Polymorphous Light Eruption

Polymorphous Light Eruption
Erhard Holzle, M.D., Gerd Plewig, M.D., Renate von Kries, M.D., Percy Lehmann, M.D.
Department of Dermatology, University of Diisseldorf, Diisseldorf, F. R. G.
Polymorphous light eruption (PLE) is a common photo­
dermatosis of unknown etiology. It affiicts mainly fair­
skinned patients, with a preponderance of young females.
There is, however, no absolute restriction as to age, sex,
or race.
Clinical variants include the papular, vesiculo-bullous,
and hemorrhagic variety, as well as plaque, erythema mul­
tiforme-like, and insect bite (strophulus)-like types. Skin
lesions appear only in certain exposed areas hours or a few
days after intense sunshine, and are nearly always mono­
morphous in the same patient. The rash subsides sponta­
neously within several days without leaving scars.
The histopathologic picture is characteristic and shows
a perivascular lymphocytic inftltrate in the upper and mid­
dle corium with subepidermal edema, vacuolization of basal
cells, and spongiosis in the lower epidermis. The most
important differential diagnoses are solar urticaria, pho­
tosensitive erythema multiforme, and lupus erythemato­
sus.
The action spectrum of PLE is under debate. Repro­
duction of skin lesions has been reported with UVB, UVA,
and, rarely, visible light, with UVA probably being the
most effective part of the spectrum.
More important than treatment of PLE is prophylaxis.
UVA- and UVB-effective sunscreens are of some help.
Phototherapy and especially photochemotherapy (psoralen
+ UVA; PUVA) offer effective ways to decrease light
sensitivity. Systemic treatment with chloroquine or f3-car­
otene has been disappointing. ] Invest Dermatol 88:325-385,
t was recognized in the early 19th century that certain ecze­
matous reactions were caused by sunlight. Willan [1] was
the first to use the term "eczema solare." In 1879 Sir Jonathan
Hutchinson [2] presented 14 patients under the diagnosis
"summer prurigo. " There is reason to believe that "prurigo
aestivalis," "prurigo adolescentium," and "acne prurigo" syn­
onymously designated similar sun-induced skin disorders. Rasch,
in 1900 [3], simplified the terminology by introducing the term
"polymorphous light eruption (PLE). " This designation was widely
accepted to characterize urticarial, papular, vesicular, and ecze­
matous reactions precipitated by light [4]; however, it did not
clarify the nosology of photodermatoses.
A variety of sunlight-evoked eruptions, such as solar urticaria,
hydroa vacciniforme, porphyrias, photoallergic dermatitis, pho­
tosensitive erythema multiforme, and lupus erythematosus (LE)
fell under this diagnosis. It was only during the past decades that
these differential diagnoses have been separated and polymor­
phous light eruption has been defmed as a clinical entity [5-8).
Until now, however, it remains difficult, if not impossible, to
distinguish in certain patients between polymorphous light erup­
tion and light-induced subacute or discoid cutaneous lupus ery­
thematosus, or photosensitive erythema multiforme.
This review describes the growing knowledge about PLE. We
put forward our concept of the disease as a distinct entity with
several morphologic variants. O ur experience is based on obser­
vations in more than 250 patients in the years from 1977 to 1985
[8-11).
1987
I
CLINICAL FEATURES
Polymorphous light eruption is a very common photodermatosis,
but the true prevalence among the population is not known. In
a survey of 271 apparently healthy subjects, 10% gave a history
consistent with PLE [12]. Many persons are aware of a transient
"sun allergy" but never visit a dermatologist because they know
how to manage the problem by themselves. Individual suscep­
tibility varies greatly, and only the most severely afflicted patients
are seen by physicians. In many patients, PLE appears during the
first days of vacationing on the beach or in high altitudes. When
the patient returns home the eruption has resolved spontaneously
and the pliysician who is asked for help can only conceive a
diagnosis by the patient's history.
