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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.
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References:
· Psoriasis, de la clinique à la thérapeutique, edited by Jean Thivolet and Jean-François Nicolas, published by John Libbey Eurotext Ltd.
· Psoriasis, edited by Louis Dubertret, published by ISED, in collaboration with Leo Laboratories.
With thanks to Professor Jean-Jacques Guilhou, Head of the Dermatology Service at the Saint-Eloi Hospital, Montpellier, France. |
Reference:
1) Dubertret, Psoriasis (Chapter 34), 1994
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