Online-only article: Differential diagnosis of psoriasis

Psoriasis is common but can be difficult to distinguish from other skin disorders. Dr Brian Malcolm describes some of the many differentials and provides a checklist of features to look out for.

Psoriasis, like many very common diseases such as acute appendicitis or myocardial infarction, is a straightforward clinical diagnosis when presenting in the conventional manner. However, just as the ‘silent' MI or retroperitoneal appendix can catch even the most astute diagnostician, the clinical manifestations of psoriasis are so protean that even the most experienced dermatologist can be challenged.

Psoriasis is one of the most prolific of all dermatoses, affecting approximately 2% of the population globally. It can present at any age but there is a significant bias towards teenage or early adult life with a smaller peak around the age of 50.

The long-term management of the more difficult spectrum constitutes a significant workload for every dermatology department.

It is axiomatic that the patient is aware from the outset, once a firm diagnosis is achieved, that we talk in terms of ‘control' and ‘treatment' rather than ‘cure'.

Let us briefly review the cardinal clinical features on which we base our diagnosis. None of these are unique or pathopneumonic; there is no definitive sign or test but an assessment of all the available evidence that confirms or refutes the diagnosis.

We will consider in turn the different clinical features and their possible differential diagnoses. The following checklist should prove useful.

Dermatology is often rather simplistically classified as a discipline based on pattern recognition. There are, however, certain classical patterns of skin involvement associated with psoriasis but all share the common ground of symmetry and margins of sharp demarcation between involved and uninvolved skin (see picture below).

Symmetrical psoriasis

Chronic plaque psoriasis is a disease of the extensor surfaces, particularly the elbows and knees, but there is a predilection for other sites such as the scalp, nails, umbilicus and genitalia. The face is relatively spared.

Another common dermatosis that presents in a symmetrical fashion is atopic eczema, which is classically flexural but there is also the so-called ‘reverse pattern' presentation where extensor involvement is the norm or may co-exist with flexor involvement. Sharp delineation of involved skin is unusual, although this can be a feature of both contact and discoid eczema. These latter conditions, however, should not be difficult to distinguish from psoriasis by good history taking and examination.

There are some crossover patterns between eczema and psoriasis that can be clinically impossible to reliably distinguish between where compromise is reached with descriptive terms such as eczematous psoriasis or psoriasiform eczema.

There is a similar hybrid in scalp and facial rashes where there may be both features of psoriasis and seborrhoeic eczema - so-called ‘sebo-psoriasis'. Indeed, a reliable differentiation between seborrhoeic eczema and psoriasis isolated to the scalp can be virtually impossible. Pragmatically, this is not necessarily of great importance as the therapeutics are very similar.

Another relatively common dermatosis that can be confused is lichen planus. The rather more violaceous colouration, presence of Wickham's striae, mucosal involvement, degree of itch and the usual ultimately self-limiting nature of this condition make the distinction less problematic. Histology can be helpful in more difficult cases.

Other patterns of skin involvement can co-exist or overlap or be the sole presenting feature. Flexural psoriasis can be difficult to differentiate from fungal intertrigo or erythrasma especially if it becomes secondarily infected or colonised.

Clues to diagnosis between psoriasis and fungal/yeast infections would be of more classically psoriasiform features of the distal margins of involvement, a lack of central clearing and absence of satellite lesions and symmetrical involvement. All of these would favour psoriasis.

Mycology may be helpful, although false positives from secondary contamination are a consideration.

Wood's light examination helps to distinguish erythrasma with the exhibition of salmon pink fluorescence mediated by porphyrin produced as a bi-product of the causal organisms - corynebacterium.

On initial presentation, differentiation between guttate, (from ‘gutta' meaning drop in Latin) psoriasis and pityriasis rosea can be very challenging, as the age group and anatomical distribution can be similar. A convincing history of a pre-existing herald patch, close examination of individual lesions demonstrating a subtle in-turning collarette of scale, sparing of the distal limbs, an upside-down Christmas tree pattern on the trunk and ultimately a history of complete resolution usually after six weeks favour a diagnosis of pityriasis rosea.

The skin scaling in psoriasis is characteristically silvery on an erythematous background. Gentle scraping across the surface exacerbates the scale and more vigorous scraping demonstrates capillary pinpoint bleeding - the so-called ‘Auspitz' sign. However, eliciting this sign is rather discouraged on health and safety grounds these days.

Koebnerisation describes the feature of lesion distribution in sites of trauma, for example surgical wounds, linear scratch marks etc. It is characteristic but not unique to psoriasis.

Other common dermatoses that manifest this phenomenon are lichen planus and viral warts.

Involvement of other sites
There are several patterns of nail involvement; 50% of patients with psoriasis have some nail involvement (rising to 80% if there is coexistent arthropathy). This includes pitting, onycholysis, discolouration, either oil drop or salmon patch and subungual hyperkeratosis. None are unique to psoriasis. Patients may exhibit any combination of these nail changes, the main pitfall is a diagnosis of tinea unguium, which may produce similar changes, although specifically the presence of pitting would favour psoriasis. The presence of tinea can be a secondary phenomenon following opportunistic colonisation of already damaged nails. A careful history about the gradual evolution and asymmetry of nail involvement in those with fungal infection should help differentiate.