Polymorphous light eruption seems to occur most frequently
among fair-skinned populations. Large series are reported from
Scandinavian countries [13,14], but the eruption is not confmed
to a certain race or skin color [7]. It has been reported in Blacks,
O rientals, and in Native North and Latin Americans [15,16].
Among the latter, the familial occurrence is a characteristic fea­
ture. This peculiar photodermatosis might represent an entity
different from PLE observed in whites [17]. Inheritance in an
autosomal-dominant mode with a reduced penetrance has been
reported in 56% of patients evaluated in Finland [18] and in 75%
of Canadian Inuit patients [19]. In other series, genetic predis­
position ranged from 3-45%. Jansen [18] suggested that patients
in other reports might not have been questioned as thoroughly
and, therefore, genetic predisposition in general might be under­
estimated.
There is agreement that the time of onset is most commonly
during the early adult life [7,13], but the eruption may also start
in childhood. In children the face and ears frequently are involved.
Similar cases have been described under the designation ''juvenile
Reprint requests to: Prof. Dr. E. Holzle, Universitatshautklinik, Moor­
enstr. 5, D-4000 Diisseldorf 1, F.R.G.
Abbreviations:
DLE: discoid lupus erythematosus
LE: lupus erythematosus
MED: minimal erythemal dose
PLE: polymorphous light eruption
PUVA: photochemotherapy (psoralen + UVA)
SCLE: subacute cutaneous lupus erythematosus
UVA: ultraviolet A
UVB: ultraviolet B
0022-202X/87/S03.50
Copyright © 1987 by The Society for Investigative Dermatology, Inc.
32s
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POLYMORPHOUS LIGHT ERUPTION
1987
spring eruption" [20,21], which is probably part of the PLE spec­
trum. Actinic prurigo [22,23], however, is in our view a separate
disorder that bears resemblance to the hereditary variant of PLE
among North, Central, and South American Natives (18,24].
Features that distinguish these disorders from PLE are involve­
ment of non-sun-exposed areas, lack of complete clearing during
the winter season, and presence of chronic eczematous lesions.
In our experience, 90% of PLE patients are women. This is in
accordance with the findings of Clorius and Jung [6] and Jansen
[13]. Others have found a preponderance of men [25,26] or an
equal sex distribution [27,28]. The reason that, in some studies,
women are more commonly afflicted is not well understood. It
is possible diat the preponderance of females does not reflect the
true prevalence, which might be equal, but rather might reflect
a higher rate of reporting of the rash by women.
Lesions of PLE are confined to sun-exposed body sites. In
contrast to others [5,7,13] we have neither observed lesions on
areas covered by clothing nor a tendency of the eruption to gen­
eralize. In our experience, PLE is confined to certain sites of
predilection even when large areas are sun-exposed. In order of
decreasing frequency of involvement, these sites are: V-neck,
dorsal forearm and back of hands, upper arms, face, shoulders,
thighs, and lower legs. Variations among different patients occur.
In exquisitely sensitive individuals larger parts of the trunk are
involved, too. Jansen [13] notes that lips and eyes are frequently
involved, and reports that patients experience general symptoms,
like chills, headache, fever, and nausea after sun exposure.
The reaction is of a delayed type. Lesions appear from several
hours to a few days after intense sun exposure. A characteristic
sequence of events is reported by most patients, and the dynamics
ofthe lesions can be observed when PLE is experimentally evoked
by phototesting. Itching occurs first; it is followed by patchy
erythema. Finally, distinct lesions emerge, which may coalesce.
The sequence from itchy, patchy erythema to sparsely distributed
and then densely aggregated skin lesions requires increasing amounts
ofradiation energy. Lesions are present for hours or several days
and resolve spontaneously without residues when further sun
exposure is avoided. Scaling, hyperkeratosis, lichenification, or
scarring are not primary lesions in PLE. These changes may occur
secondarily due to rubbing or scratching and prolonged sun ex­
posure. Also the clinical picture might be altered by a concomitant
sunburn reaction. The majority of patients experience seasonal
recurrences over many years, with decreasing sensitivity after
repeated sun exposures. Finally, the problem may resolve spon­
taneously after many years [13,29].
