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.

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