The dijferential diagnosis o.f psoriasis Continuous dermatologica l education 
The dif.ferentwl diagnosis of psoriasžs 
P.C.M. van de Kerkhof 
SUM MARY------ ----- ---------- -----
In general the diagnosis psoriasis can be rnade on clinical grounds. The differentiation frorn eczerna and 
other erythernatosquarnous eruptions is of irnportance. 
lf a biopsy is indicated, it is irnportant to take it frorn the rnargin of the lesion. Characteristic phenornena 
such as rnicropustules of Kogoj, rnicroabscesses of Munro and the characteristic topography are usually 
seen in the material taken frorn the rnargin of the lesion and not frorn the central areas. 
1. Clinicaljeatures and histo-
pathological aspects ofpsoriasis 
Definition 
Psoriasis is a common and chronic skin disease, 
characterized by cutaneous inflammation and epidermal 
hyperproliferation. The prevalence of psoriasis in 
Caucasians is between 1.5 and 3% (1). The age of onset 
is variable, with two peaks between 16 and 22 years 
and between 57 and 60 years (2). The incidence of 
psoriasis is equally distributed between both sexes (3). 
The clinical course is unpredictable with remissions and 
exacerbations. 
Clinical features 
Por a detailed description of the many clinical 
variants of psoriasis various reviews are available (1,4). 
JO 
Despite the fact that psoriasis has many different clinical 
presentations, it is regarded as a single entity. Different 
forms of psoriasis can coexist in one patient, or may 
appear at different moments in the same patient. In the 
same family the members may show different forms. In 
brief, the most important clinical manifestations are 
described: 
1. Erythematosquamous variants 
Chronic plaque psoriasis is the most common pre-
sentation, characterized by coin-sized to palm-sized 
erythematosquamous plaques with a sharply delineated 
edge. Annular lesions may occur asa result from central 
healing. Predilection sites are the scalp , the trunk, the 
sacral region and the extensor surfaces of knees and 
elbows. Guttate psoriasis (Figure 1) is a variant with 
erythematosquamous papules. Systemic triggering 
factors often are relevant to this form. 
acta dermatovenerologica A.P.A. Vol 8, 99, No 2 
Continuous dermatological education 
Figure1. Guttate psoriasis. 
Figure 2. Flexural psoriasis. 
Figure 3. Pustulosis of the palms. 
The differential diagnosis oj psoriasis 
Palmoplantar psoriasis is rather common and may occur 
with or without psoriatic lesions on other parts of the 
skin. Characteristic features are sharply delineated 
erythematosquamous plaques with adherent scales and 
fissures. 
Flexural psoriasis or psoriasis inversa occurs in 2-6% 
of the patients as an isolated symptom, much more 
frequently it occurs in association with non-flexural 
symptoms of psoriasis. Scaling is mostly absent or 
minimal (Figure 2). 
Erythrodermic psoriasis is the most severe form chara-
cterized by generalized erythema and scaling. Systemic 
disregulation with hypo- and hyperthermia, loss of pro-
teins, dehydration and even renal and cardiac failure 
may occur and hospitalization is indicated. 
2. Pustular variants 
Persistent palmoplantar pustulosis occurs in up to 5% 
of ali psoriatic patients. It is stili a debate whether this is 
a manifestation of psoriasis or a separate entity. 2 to 
19% of the patients have further signs of psoriasis (5 ,6). 
Erythematosquamous plaques may accompany the 
sterile postulation (Figure 3). The disorder is very 
resistant to therapy. 
Acrodermatitis continua o/Hallopeauis a rare disorder 
with erythema, scaling and pustules on the distal pha-
langes of fingers or toes, sometimes with nail defor-
mation and bone changes. Treatment failures are 
common (Figure 4). 
Generalized pustular psoriasis (Zumbusch) is a relati-
vely rare and severe form of psoriasis with erythema, 
scaling and the development of sterile pustules (7) 
(Figure 5). Most patients have a history of chronic plaque 
psoriasis. There are systemic symptoms like malaise, 
Figure 4. Acrodermatitis continua Hallopeau. 
acta dermatovenerologica A.P.A. Vol 8, 99, No 2 -------------------------------- JJ 
The d/fferential diagnosis of" psoriasis 
fever and leukocytosis. Generalized pustular psoriasis 
is a serious disease with a considerable mortality. 
3. Extracutaneous symptoms 
The nails are in vol ved in 10-55% of the psoriatic patients 
(8). Pitting ofthe nails , subungual hyperkeratosis, distal 
onycholysis and brownish red discolorations due to 
accumulation of parakeratotic material in the nailbed 
are the most common features (Figure 6). 