Many patients experience the elicitation of skin lesions by sun­
light through window glass, e.g., in airplanes, cars, and trains.
This was already observed by early authors [25] and indicates
that the action spectrum is not confined to sunburn radiation.
MORPHOLOGIC VARIANTS
Most authors agree that there are different morphologic variants,
with the papular variety being the dominant type [13,28,30].
Lamb et al [31] and Epstein [32,33] in their series distinguished
plaque, prurigo-like, and erythema multiforme-like types from
the papular, eczematous, and erythematous variety. The latter
was designated erythema solare perstans. In his review [7] Epstein
refers to small papules, large papules that may coalesce and form
plaques, papulovesicular lesions, erythema multiforme-like le­
sions, prurigo-like nodules, and an eczematous response that may
become secondarily lichenified by rubbing and scratching. Re­
cently, a vesiculobullous variant of PLE was described [34,35].
In our view, diffuse erythematous response, eczematous re­
actions, and prurigo-like nodules are not part of the PLE spec­
trum. These lesions characterize separate light-induced disorders,
which comprise differential diagnoses of PLE. Widespread diffuse
erythema following sun exposure might occur in systemic LE or
in the presence of a phototoxic photosensitizer. Eczema in ex­
posed areas is a feature of photoallergy, photosensitive eczema,
persistent light reaction, light-aggravated atopic eczema, or air-
33$
borne contact dermatitis. Prurigo associated with sun exposure
might by seen in actinic prurigo, which represents an entity sep­
arate from PLE. The reason we do not observe eczematous lesions
in our patients might be the lack of chronic exposure to high­
intensity solar radiation. In the F. R. G. the summer season is
short and longer periods of sunny weather conditions are rare.
In our patients [8,11] the papular type, along with the hem­
orrhagic and insect bite (strophulus)-like varieties, the plaque
type, the vesiculobullous type, and an erythema multiforme-like
type make up the spectrum of PLE. We do not share the opinion
that the clinical picture might be polymorphous in the same in­
dividual [5,6] or that combination or transition between the var­
ious types might occur [7]. In our series skin lesions were mon­
omorphous in any one patient.
Papular Type It is the most common type of PLE. Small
papules or papulovesicles are disseminated or densely aggregated
on a patchy erythema (Figs lA,2A).
Hemorrhagic Type
rhagic.
Rarely, the papular lesions are hemor­
Insect bite (Strophulus)-like Type This is a peculiar, rare
variety of the papular type. The lesions are few, scattered, and
consist of small urticarial papules topped by a tiny vesicle.
Plaque Type This is the second most common variety in our
patients. It occurs frequently on the face [7] and resembles sub­
acute cutaneous lupus erythematosus (SCLE), with its sharply
demarcated, erythematous, elevated, often urticarial plaques.
Erythema Multiforme-like Type
sions and occurs less frequently.
It shows typical target le­
Vesiculobullous Type It consists of tense vesicles and small
bullae on an erythematous base. It seems to be frequent among
North American women vacationing to Hawaii [34]. In Europe
it is very rarely observed [35].
HISTO PATHOLOGY AND
IMMUNOHISTO PATHOLOGY
In 1960 Wright and Winer [36] gave a description of the histo­
pathologic pattern seen in PLE that has gained wide acceptance.
They found parakeratosis, subepidermal edema, vascular dilata­
tion, and lymphocytic infiltration. Sometimes acanthosis, liq­
uefaction degeneration of the basal layer, and diminished elastic
fibers were observed. McGrae and Perry [5] stressed the similar­
ities to lymphocytic infiltration Jessner. Lamb et al [26], as well
as Lever and Schaumburg-Lever [37] distinguished the plaque
type of PLE from LE by absence of vacuolar alteration of basal
cells, a view that did not hold true [7,8,38]. The main features
characteristic for PLE are observed in the common papular type
[7,8]. In other morphologic variants, this basic pattern is also
prominent, but certain features might be pronounced or addi­
tional features superimposed.
