Which other diseases can look like psoriasis?

Most of the time, the diagnosis of psoriasis is straightforward.

However, a doctor may have doubts. The most common form of skin conditions that psoriasis is often confused with are the following:

Seborrheic dermatosis

Eczema
Parapsoriasis

Pityriasis rosea

Pityriasis rubra pilaris
Lichen planus
Bacterial or fungal intertrigo

Secondary syphilis

Vitiligo

Lymphoma cutis

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Forms of psoriasis that are difficult to diagnose

Seborrheic dermatosis

This skin condition is characterised by poorly demarcated erythemato-squamous (red and scaly) plaques. It principally involves the face (the nasolabial folds, the ala of the nose and the area between the eyebrows) and the frontal and parietal scalp. Lesions are particularly common in winter. They are frequently brought on by stress, and are moderately itchy.

The presence of psoriatic lesions on various parts of the body makes diagnosis easier. However, when lesions caused by seborrheic dermatosis are confined to the scalp, it is difficult to differentiate it from psoriasis of the scalp as extensive flaking is observed in both conditions. However, scale caused by seborrheic dermatosis is more oily than those present in psoriasis.

It should be noted that there are "overlap" forms of seborrheic dermatosis and psoriasis. For example, a patient with psoriasis may develop seborrheic dermatosis. The converse is also possible; psoriasis may appear in a patient with seborrheic dermatosis.

In these mixed forms, lesions are usually very scaly, cover the entire scalp and extend beyond the hairline.

Babies may also suffer from seborrheic dermatosis. In this case, the scalp and the buttocks are particularly affected.

Psoriasis is rare in newborns, but some seborrheic dermatoses do turn out to be psoriasis or eczema.

The course of the skin condition in the months following the first flares allow a diagnosis to be made. In seborrheic dermatitis, lesions gradually disappear whereas in psoriasis, they tend to be persistent.

Eczema

Eczema is a very common skin disorder characterised by red plaques covered by small blisters. It causes intense itching. The blisters frequently weep and break up to form small scabs. The condition is often caused by an external factor, in which case it is known as contact eczema.

Atopic eczema (or atopic dermatitis) is systemic and appears often during the first months or first years of life. This eczema appears in atopic patients (patients with a tendency to develop many allergies) and is often associated with asthma and hay fever. There is a genetic predisposition in one out of two or three cases. Red and scaly plaques are found mainly in the skin creases and on the cheeks.

The fact that blisters are present in eczema allows it to be distinguished from psoriasis. Moreover, lesion topography (face and skin folds) and association with other conditions (such as asthma and hay fever) will confirm the diagnosis of atopic dermatitis.

Nummular eczema, which is a clinical form of eczema presenting as coin-shaped lesions, can sometimes be confused with nummular psoriasis.

Nummular psoriasis and nummular eczema both present with red, itchy plaques which are the same size and shape as a coin. If these lesions are associated with classic eczema lesions, there is no need for differential diagnosis.

If doubts persist, the presence of weeping blisters will confirm the diagnosis of nummular eczema.

Lastly, in some patients eczema lesions appear on top of their psoriatic lesions. Eczematous psoriasis can then be diagnosed.

Diagnosis is by no means easy in this case as eczematous psoriasis may include weeping or cracked lesions which could be mistaken for eczema.

Parapsoriasis

This term is used for skin disorders which are characterised by red patches or papules and by scale which flakes away from the skin.

Although these dermatoses have certain similarities to psoriasis, they are distinct clinical conditions. Distinction is made between guttate parapsoriasis and plaque parapsoriasis.

Guttate parapsoriasis is a disorder characterised by an eruption of papules which become scaly. Lesions primarily involve the trunk, arms and legs. After desquamation (flaking), patches of pigmentation remain on the skin. The cause of this condition, which appears mostly in young adults, is unknown.

Chronic guttate parapsoriasis is sometimes difficult to differentiate from guttate psoriasis, although in parapsoriasis the lesions are more numerous, more extensive and have a brownish orange colour. The diagnosis can be confirmed by examining tissue under a microscope.