Psoriatic arthritis is present in approximately 7% of 
patients (9). Arthritis of the distal phalanges is the most 
characteristic manifestation. Although the incidence of 
peripheral oligoarthropathy is much higher, axial 
changes or severe deformities (arthritis mutilans) may 
occur in some patients (1). 
Histopathology 
The most important epidermal changes in a fully 
developed psoriatic lesion are elongation of the epi-
dermal rete ridges and the dermal papillae with thinning 
ofthe suprapapillary plate (Figure 7). In the epidermis 
polymorphonuclear leucocytes (PMNs) accumulate into 
micropustules ofKogoj with their characteristic spongi-
form appearance. Absence of a granular layer, para-
keratosis, and intracorneal "Munro microabscesses" 
filled with PMNs constitute the changes of the supra-
basal compartment (10). In pustular psoriasis macro-
pustules , filled with PMNs, are present in the upper 
stratum spinosum. Figure 7 illustrates the histological 
appearance ofthe psoriatic plaque. 
Table l. Eczema versus psoriasis 
Eczema Psoriasis 
ltch + 
Sharp demarcation ± + 
Erythema + + 
Pinpoint bleeding + 
Vesiculation + 
Sterile pustules + 
Excoriations + ± 
Lichenification + 
Scaling ± + 
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Continuous dermatological education 
2. Eczema 
Eczema is a morphological diagnosis. It designates 
a morphological entity which might represent different 
etiological entities. Eczema can be defined asa polymor-
phic dermatosis which itches and is characterized by 
an acute and chronic phase. The acute phase is charac-
terized by ill-defined erythema, vesiculation, papules 
and excoriations. The chronic phase is characterized 
by ill-defined lichenification, some scaling and exco-
riations. The acute and chronic phase usually are seen 
simultaneously in an individual patient and even in one 
single lesion. 
The histopathological picture reveals vasodilatation, 
dermal oedema and characteristic spongiosis. A predo-
minant T-lymphocytic infiltrate is seen at perivascular 
location, diffuse in the dermis and in the epidermis . 
Table I highlights the differential diagnosis eczema 
versus psoriasis. 
Atopic dermatitis 
Atopic dermatitis (AD) is a disease entity, which is 
expressed as eczema. AD is part of the atopic syndrome. 
The face is involved predominantly in infants. In 
children and adults, the symmetrical involvement of the 
flexures is highly characteristic. The typical distribution 
of the eczema yields into a certain diagnosis in most 
cases. Other clinical manifestations of the atopic 
syndrome, positive intracutaneous tests and increased 
semm IgE may provide further evidence for an atopic 
constitution. 
Eczema Psoriasis 
Vasodilatation + + 
Tortuosity of capillaries + 
Suprapapillar thinning + 
Spongiosis + 
Micropustules of Kogoj + 
Microabcesses of Munro + 
T-cell infiltrate + + 
Neutrophil infiltrate + 
Acanthosis + + 
Parakeratosis + 
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Co n t i nuous d e rmatolo g i ca l ed u ca tion 
Figure 6. Psoriasis of the nails: pitting. 
Occasionally the lesions may not be preferentially 
localized in the flexures, a sharp demarcation may occur 
and sometimes scaling may be more prominent. In such 
cases the differential diagnosis with psoriasis may be 
difficult. The occurrence of vesicles and lichenification 
as well as the p revious history of vesicu lation may 
provide further information . A biopsy should be taken 
form the margin oj the lesion to differentiate between 
eczema and psoriasis if differentiation is impossible by 
clinical hallmarks . 
Figure 7. Elongation of rete ridges and dermal 
papillae with thinning of the suprapapillary plate. 
acta dermatovenerologica A.P.A. Vo l 8, 99, No 2 
The differential diagnosis of psoriasis 
Figure 5. 
Generalized 
pustular 
psoriasis. 
Figure 8. Seborr-
hoeic dermatitis 
of the tace. 
JJ 
Continuous dermatological education 
Allergic contact dermatitis 
Allergic contact dermatitis is a delayed hyper-
sensitivity, which is expressed as an eczema. The 
location is at sites which have been exposed to the 
allergen and or at sites which have a relatively thin 
skin barrier such as the eyelids and the back of the 
hands. History and allergic patch testing will reveal the 
diagnosis. 
In the chronic phase of eczema, differentiation from 
psoriasis can be difficult. However, the development 
of the lesions, starting with vesiculation subsequently 
yielding into lichenification and scaling will permit a 
certain diagnosis in most cases. Sometimes a biopsy 
from the margin zone is needed. 
Occasionally, psoriasis may be provoked by an 
underlying allergic contact dermatitis without vesicu-
lation macroscopically. If psoriasis occurs predo-
minantly at the predilection sites for allergic contact 
dermatitis, such as band and faces, patch testing is 
indicated, even if vesicles are absent. 