The pattern of PLE, seen in the papular type, shows a superficial
and deep perivascular lymphocytic infiltrate with an interstitial
component in the upper dermis. There are subepidermal edema
and vacuolization of basal cells, exocytosis of lymphocytes, and
focal spongiosis. Sunburn cells are notably absent or sparse (Figs
lB,2B).
In the plaque type the infiltrate tends to be lichenoid, subepi­
dermal edema is pronounced, and spongiosis is widespread, in­
volving the lower epidermis.
The erythema multiforme-like type exhibits a prominent su­
bepidermal edema sometimes forming a blister, and few extra­
vasated erythrocytes. Epidermal necrosis may or may not be
present.
In the vesiculobullous type spongiotic vesicles and subepider­
mal blisters are formed.
The insect bite (strophulus)-like type shows focal necrosis in
the upper malpighian layer, and in the hemorrhagic type there is
notable extravasation of erythrocytes.
Figure 1. PLE, papular type, genuine lesions. A, Papules scat­
tered on chest 48 h after sun exposure; B, Histopathology from
another patient with similar lesions shows perivascular and in­
terstitial lymphocytic infiltrate in the upper corium with subep­
idermal edema, vacuolization of basal cells, and spongiosis.
Figure 2. PLE, papular type, experimentally induced lesions.
A, Urticarial papules that coalesce emerged in the moderately
pigmented test area on the upper arm 24 h after irradiation with
toO J/cm2 UVA (UVASUN 3000); B, Superficial and deep per­
ivascular lymphocytic infiltrate with subepidermal edema, vac­
uolization of basal cells, and spongiosis.
A
B
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POLYMORPHOUS LIGHT ERUPTION
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Neutrophils and eosinophils are only present in a few cases [7].
We have noted them only in the vesiculobullous type. This is,
however, inconsistent with other reports [34].
Using monoclonal antibodies to defme subsets of infiltrating
mononuclear cells in papular PLE, Muhlbauer et al [39] found
that the perivascular infiltrate was predominandy composed of
T cells with a slight but inconsistent preponderance of cyto­
toxic/suppressor cells over helper/inducer cells. Macrophages
represented less than 5% of the infiltrating cells. Langer­
hans/indeterminate cells were found to be increased in number.
The same group [40] studied PLE by direct immunofluores­
cence technique and detected intervascular and focal perivascular
deposition·of fibrin. In some patients, C3 and IgM was found in
the walls of venules. The lupus band test was negative in all
subjects. Recendy, Moncada et al [41] found a predominance of
T helper cells and Ia + cells in the dermal infiltrate of PLE.
evoked by sun exposure through window glass while patients are
riding on trains, airplanes, or in cars. Furthermore, sunburn is
not a prominent feature in PLE, neither clinically nor micro­
scopically.
The question as to whether erythemal or pigmentary responses
to solar radiation may differ from normal reactions in patients
with PLE has stimulated research that produced confticting. ex­
perimental data. Thune [14] as well as others[54] found the MED
of sunburn radiation lowered in patients with PLE. In contrast,
Jansen [55] reported normal values, perhaps with a slighdy de­
layed fading of the erythema; pigmentary responses were also
found to be normal.
We could not discern any differences in MED-UVB or pig­
mentary reactions induced by UVA between patients with PLE
and normal controls [9]. Others, too, have questioned the use­
fulness of MED-UVB as a diagnostic parameter [7].
ACTION SPECTRUM
DIFFERENTIAL DIAGNOSIS
The action spectrum ofPLE is not known. There have been many
debates about the wavelength and energy requirements for elic­
iting skin lesions. Experimental results are conflicting and some­
times contradictory. Factors that may relate to the reported var­
iability include differences in individual susceptibility, seasonal
variations, differences in test sites, light sources, and interpreta­
tion of test results, as well as the clinical variety ofPLE, including
geographic differences. Many investigators report the induction
of skin lesions with sunburn radiation between 280 and 320 om
[32,33,42,43,44]. Most of these authors used radiation from high­
pressure mercury or xenon lamps or carbon arc sources with
appropriate filters. In some instances responses to UVC were
noted [33].
