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Paolo Lisi, MD
Dermatologia Clinica, Allergologica e Venereologica
Policlinico - 06122 Perugia, Italy
E-mail: plisi@unipg.it
DIFFERENTIAL DIAGNOSIS OF PSORIASIS
P. LISI
Section of Clinical, Allergological and Venereological Dermatology, Department of Medical and Surgical Specialities
and Public Health, University of Perugia, Perugia, Italy
INTRODUCTION
The clinical diagnosis of psoriasis (P) is relatively
easy, especially when the lesions consist of ery-
thematous (“salmon pink”), silvery white scaly,
sharply demarcated, indurated plaques. They have
an oval or irregular shape, vary from one to sever-
al centimetres in diameter and are distributed sym-
metrically on the extensor surfaces of limbs (main-
ly elbows and knees), the lower back and the scalp.
Itching is very variable, but it is usually absent.
The degree of itching is often conditioned by the
emotional state of the patient and, if severe, may
be a symptom of anxiety or depression (1). These
clinical features reflect the histopathological find-
ings observed in active lesions, characterized by
hyperkeratosis, parakeratosis, diminution or loss
of the granular cell layer, acanthosis of the epider-
mal ridges, tortuous and dilated blood vessels, and
perivascular leukocytic infiltrate in the dermal
papillae. Typically, the epidermal ridges are even-
ly elongated and club-shaped at the tips. Two oth-
er changes are diagnostic: the Munro microabscess
and the spongiform pustule of Kogoj, due to the
small accumulation of neutrophils, respectively
within the parakeratotic stratum corneum and the
spongiotic epidermis.
DIFFERENTIAL DIAGNOSIS
The clinical and histological aspects of chronic P
vulgaris are generally sufficient to make the diag-
nosis. Diagnostic doubts, however, may arise in
several clinical variants and atypical cases or when
the psoriatic lesions are localized in particular sites,
especially in patients with type II P. In these pa-
tients the cutaneous manifestations appear in their
50s (2) and, unlike type I, do not show familial
segregation and are much less significantly associ-
ated with specific HLA phenotypes (Cw6, B13,
B17, DR7) (3). The most common onset of type I
P is in young adults.
Clinical variants and atypical cases
Guttate psoriasis is the most frequent P variant ob-
served in children and young adults. It is charac-
terized by an acute eruption of numerous, small
(0.3-1.0 cm diameter), round or slightly oval, ery-
thematous and scaly papules and plaques which
are widely disseminated, particularly on the trunk
and proximal part of the extremities. The face could
SUMMARY
The clinical diagnosis of psoriasis is relatively easy, especially when the lesions consist of erythematous, silvery white
scaly, sharply demarcated, indurated plaques, distributed symmetrically on the extensor surfaces of limbs, the lower
back and the scalp. These clinical features reflect the histopathological findings observed in active lesions, charac-
terized by parakeratosis, acanthosis of the epidermal ridges, tortuous and dilated blood vessels, and perivascular leuko-
cytic infiltrate; the Munro microabscess and the spongiform pustule of Kogoj are diagnostic. Diagnostic doubts, how-
ever, may arise in several clinical variants and atypical cases (guttate psoriasis, follicular or spinulosic psoriasis, ery-
throdermic psoriasis, pustular psoriasis) or when the psoriatic lesions are localized in particular sites (palms, soles,
scalp, body folds, penis, nails). The value of erythemato-papulosquamous psoriasiform eruptions occurring during or
after the administration of a diagnostic or therapeutic agent especially in psoriatic subjects is discussed.
Key words: Psoriasis, differential diagnosis, skin diseases, clinical features, histopathological aspects
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Differential diagnosis of psoriasis 57
also be involved. The lesions typically appear one
or two weeks after a severe streptococcal infection
of the upper respiratory tract.
The principal differential diagnosis includes some
papulosquamous or erythematosquamous disor-
ders, such as pityriasis lichenoides chronica, sec-
ondary syphilis, lymphomatoid papulosis and pityr-
iasis rosea. In the first, the recurrent lesions are
usually less evenly scattered, have a brownish red
or orange-brown colour, are capped by a single de-
tachable opaque mica-like scale, and often leave
hypopigmented macules. Papules of pityriasis
lichenoides chronica as those of guttate P are
asymptomatic, are localized on the trunk and up-
per limbs and appear prevalently after a pharyngo-
tonsillar infection in pediatric population.