Table II. Lupus erythematosus discoides (LED) 
versus psoriasis 
Localization: scalp 
tace 
Atrophy 
Follicular hyperkeratoses 
+ 
+ 
+ 
+ 
+ 
Table III. Parapsoriasis versus psoriasis 
Unsharp demarcation 
Atrophy 
Reticular pigmentations 
+ 
+ 
+ 
Table IV. Syphilis versus psoriasis 
Sudden appearance 
Palms/soles 
Plasma cells 
Syphilis serology 
+ 
+ 
+ 
+ 
± 
± 
The d/fferential diagnosis (l psoriasis 
Orthoergic dermatitis 
Orthoergic dermatitis due to exposition to factors 
which damage the skin barrier are localized predo-
minantly on the hand. In general, the lesions are chara-
cterized by non-sharply demarcated lichenifications and 
erosions. Occasionally, vesicles may complicate the 
picture. 
Nummular eczema 
The clinical picture is characterized by relatively 
sharply demarcated plaques with papules and vesicles. 
The etiology of nummular eczema is not known. 
In the chronic phase, scaling may complicate the 
picture and differentiation from psoriasis may be 
difficult. In case of doubt a biopsy from the margin zon.e 
is needed. 
Dyshidrotic eczema 
Vesicles, bullae and scaling, localized on the hands, 
are characteristic features of dyshidrotic eczema. In 
general, the condition is complicated by erosions, ery-
thema, scaling and fissuring. Allergic contact dermatitis 
has to be excluded in these conditions. It is the common 
belief that vesiculation excludes the diagnosis psoriasis. 
However, a contact dermatitis may provoke psoriasis. 
3. Erythematosquamous 
dermatoses 
Psoriasis belongs to the morphological directory of 
erythematosquamous dermatoses. These dermatoses 
have in common inflammation in conjunction with 
abnormal differentiation of the epidermis, resulting in 
erythema and scaling. Of course erythema and scaling 
may occur in a large variety of dermatoses as a secon-
dary, not obligatory, hallmark. For example, eczema in 
the chronic phase may show scaling. By definition 
eczema is not an erythematosquamous dermatosis as 
scaling is not obligatory and often absent or incon-
spicuous. 
Seborrhoeic dermatitis 
Seborrhoeic dermatitis in infants is localized on the 
scalp and predominantly in the large flexures. In adults, 
sebarrhoeic dermatitis is localized on the scalp and on 
the face, predominantly between the eyebrows, in the 
nasolabial fold (Figure 8), occasionally in the inter-
scapular area and in the presternal region. The scaling 
has a yellow color. The histopathological picture closely 
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The d/fferential diagnosis of psoriasis 
resembles psoriasis. However, a major difference is the 
prominent exudation, which is characterized by 
spongiosis. This exudation results in the yellowish color 
of the scales. 
The etiology of seborrhoeic dermatitis is not clear. 
However, an overgrowth of Pityrosporon ovaleplays a 
significant role. Indeed, treatments with antimycotic 
drugs result in a major improvement in the majority of 
patients. In children the occurrence of flexural and scalp 
psoriasis is extremely seldom. In adults, however, 
psoriasis inversa may at11ict the areas, which are affected 
by seborrhoeic dermatitis. The yellowish scaling, instead 
of the silvery scaling in psoriasis , is the most useful 
hallmark for differentiation. The histological equivalent 
of this is spongiosis. The occurrence of psoriatic lesions 
elsewhere and seborrhoeic dermatitis-specific features 
on the trunk may be helpful as well. 
Sometimes it is impossible to differentiate psoriasis 
from seborrhoeic dermatitis. In this situation the 
condition might be designated as sebo-psoriasis. 
Pityriasis rosea 
Pityriasis rosea (Gilbert) is an erythematosquamous 
dermatosis characterized by nummular sharply demar-
cated erythematosquamous plaques. The lesions are 
localized on the trunk and the proximal parts of the 
limbs. The lesions usually start with one single lesion, 
which may enlarge to palm-sized plaques. The lesions 
are round-oval and are arranged along the split lines of 
the skin (Figure 9). The scaling is a collarette within the 
e1ythematous plaque. 
Histopathology is different from psoriasis. In the 
acute phase the epidermis shows vacuolation and in 
the chronic phase an aspecific dermatitis with mild 
mononuclear infiltrate and slight acanthosis is seen. The 
etiology is not known. But the course is restricted to 
about 6 weeks. 
Nummular psoriasis may closely look like Pityriasis 
rosea. The course of the disease (6 weeks only), the 
scaling collarette and the arrangement according to 
splitlines is highly characteristic. Seldom a biopsy from 
the margin of the plaque is needed. 