On the other hand, Langhof and Sprossig [45], Wiskemann
and Wulf[46], and Gschnait et al [47] were able to reproduce skin
lesions ofPLE with long-wave UV light (UVA). In a multitude
of experimental studies, various authors came to the conclusion
that the action spectrum might range from UVB, to UVA, and
into visible light[14,27,44,48-51]. In some instances, an additive
effect of infrared radiation was observed [13,14].
During the years from 1977 to 1985 we tried to induce skin
lesions experimentally in more than 250 patients. Until 1982, 180
patients were tested by using various procedures [8]. Monochro­
matic light, both UVA and UVB, as well as polychromatic UVA
in doses up to 40 J/cm2 failed to elicit PLE. When UVA from a
high-intensity apparatus (UVASUN 3000; Mutzhas, Munich, FRG)
[52] was given in doses of 60-100 J/cm2, up to 90% of patients
developed lesions ofPLE if stringent requirements were met (Ta­
ble I). Adhering to the same experimental procedure but using
polychromatic UVB in minimal erythemal doses (MED) up to
4, we were unable to reproducePLE at that time; however, only
a few patients were tested.
Recendy, we have extended our experience to another 90 pa­
tients. These were tested with UVA and UVB (UV 800 equipped
with fluorescent bulbs, Philips TL 12; Waldmann, Villingen­
Schwenningen, FRG) on a contralateral or adjacent skin site, fol­
lowing the method described (Table I). About 60% of patients
showed lesions ofPLE; out of this group 75% revealed sensitivity
to UVA, 10% to UVB, and 15% reacted to both UVA and UVB
[11]. The smaller yield of positive reactions compared with the
previous study [8] might be due to the fact that some patients
were tested in summer or fall, at a time when their skin was less
UV sensitive due to hardening.
Similar results were found by Ortel et al [53], who were able
to reproduce skin lesions in 50% of 142 patients. In 56% the
eliciting wavelengths were found in the UVA range, in 17% in
the UVB spectrum, and in 26% in both.
In view of these data it appears that, in the majority of PLE
patients, long-wave UV is most effective in eliciting skin lesions.
A small subgroup of patients shows additional sensitivity to UVB
or may react to UVB exclusively. These findings are in accord­
ance with the observation that eruptions of PLE are frequendy
The diagnosis of PLE is based on a patient's histo.ry and clinical
features of lesions. The time course of PLE, with its onset hours
after intense sun exposure and spontaneous fading (without res­
idue) of lesions within hours or days is very characteristic. It aids
in establishing the diagnosis when the patient presents without
skin lesions. A diagnosis can be made almost with certainty when
the patient presents with a typical rash or whenPLE is reproduced
by phototesting in the laboratory. Biopsies of genuine or exper­
imentally induced lesions may confirm the diagnosis, if other
differential diagnoses cannot be excluded clinically. Staining with
hematoxylin and eosin suffices; special stains are of little help,
perhaps with the exception of direct immunofluorescence tech­
nique, if lupus erythematosus is suspected.
It is our belief that chronic skin lesions with pronounced epi­
dermal changes are not part of the PLE spectrum. Thus, light­
induced disorders showing eczematous lesions, e.g., photoallergic
dermatitis, photosensitive eczema, and persistent light reaction
are excluded from PLE by their morphology.
Solar urticaria may mimic the plaque type ofPLE. Time course
of whealing and the reproduction of typical hives using low doses
of electromagnetic radiation are characteristic for solar urticaria.
Photosensitive erythema multiforme is a rare condition[56]. It
is difficult to separate this disorder from the erythema multiforme
type ofPLE. Clinical and histologic pictures are very similar[57].