The non-pruriginous papulosquamous lesions of
late secondary syphilis may mimic a guttate P, but
the coppery red colour and the collarette of scales
of papules, the presence of other cutaneous signs
of papular syphilis (e.g. condylomata lata, P-like
plaques of the palms and soles, corona veneris) and
the generalized lymphadenopathy with enlarged
indolent lymph nodes remove any diagnostic
doubts even before the results of serological test-
ing are known.
Lymphomatoid papulosis, which sometimes may
be confused with guttate P, is a chronic, recurrent,
self-healing papulonecrotic or papulonodular skin
disease with histologic features suggestive of a ma-
lignat lymphoma (CD30-positive). The coexistence
of red-brown papules, nodules with ulcerated
necrotic centres, hypopigmented or hyperpigment-
ed macules and, occasionally, fine atrophic circu-
lar or varioliform scars predominantly affecting the
trunk and limbs is diagnostic.
Pityriasis rosea is a common acute self-limited
papulosquamous eruption, probably viral, which is
more frequent in adolescents and young adults. The
classical lesions are pink or salmon in colour, oval
in shape and with a long axis oriented along the
Langer’s cleavage lines; they are usually distributed
on the trunk and proximal extremities. These le-
sions are easily distinguishable from those of gut-
tate P because of their appearance; they are larger
in size, with small fine scales, squamous marginal
collarette and a “fir-tree” or “Christmas tree” dis-
tribution on the posterior trunk. The onset of a soli-
tary herald patch which is wider than the lesions of
the later eruption and the spontaneous resolution in
6-8 weeks are pathognomonic for pityriasis rosea.
The erythemato-papulosquamous psoriasiform
eruptions occurring during or after the administra-
tion of a diagnostic or therapeutic agent may mim-
ic an eruptive P, especially in patients with P or a
psoriatic diathesis. In these cases the bright red
colour of the lesions, less abundant scaling, more
severe itching, eosinophilia and the variable num-
ber of eosinophils in the dermal infiltrate are more
typical of a drug reaction. However, the diagnosis
is often retrospective. Several drugs are known to
exacerbate a pre-existing P (antimalarials and,
sometimes, non-steroidal anti-inflammatory drugs,
angiotensin-converting enzyme inhibitors) or trig-
ger off the onset of P lesions in normal and psori-
atic subjects (lithium salts, beta-blocking agents,
interferon-alfa) after a variable latent period.
Follicular or spinulosic psoriasis is rare but more
frequent in children. It may be mistaken for pityr-
iasis rubra pilaris which is characterized by rough
follicular papules on an erythematous base (dark
red), especially on the dorsal aspect of the fingers,
and by large orange-red plaques with distinctive
“islands of sparing”. An orange-red waxy palmar
and plantar keratoderma is also peculiar. Histolog-
ically pityriasis rubra pilaris differs from P due to
the presence of alternating parakeratosis and hy-
perkeratosis in both vertical and horizontal direc-
tions (spotty parakeratosis). Neutrophil infiltration
of the stratum corneum is not a feature of pityria-
sis rubra pilaris unless there is a secondary infec-
tion (4).
Erythrodermic psoriasis is a generalized redness
and scaling, often associated with oedema, which
involves more than 90% of the skin surface. Its on-
set can be precipitated by the rapid withdrawal of
systemic or topical corticosteroid after prolonged
use, treatment with antimalarials, acute infections,
hypocalcaemia, over-exposure to sunlight or UV ir-
radiation when the disease is in a Koebner-positive
phase. The differential diagnosis with other ery-
trodermas (Tab. I) is not always easy, but previous
plaques in classic locations, nail involvement, fa-
cial sparing, inflammatory arthritis and family his-
tory of P are diagnostic clues. Itching may be se-
vere, but it is more typical and severe in erythro-
dermas due to eczematous dermatitis or Sèzary
syndrome, in which lymphadenopathies are also a
more common finding. In spite of multiple skin
biopsies and clinical investigations, the underlying
cause of erythroderma is not found in about one-
third of patients (5).
Pustular psoriasis (PP) (generalized PP or von
Zumbusch pattern, generalized PP of pregnancy or
impetigo herpetiformis, localized PP on palms and
soles or Barber pattern) and its variants (circinate
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58
P. Lisi
or Bloch-Lapière‘s pattern, exanthematic, acroder-
matitis continua of Hallopeau) are characterized by
non-follicular sterile pustules, which represent the
macroscopic aspect of the massive neutrophil infil-
tration of epidermis. Focal infections, corticosteroid
therapy, topical irritants, pregnancy, hypocalcaemia
and stress have been reported as triggering factors.
