two diseases,
patients with SU have a reaction immediately while patients with lupus
erythematosus will have a delayed reaction. Patients who have experienced
solar urticarial symptoms from a young age could mistakenly be thought to have
erythropoietic protoporphyria. However, the main symptom for this disease is
pain and patients with have been found to have abnormal levels of
protoporphyrin in their blood while these levels are normal in SU patients.
Finally, cholinergic urticaria, or urticaria induced by heat, can
occasionally appear to be solar urticaria because the heat from the sun
will cause a person with the disease to have a reaction.
Management = Antihistamines =
Histamines are proteins associated with many allergic reactions. When the UV
radiation or light comes in contact with a person with solar urticaria, histamine
is released from mast cells. When this occurs, the permeability of vessels near
the area of histamine release is increased. This allows blood fluid to
enter the vessels and cause inflammation. Antihistamines suppress
the activity of the histamine. Diphenhydramine, a first-generation H1
receptor antagonist or medicine that combats the H1 receptor that is
associated with many allergic reactions, has been found to be the most potent
antihistamine for this particular disease. Patients prescribed 50
milligrams four times per day have been able to sustain normal exposure to the
sun without suffering a reaction. Patients with less potent forms of solar
urticaria such as fixed solar urticaria can be treated with the medication
fexofenadine, which may also be used prophylactically to prevent recurrence.
= Desensitization = This form of treatment is meant to
reduce the intensity or altogether eliminate the allergic reactions people
have by gradually increasing exposure to the form of radiation that brings about
the reaction. In the case of solar urticaria, phototherapy and
photochemotherapy are the two major desensitization treatments.
Phototherapy can be used for prevention. Exposure to a certain form of light or
UV radiation enables the patient to build up a tolerance and outbreaks can
be reduced. This type of treatment is generally conducted in the spring.
However, the benefits of this therapy only last for two to three days.
Photochemotherapy, or PUVA, is considered superior to phototherapy
because it produces a longer-lasting tolerance of the radiation that
initiates the outbreak. When treatment first begins, the main goal is to build
up the patient's tolerance to UVA radiation enough so that they can be
outdoors without suffering an episode of solar urticaria. Therefore, treatments
are regulated at three per week while constantly increasing the exposure to
UVA radiation. Once the patient has reached an adequate level of
desensitization, treatments are reduced to once or twice per week.
= Immunosuppression = Doctors will sometimes prescribe
immunosuppressive drugs such as prednisolone and ciclosporin if the
patient is suffering from an intense form of solar urticaria. However, the
side effects of these medicines can be severe which is why they are reserved
for the most extreme of cases. = Plasmapheresis =
In more extreme cases, plasmapheresis can be considered. This technique is
used to remove the blood plasma or fluid in the red blood cells and then return
the cells to the body. It "removes a circulating factor from the blood that
may be involved in causing the urticaria," but is still being tested
and is not always effective. When the treatment is a success, the patient's
photosensitivity is decreased to the degree that they can undergo PUVA which
can result in the relief of the urticarial outbreaks for an extended
period of time. The major setback to this treatment is that the side effects
can be severe and may include anaphylactoid reactions.
Epidemiology In the United States, only about 4% of
patients with photosensitive disorders are reported to have been diagnosed with
solar urticaria. Internationally, the number is slightly larger at 5.3%. Solar
urticaria may occur in all races but studies monitoring 135 African Americans
and 110 Caucasians with photodermatoses found that 2.2% of the African Americans
had SU and 8% of the Caucasians had the disease showing that Caucasians have a
better chance of getting the disease. Globably 3.1 per 100,000 people are
affected and females are more likely to be affected than males. The age ranges
anywhere from 5–70 years old, but the average age is 35 and cases have been
reported with children that are still in infancy. Solar urticaria accounts for
less than one percent of the many documented urticaria cases. To put that
into a better perspective, since its first documented case in Japan in 1916,
over one hundred other instances of the disease have been reported.
History Solar urticaria was first identified by
P. Merklen in 1904. Just a year later, in 1905, Ward became the first to induce
urticaria through exposure to the sun in a controlled environment. The first
documented case came in Japan in 1916. The name "solar urticaria" was proposed
in 1923. In 1928 urticaria was induced for the first time. This was carried out
by phototesting with increasing amounts of radiation of varying wavelengths. In
1942 the disease was passively transferred to normal volunteers using
serum from patients with solar urticaria.
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