The following features may help to differentiate light-sensitive
erythema multiforme fromPLE: (1) The former occurs 1-14 days
after sun exposure. (2) Immune complexes may be detected in
the initial stage, and the lymphohistiocytic infiltrate is perivascular
and interstitial in the upper corium. (3) Sometimes eosinophils
and extravasated erythrocytes are prominent, and the epidermis
shows focal necrosis.
The deposition of immune complexes in venules of the skin
triggered by UV erythema has been discussed as the underlying
pathogenetic mechanism in photosensitive erythema multiforme
[58].
Lupus erythematosus is the most important differential diag­
nosis, clinically and histologically. Subacute cutaneous lupus er­
ythematosus and the tumid type of discoid LE (DLE) may present
with lesions very similar to the plaque type ofPLE. In LE, how­
ever, waxing and waning of lesions are not as closely correlated
to sun exposure as in PLE. Lesions usually persist for several
weeks or months even without further UV exposure. When LE
lesions are induced by experimental UV irradiation [59] it takes
at least several days, in most cases 1-2 weeks or longer, for lesions
to appear. This prolonged time course differs from PLE.
Histologically, the lymphocytic infiltrate in LE frequendy ex­
tends perivascularly and around the adnexa in the deeper corium.
There might be mucin deposition between collagen bundles[60].
Subepidermal edema is not pronounced, and liquefaction degen­
eration, if present, is confined to the epidermal basal cell layer.
Spongiosis is absent. Sometimes neutrophils are seen in vessels
of the superficial plexus. In most cases of PLE the perivascular
36$
HOLZLE ET AL
Table I.
THE JOURNAL OF INVESTIGATIVE DERMATOLOGY
Recommendations for Provocative Photo testing
in PLE
Test time
Test site
Size of test area
Light source
UV dose
Before the PLE season starts
Previously involved body areas. Avoid
pigmented skin
No less than 5 x 10 cm
Use polychromatic UV in large doses
UVA: 60-100 J/cm2
UVB: 2 MED
Repeat irradiations on day 2 and 3, if necessary
lymphocytic infiltrate reaches only into the mid-dermis. Subep­
idermal edema is a prominent feature. Vacuolization accompanied
by spongiosis occurs in the 2 or 3 suprabasal cell layers. Intra­
cellular and intercellular edema of the lower part of the epidermis,
in combination with subepidermal edema, gives the dermoepi­
dermal interface a spongy appearance. The histopathologic dif­
ferences mentioned are prominent only in fully developed lesions.
In the initial stage of LE or PLE it is extremely difficult, sometimes
impossible, to distinguish between the two diseases on histo­
pathologic grounds. Direct and indirect immunofluorescence are
of limited value because they may yield negative results in early
stages of LE or may be found positive in some patients with PLE
who, however, do not meet other criteria of LE [29]. It is our
impression that there might be a subgroup of patients that cannot
be separated clearly either from DLE, SCLE, or PLE. It is con­
ceivable that these patients eventually progress into LE. This view
is, however, not substantiated by follow-up studies [29,30], in
which PLE patients were not found to develop LE.
THERAPY AND PROPHYLAXIS
Some patients may experience a cooling and beneficial subjective
effect by local application of zinc lotion in acute-stage PLE. Top­
ical or systemic steroids may ameliorate inflammation or itching
and shorten the eruption. In few patients eruptions of PLE can
be suppressed by prophylactic systemic steroid treatment. Con­
trolled trials are lacking and it seems inappropriate to treat this
chronic, intermittent, self-limited, and benign disease with sys­
temic steroids. More important than therapy of the acute eruption
seems the prevention of further attacks by prophylactic measures.
Light protection, phototherapy, photochemotherapy and, to some
extent, systemic drugs offer ways to decrease UV sensitivity.
Sun protection by protective clothing or staying in the shade
may prevent PLE. The usefulness of topical sunscreens is limited,
although sun blockers have been found effective in UVB-sensitive
patients [62], and broad-band screens (UVB plus UVA) or phys­
ical sunscreens may be beneficial in other patients [8,51,63,64].
Systemic treatment with J3-carotene yielded different results.