Lupus erythematosus 
Lupus erythematosus (LE) is an autoimmune disease 
of the skin, characterized by e1ythematous changes. In 
case of lupus e1ythematosus discoides (LED) follicular 
hyperkeratoses and atrophy of the skin are characteristic 
features (Figure 10). Histologically, atrophy of the 
epidermis with hydropic degeneration of the basal layer 
and follicular keratoses is the well-established picture 
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Continuous dermato l o g i cal education 
of LED. Immunohistochemically, a deposition of 
immunoglobulins at the dermoepidermal junction is 
diagnostic. In systemic LE, the follicular changes and 
the atrophy are less conspicuous and involvement of 
the interna! organs occurs, in particular involvement of 
the joints. 
Sometimes it is difficult to differentiateLED of the 
scalp from psoriasis. However, follicular scaling and the 
histopathological picture helps to differentiate LED from 
psoriasis. Atrophy is not restricted to LED of the scalp. 
Scalp psoriasis may also show extensive atrophy. Table 
II summarizes the differential diagnosis of LED versus 
psoriasis. 
Fungal infections 
Epidermomycoses are frequent skin disorders. 
These are characterized by sharply demarcated erythe-
matous plaques with an elevated border. If localized 
on palms and soles, scaling is especially prominent in 
the skin folds of the palms and soles. Fungal infections 
ofthe scalp (trichomycoses) are characterized by sharply 
demarcated erythematosquamous lesions; in addition 
the hair is often broken and follic,ular pustules can be 
observed. 
Fungal infections can be misdiagnosed as psoriasis; 
direct examination of the scales and cultures will show 
the fungus. 
Parapsoriasis 
Parapsoriasis is a skin disorder, which looks like 
psoriasis but still is very different from psoriasis. 
Parapsoriasis is characterized by ill-defined erythe-
matosquamous plaques with a characteristic cigarette 
paper-like atrophy. Parapsoriasis can be manifest as 
"small plaque parapsoriasis". This condition is also 
designated as "chronic superficial dermatitis". Histo-
pathology reveals an aspecific chronic dermatitis. The 
course of the condition is chronic over years and usually 
no progression to mycosis fungoides occurs. 
Parapsoriasis can be manifest as "large plaque 
parapsoriasis". This condition is also designated as 
"poikiloderma atrophicans vasculare" and is complica-
ted by a more severe atrophy of the skin with reticular 
hyperpigmentations. Parapsoriasis of the large plaque 
type progresses to mycosis fungoides in about 20% of 
the patients. The histopathological picture is characte-
rized by a dense lymphocytic infiltrate with epider-
motropism. 
The differentiation from psoriasis is not difficult: the 
unsharp demarcation, the atrophy of the skin and the 
acta dermatovenerologica A.P.A. Vol 8, 99, No 2 
Continuous dermatological ed u cat ion 
Figure 1 O. Lupus erythematosus discoides (elbow). 
Figure 11. Pityriasis rubra pilaris. Follicular 
hyperkeratosis, islands of normal epidermis 
within erythematous lesions 
The differential diagnosis oj psoriasis 
Figure 9. Pityriasis rosea. Note the erythematous 
lesions aranged according to the split lines of 
the skin. 
histopathological picture permit to make the right 
diagnosis (Table III). 
Pityriasis rubra pilaris 
The classical manifestations of pityriasis rubra pilaris 
(PRP) are characterized by follicular hyperkeratoses 
(Figure 11) and erythematous lesions, which may extend 
to erythroderma. The nail may show linear streaks with 
splinter haemorrhages. Within erythematous lesions 
sharply demarcated islands of normal skin can be seen. 
Palmaplantar keratoderma may occur. 
Histologically, follicular hyperkeratosis with a su-
rrounding parakeratotic area is diagnostic for PRP. 
Syphilis 
Secondary syphilis often has important skin mani-
festations. These may be localized in the genital area 
such as condylomata lata. Often these lesions are more 
disseminated such as roseola. Secondary syphilis also 
may adopt a picture, which is close to nummular pso-
riasis. A characteristic feature is the pronounced involve-
ment of the palms and soles (Figure 12). 
In case of a "sudden appearance" of nummular psori-
asis 'de novo' with involvement of palms and soles . 
Syphilis serologywill reveal the diagnosis. The histopa-
thological p icture of lesional skin will show numerous 
plasma cells. 
Figure 12. Syphilis II of the palms. 
acta dermatovenerologica A.P.A. Vol 8, 99, No 2 - ----------------------------- 57 
The d/fterential diagnosis c1f psoriasis Continuous derma t o l ogica l education 
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ADDRESS 
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Peter C.M . van de Kerkhof, Projessor oj Dermatology, Department oj 
Dermatology, University HospitalNijmegen, PO Box 9101, 65 00 HB Nijmegen, 
The Netherlands 
acta dermatovenerologica A.P.A. Vol 8, 99, No 2