Plaque parapsoriasis is a chronic skin condition which represents a potentially premalignant state of lymphocyte proliferation (lymphoma). It presents as numerous scaly plaques on the trunk, arms and legs. The plaques may be small or large, measuring up to 20 cm in diameter.

There is a risk that atrophic large plaque parapsoriasis will progress towards lymphoma. This type of parapsoriasis rarely poses a problem of differential diagnosis with psoriasis.

Pityriasis rosea

Pityriasis rosea, or Gibert's disease, is a dermatosis which produces red and scaly lesions and tends to disappear in four to eight weeks. It is probably virally induced and mostly affects children and young adults.

The condition is characterised by an oval patch which appears on the trunk. This patch is paler in the centre and is followed by a pink rash with fine scales that covers the body from the neck to the knees. These lesions are sometimes itchy.

As the scale is finer than in psoriasis and the condition clears up spontaneously in a few weeks, it is easy to differentiate it from psoriasis.

Pityriasis rubra pilaris

This is a very rare skin condition which presents as follicular hyperkeratosis (thickening of the stratum corneum of the epidermis at the level of hair follicles) and squamae.

Lesions frequently involve the hands and feet. As in psoriasis, the nails are often affected. The condition is chronic.

In the early stage of this condition, it can be difficult to distinguish from palmoplantar psoriasis. However, in pityriasis rubra pilaris, lesions are salmon coloured and the skin looks chalky as a result of hyperkeratosis.

Lichen planus

Lichen planus is a rare inflammatory dermatosis of unknown cause. It presents as mauve papules which are smooth on top. These papules are several millimetres in diameter and sometimes covered with whitish ridges.

Lesions are chiefly found on the inner wrist, forearm and leg but can also involve the lower back and other parts of the body such as the hands, feet and nails.

In one out of three cases, the mucous membranes are affected (e.g. the mouth and genital mucous membranes).

The Koebner phenomenon (i.e. the appearance of a lesion at sites following trauma)  is found in lichen planus, as in psoriasis. Certain clinical forms of lichen planus are difficult to differentiate from psoriasis.

• Palmoplantar lichen planus can resemble palmoplantar psoriasis, particularly when red and scaly lesions are present.
• Ungual lichen planus is rarely confused with psoriasis of the nail. However in some atypical cases, hyperkeratosis (thickening of the nail) and onycholysis (detachment of the nail) which strongly resemble psoriasis are present.
• Oral lichen planus is characterised by whitish lesions and is rarely confused with psoriasis.
• Genital lichen planus may pose diagnostic problems. As psoriatic lesions do not tend to be scaly in this part of the body, they can be difficult to distinguish from lichen planus lesions.

Bacterial or fungal intertrigo

Intertrigo is an inflammatory condition affecting skin folds. As a result of friction between skin surfaces, this condition is particularly common among obese patients and babies. An isolated area of intertrigo can have multiple causes, such as eczema, cutaneous candida, dermatophysis (infection caused by dermatophytes, a type of fungus) and bacterial infections.

Candidal intertrigo appears on the same skin folds as the lesions of inverse psoriasis. This condition is sometimes difficult to differentiate from undiagnosed flexural psoriasis as the lesions are very similar. In contrast to lesions on other parts of the body, the lesions of flexural psoriasis are only slightly scaly, which complicates differential diagnosis.

When psoriasis is known to be present but lesions are confined to skin creases, diagnosis can also be tricky. The presence of psoriatic lesions on other parts of the body (in particular the nails and scalp) should exclude any doubts.

A mycological examination will allow a diagnosis of candidal intertrigo or dermatophytes to be established. Similarly, a bacterial examination will confirm if the lesions have been caused by bacteria. However, inverse psoriasis can be superinfected by bacteria, which complicates the problem.

Dermatologists should consider psoriasis as a possibility when confronted with any chronic intertrigo, especially when antibiotics, antifungals and antiseptics have not been effective.

Secondary syphilis (1)

Secondary syphilis is justifiably nicknamed "the great pretender". It is an infectious disease with numerous possible symptoms and often poses problems of differential diagnosis.
It is caused by treponema pallidum and is mostly transmitted by sexual intercourse.
Syphilis develops in several phases: there is an incubation period, followed by the primary, secondary and tertiary phases.