Smoking may aggravate the course of lesions lo-
calized on the palmoplantar surfaces.
All of these variants must be distinguished from
other amicrobic pustular eruptions, in which the
histological aspect of the lesions is sometimes
spongiform or multilocular as in pustular P, but
more often it is non-spongiform or unilocular
(Tab. II). The most important diagnostic problems
are raised by acute generalized exanthematous
pustolosis, particularly in patients with a family
and/or personal history positive for P, and sub-
corneal PP.
Acute generalized exanthematous pustolosis
(AGEP) is an acute febrile drug eruption charac-
terized by numerous small, primarily non-follicu-
lar sterile pustules, arising within large areas of
edematous erythema, which also happens in gen-
eralized pustular psoriasis.
The acuteness of lesions, their first appearance on
the cutaneous folds and face, the following rapid
dissemination, the frequent association with other
skin manifestations (purpura, vesicles, erythema
multiforme like lesions), the intense itching, the
high fever, and the relatively short latent time be-
tween the drug administration and the skin eruption
(1-2 days) make the diagnosis of AGEP possible.
Neutrophilic leucocitosis and the mucous involve-
ment, on the contrary, are not diriment because
they are present in both diseases; in particular, the
circinate scaling of the tongue is a typical sign of
PP (benign migratory glossitis). Histological fea-
tures can also be helpful. Massive edema and
mixed perivascular inflammatory infiltrate in the
superficial dermis, exocitosis of eosinophils and
necrosis of several keratinocytes are more sugges-
tive of AGEP, while parakeratosis and papilloa-
canthosis are more characteristic of PP.
Subcorneal pustular dermatosis (Sneddon-Wilkin-
son disease) may be easily differentiated from PP,
whether clinically or histologically. The annular,
polycyclic or serpiginous lesions with a scaly
edge, the relapsing flaccid vesicopustules with
clear fluid in the upper half and pus in the lower
half, and the absence of spongiform changes are
diagnostic, as is the positive response to dapsone.
In other cases, mainly when the pustules are asym-
metrically localized on the palmar and plantar re-
gions, infective conditions (e.g. pustular dermato-
phytoses, bacterial impetigo) and eczematous der-
matitis (e.g. dyshidrosis, irritant or allergic contact
dermatitis) must be excluded by carrying out PAS
or GRAM stains and cultures of biological mater-
ial, and patch test.
Table II - Histological classification of amicrobic pustular eruptions (6).
Spongiform or multilocular intraepidermal pustules Non-spongiform or unilocular intraepidermal pustules
Pustular psoriasis Subcorneal pustular dermatosis or Sneddon-Wilkinson disease
Post-streptococcal acute generalized pustolosis IgA intraepidermal neutrophilic dermatosis Infantile acropustolosis
Acute generalized exanthematous pustolosis Fiessinger-Leroy-Reiter syndrome Pustular vasculitis of the hands
Generalized pustolosis associated with hypocalcaemia caused
by post-surgical hypo-parathyroidism, chronic renal failure, ..
Table I - The main underlying causes of erithrodermas in neonates, infants and adults (from Sterry and Muche (5), modified).
Common causes Less common causes Rare causes
Atopic dermatitis (also in infants) Contact dermatitis Ofuji’s papulo erithroderma
Psoriasis (also in infants) Seborrheic dermatitis Autoimmune connective tissue disorders
Drug reactions Chronic actinic dermatitis (dermatomiositis, subacute lupus erythematosus)
Cutaneous T-cell lymphoma Pityriasis rubra pilaris Norwegian scabies
(Sèzary syndrome, mycosis fungoides) Bullous dermatoses (pemphigus foliaceus, Hypereosinophilia syndrome
Congenital ichthyoses (bullous and not) bullous pemphigoid, paraneoplastic pemphigus) Lichen planus
(in neonates and infants) Netherton syndrome (in infants) Dermatophyte infection (also in infants)
Staphylococcal scalded skin syndrome (in neonates) Wiskott-Aldrich syndrome (in infants)
Omenn’s syndrome (in neonates) Graft-versus-host disease (in infants)
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Differential diagnosis of psoriasis 59
Morphological modifications by lesion
localization
Psoriatic lesions exhibit a considerable degree of
morphological uniformity, even if they might show
regional variations. Those observed on palms and
soles are the most significant, probably because of
the skin thickness and the trauma related to occu-
pational activities and/or hobbies. The sharply de-
fined edge of plaques, the orange-red colour, the
absence of vesiculation and the dull red thickening
of the skin on the knuckles are helpful to exclude
keratotic eczema, lichen simplex or secondary
syphilis. Nail changes are diagnostic. Palmo-plan-
tar P, nevertheless, may be eczematous, mainly in
children and in those with a positive family histo-
ry for P and/or atopy, or lichenoid. The violaceous
colour, glistering surface and oral changes are sug-
gestive of lichen planus.