Some authors found only modest improvement of UV tolerance
not sufficient to enable the patient to perform normal outdoor
activities [65,66], while others report complete remission [14,67]
(uncontrolled studies). J3-Carotene was also used in combination
with canthaxanthin [68], a combination recently banned by the
German Federal Department of Health. In controlled trials[69,70]
J3-carotene was ineffective in many cases. The same holds true
for chloroquine [68], although antimalarials, mainly chloroquine
and quinacrine, have been commonly used to treat PLE[61,71,72].
Results have been unimpressive.
Some authors [73,74] assume that the underlying mechanism
in PLE is a disturbance in the tryptophan metabolism, with ac­
cumulation of kynurenic acid, which may act as a photosensitizer.
They advocate treatment with high doses of nicotinamide to re­
duce the level of kynurenic acid and report good therapeutic
results [74]. There is no proof of this hypothesis.
In actinic prurigo [75] and PLE [76], thalidomide was helpful,
but is obsolete in view of the teratogenic effect and neurotoxic
side effects. Ketoprofen, a nonsteroidal anti-inflammatory agent,
was shown to be ineffective [69].
Phototherapy and photochemotherapy provide a good protec-
tive effect. Patients with PLE frequently show "hardening" with
decreasing UV sensitivity during spring or early summer. It seems
worthwhile to induce this protective mechanism by controlled
exposures to sunlight [77] or artificial UV sources [78-80]. Also,
PUVA treatment was used to prevent further attacks of PLE
[47,81-84], and was found to be superior to phototherapy
[53,62,83,85]. Until more is known about the long-term hazards
of PUVA therapy, patients should be monitored carefully, al­
though the number of exposures and the cumulative UVA doses
are quite small. We do not advocate PUVA therapy in PLE as
the treatment of first choice, but inform our patients about the
benign nature of PLE and recommend sun-protective measures
in combination with skin "hardening" using natural sunlight. If
this fails, phototherapy is used. O nly those patients who are
exquisitely sensitive to UV are treated with PUVA therapy in
our department. The mechanism by which phototherapy or pho­
tochemotherapy induce protection is not completely understood.
Besides increased melanization and thickening of epidermis and
stratum corneum, factors that enhance UV absorption in the ep­
idermis and provide a protecting shield for the underlying dermis,
immunologic mechanisms cannot be excluded.
HYP O THESES ON ETIOLOGY AND PATHOGENESIS
OF PLE
Apart from the fact that PLE is induced by electromagnetic ra­
diation emanating from the sun, nothing is known about its etiol­
ogy, despite many investigative studies.
In their search for an endogenous photosensitizer, European
authors focused on gastrointestinal abnormalities in PLE patients
[45,86]. Also urinary excretion of an indole derivative, called
"Kimmig's light band" [87] was described but was not found to
be specific for PLE [88]. O thers [73,74] put forward the hypoth­
esis that disturbances in the tryptophan metabolism might be the
cause of photosensitization. None of these theories account for
the pathomechanism of PLE.
Many investigators hold the view that PLE is related to a cell­
mediated immunologic reaction [7]. The time course of the re­
action, with a delay after the initiating sun exposure, the clinical
picture, and histopathologic findings bear similarities to the ecze­
matous reaction of delayed-type hypersensitivity. A relevant pho­
toallergen has not been identified. Functional disturbances of lym­
phocytes have been found only inconsistently. Horkay and Meszaros
[89] described UV-induced lymphocyte transformation in PLE.
Also, a defect in the excision repair mechanism in lymphocytes
of PLE patients was reported [90], but was not confirmed by
others [91,92].
In recent studies on cell-mediated immunity in PLE, a transient
decrease [93] as well as an increase[41] in number of lymphocytes
was found; no inhibition of leukocyte migration was detected
[94]. The number of peripheral T cells and response of peripheral
lymphocytes to phytohemagglutinin, concanavalin A, and pu­
rified protein derivative of tuberculin was found normal before
and after PUVA treatment [95]. This argues against the distur­
bance in lymphocyte function as a cause of PLE.
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