Scalp As previously stated, scalp involvement alone can be very difficult to distinguish from seborrhoeic dermatitis/eczema. Features favouring psoriasis would be a predilection for the margins, involvement of the inner concha of the ear and external auditory meatus (so-called ‘Schuster's sign') and lack of facial involvement.

Genitalia As with flexural psoriasis, there may be a more characteristic silvery scale on the periphery of any area of genital involvement more distant from opposed or occluded surfaces. Both tinea cruris and psoriasis can involve the inner thighs but tinea tends to spare skin with a more highly sebaceous content, such as the scrotum or penile shaft, while areas of psoriasis may often be more confluent.

Psoriasis of the penis itself has to be distinguished from other causes of balanitis - Zoon's (plasma cell) balanitis for example has a more shiny sprinkled cayenne pepper appearance. In cases of doubt, especially of dysplasia/neoplasia is considered, histological examination may be required.

This is a rather more unreliable feature although characteristically psoriasis is rather less itchy than either lichen planus or eczema but this is an inconsistent finding.

Family history
A positive family history of psoriasis in a first-degree relative will be evident in up to 40%. Eczema may also be associated with both familial elements and coexistent atopy such as asthma, hay fever or urticaria.

Exacerbating factors
Psoriasis is recognised to be initiated or exacerbated by a range of stressors including infection (especially the association between streptococcal tonsillitis and guttate psoriasis in adolescence), drugs such as beta-blockers, lithium and antimalarials, psychological stress, alcohol and obesity (due to increased levels of TNF-alpha). Such features may lend some additional supportive evidence but are not specifically reliable to establish or refute the primary diagnosis.

Coexistent arthropathy
Around 8-10% of patients with psoriasis have a coexistent sero-negative arthropathy. As with nail involvement, there are a number of characteristic patterns but none is pathopneumonic; these may overlap or coexist:

  • Oligoarthropathy;
  • Symmetrical rheumatoid-like polyarthropathy;
  • Sacroileitis;
  • Arthritis mutilans.

Additional pattern
Psoriasis can also present with some other striking clinical patterns:

Erythroderma Defined as 90% or more of skin involvement with erythema. A number of conditions can manifest this way, including psoriasis itself, eczema, drug reactions, para-neoplastic conditions and pityriasis rubra pilaris. The latter is considered by some to be a psoriasis variant.

When there is pre-existing psoriasis, the diagnosis may be clear but if erythroderma is the first presentation, the diagnostic process becomes more challenging!

Pustular psoriasis Pustular psoriasis can be generalised (Von Zumbusch) or superimposed on existing psoriasis and can constitute a medical emergency but more commonly presents as a recognised sub-pattern of palmar plantar pustulosis most prevalent in female smokers (see picture below).

Plantar pustular psoriasis

The main differential here is of pompholyx eczema, which can usually be distinguished by the presence of small ‘sago grain' sub epidermal vesicles although larger bullae are possible. Pustular psoriasis again will demonstrate the common features of very concise lines of delineation between normal and abnormal skin, with uniform vesicles filled with sterile pus, which can vary in colour from yellow to brown depending on their age.

There are patterns of psoriasis that can be devilishly difficult to differentiate from other clinical conditions, especially when the presentation may be atypical, for example asymmetric or just a single area of involvement.

Mycosis fungoides
Plaques of mycosis fungoides (cutaneous T cell lymphoma: see figure 3) can, especially in the early stages, look very eczematous or psoriasiform.

Ulcerating plaque stage mycosis fungoides

This diagnosis is important not to miss. When there is doubt or concern, histology is indicated and this may have to be repeated at intervals before a firm diagnosis can be reached. Histology in the diagnosis of psoriasis is rarely necessary - the features of parakeratosis, focal orthokeratosis and microabscess formation are characteristic but not diagnostic.

Tinea of the skin can also be confused for psoriasis, especially if the pattern is annular. Mycological diagnosis is dependent on the quality of the specimen received and the local level of service. Tinea incognito is the term used when the morphological appearances are altered by steroids, although this is more commonly misdiagnosed as eczema, it can also be a problem with psoriasis when topical steroids are in use (see picture below).

Tinea incognito with psoriasis

Psoriasis is very common; the diagnosis is usually straightforward but always must be considered in any puzzling dermatological presentation, especially in the presence of erythema and/or scaling.

A diligent whole body examination should be undertaken to look for other areas of involvement as the patient may be unaware and asymptomatic in this respect.

Particular attention should be paid to the examination of the scalp, nails and genitalia. Histology can occasionally be helpful in more difficult cases.


  • Dr Brian Malcolm is an associate specialist and GPSI in dermatology in Barnstaple, Devon, honorary associate lecturer, Cardiff University, and executive treasurer of the Primary Care Dermatology Society

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