It may be confused with psoriasis during the second phase. Patients with syphilis present with a rash of round or oval spots on the trunk.

This initial eruption is succeeded by dark red or brownish lesions which are several millimetres in diameter. These papular syphilitic lesions are often scaly and resemble those of guttate psoriasis, which is also highly eruptive. Lesions may appear anywhere on the body, and are particularly common on the palmoplantar areas. However, when the lesions are palpated while wearing gloves, there is a feeling of firmness and infiltration within the skin which is absent in psoriasis.

The appearance of a chancre (a single lesion, often in the genital area) two months before the eruption and positive treponemal serology (Treponema Pallidum is the causative agent of syphilis) steers the diagnosis towards syphilis.

Vitiligo

This skin condition causes loss of skin pigmentation and produces smooth, white patches, the edges of which are highly pigmented.

It has multiple causes, and involves genetic and immunological factors. Psychological stress may also be a factor in triggering its initial appearance and subsequent flares. As in psoriasis, the Koebner phenomenon often plays a role (this is the name given to the phenomenon whereby a lesion appears on an area of skin that has been traumatised).

When psoriasis is in remission, residual loss of pigment in the form of white patches is left by lesions which have otherwise cleared. This loss of pigmentation is sometimes exacerbated by local corticosteroids. It is during a remission phase that psoriasis can sometimes be mistakenly diagnosed as vitiligo. However, there are differences. In psoriasis, not all lesions become depigmented, and red patches are present before the appearance of depigmentation.

Lymphoma cutis

This term is used to cover various cutaneous manifestations of cancer of the blood cells, the proliferation of abnormal cells in the skin. It is most commonly observed in cancers of the lymphocytes (leukaemia and lymphoma). Mycosis fungoides is the most typical form of lymphoma cutis.

Some forms of psoriasis are more problematic to diagnose.

* Palmoplantar psoriasis

Isolated psoriatic lesions on the hands and feet often pose diagnostic problems. Although palmoplantar keratoderma (very thick skin on the soles and palms) is a clinical sign of psoriasis, it may be caused by other skin conditions such as eczema, lichen planus, dermatophysis and pityriasis rubra pilaris. As a result, making the diagnosis with certainty is not always straightforward.

* Psoriasis of the nails

Pitted nails are highly characteristic of psoriasis of the nail and are rarely present in other skin conditions such as alopecia areata, eczema or lichen planus. However, psoriasis of the nail is sometimes confused with infections caused by dermatophytes (parasitic fungi) as onycholysis (detachment of the nail from its bed) may be observed in both conditions.
As psoriasis of the nail may be infected by dermatophytes, a mycological examination is often useful as an infection will need specific treatment.

* Erythrodermic psoriasis

Erythroderma is a skin disorder where large areas of skin become red and inflamed. Lesions are generally covered with scale.

Psoriasis is the principal cause of erythroderma. However, erythroderma may be caused by other conditions, such as eczema, lymphoma and less frequently by pityriasis rubra pilaris. Certain drugs can also result in erythroderma (this is known as toxiderma).

* Psoriatic arthritis

The peripheral form of psoriatic arthritis is often difficult to differentiate from rheumatoid polyarthritis. Rheumatoid polyarthritis is a chronic inflammatory arthritis which affects the joints of the hands and wrists. It is painful, tends to wax and wane and leads to a general deterioration in health. The symptoms of psoriatic polyarthritis are very similar, but there are clinical signs which allow the two conditions to be distinguished. In rheumatoid polyarthritis, joints are affected symmetrically and the distal interphalangeal joints (the end joints of the fingers) tend not to be involved, which is not the case in psoriatic arthritis.

The central form of psoriatic arthritis often resembles ankylosing spondylitis.

Ankylosing spondylitis is a chronic inflammatory arthritis which involves the spine and sacroiliac joints. The condition tends to wax and wane.

Differential diagnosis with psoriatic arthritis is particularly difficult when there are no psoriatic lesions on other parts of the body.