The scalp is one of the most common sites for P.
Its involvement may be diffuse or in multiple dis-
crete plaques of varying size. In contrast to sebor-
rhoeic dermatitis, psoriatic patches are well-de-
fined, silvery white, xerotic, pruriginous, and often
advance beyond the hairline. Sometimes, the scales
have an asbestos-like appearance and are firmly at-
tached to the scalp hairs (pityriasis amiantacea).
However, the differential diagnosis may be diffi-
cult, especially in subjects without psoriatic lesions
on classic sites. In these cases also histological as-
pects are not decisive, although the spongiotic com-
ponent is less cospicuous or absent in P.
Flexural or inverse P is characterized by the de-
velopment of shiny, pink to red, sharply demarcat-
ed thin plaques in the body folds (axillae, sub-
mammary creases, inguinal folds, intergluteal cleft,
retroauricolar folds), which are commonly fissured
at the depth. As this P pattern is one of the causes
of intertrigo, it must be differentiated from other
etiologies as bacterial intertrigo, Candida intertri-
go, seborrhoeic dermatitis and, less often, ery-
thrasma, contact dermatitis, Hailey-Hailey disease,
flexural Darier’s disease (Table III). The potassium
hydroxide examination and the culture of scales,
Wood’s lamp examination may be useful to ex-
clude fungal and/or bacterial infections.
Scaling is also greatly reduced or absent in P of the
penis, but the colour of patches and their well-de-
fined edges are usually distinctive. In some cases,
especially when only a lesion is present, a diag-
nostic biopsy is necessary to exclude Zoon’s plas-
ma cell balanitis, erythroplasia, Bowen’s disease or
extramammary Paget.
Toenails and mainly fingernails are frequently in-
volved in all types of P, especially in psoriatic
arthropathy. Pitting, oil spots, subungual hyperk-
eratosis and onycholysis are the most common but
are not patognomonic disorders. However, pits or
punctate depressions of the nail plate due to a de-
fect of the nail matrix are deeper in P than in alope-
cia areata, contact dermatitis or occupational trau-
ma. Also oil spots, caused by a separation between
the nail plate and nail bed due to the accumulation
of large serum-like exudate containing glycopro-
teins, may be found in inflammatory and eczema-
tous diseases.They have been reported in systemic
lupus erythematosus (7). Subungual hyperkerato-
sis is very frequent in P and results from repeated
trauma and numerous other chronic inflammatory
conditions involving the nail bed or hyponychium,
such as onychomycosis, contact dermatitis and
Tabella III - The most common causes of intertrigo and their clinical features.
Skin disorders Clinical features
Infections:
bacterial intertrigo Eczematoid dermatitis, fissuring in the depth of affected flexure
erythrasma Red-brown patches, furfuraceous scales in the margins
Candida intertrigo Glistening deep red colour and little moist exudation of the fold, scaling confined to the edge,
small satellite pustules and tiny erosions outside the main area, soreness and itching
Eczematous dermatitis:
contact dermatitis Reddening, erosions, exudation, maceration, crusts, burning
atopic dermatitis Poorly demarcated erythema, papules, crusts, lichenification, itching
Miscellanea:
psoriasis Shiny, pink to red, sharply demarcated thin plaques, itching
seborrhoeic dermatitis Yellowish-red lesions, bran-like scaling
Hailey-Hailey disease Vesicles and scaling crusts in itchy patches
flexural Darier’s disease Keratotic papules, dirty-gray/yellowish-brown in colour, fetid smell
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60
P. Lisi
pityriasis rubra pilaris. Finally, onycholysis is the
most aspecific nail alteration in psoriatic patients
because the detachment of the nail from the bed is
due to many systemic, congenital, cutaneous, phys-
ical, chemical and infective causes.
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