Psoriasis: Differential diagnosis, Investigations

Differential diagnosis

Discoid eczema

Lesions are less well defined and may be exudative or crusted, lack ‘candle grease’ scaling, and may be extremely itchy. Lesions do not favour scalp, extensor

Seborrhoeic eczema

Scalp involvement is more diffuse and less lumpy.

Intervening areas of normal scalp skin are unusual.

Flexural plaques are less well defined and more exudative. There may be signs of seborrhoeic eczema elsewhere, such as in the eyebrows, nasolabial folds or on the chest.

Pityriasis rosea

This may be confused with guttate psoriasis but the lesions, which are oval rather than round, tend to run along rib lines. Scaling is of collarette type and a herald plaque may precede the rash. Lesions are usu-ally confined to the upper trunk.

Secondary syphilis

There is usually a history of a primary chancre. The scaly lesions are brownish and characteristically the palms and soles are involved. Oral changes, patchy alopecia, condylomata lata and lymphadenopathy complete the picture.

Cutaneous T-cell lymphoma

The lesions, which tend to persist, are not in typical loca-tions and are often annular, arcuate, reniform or show bizarre outlines. Atrophy or poikiloderma may be pre-sent and individual lesions may vary in their thickness.

Tinea unguium

This is often confused with nail psoriasis but is more asymmetrical and there may be obvious tinea of neigh-bouring skin. Uninvolved nails are common. Pitting is not seen and nails tend to be crumbly and discoloured at their free edge.

Investigations

1 Biopsy is seldom necessary.

2  Throat swabbing for β-haemolytic streptococci is needed in guttate psoriasis.

3 Skin scrapings and nail clippings may be requiredto exclude tinea.

4 Radiology and tests for rheumatoid factor are help-ful in assessing arthritis.