Diagnosis

In well-marked and typical cases there cannot be the least difficulty in diagnosis, especially to anyone who has already seen an example of the disease. Unfortunately, however, cases are not always typical; and we must learn to distinguish psoriasis from syphilis, eczema, and dermatitis exfoliativa. As we have already stated, eczema may closely resemble psoriasis. In like manner the latter disease may closely counterfeit the former in its out­ward appearance; and in this particular case the diagnosis will be by no means easy, nor arrived at a glance, but only by careful consideration of the case in all its bear­ings. A squamous syphilide may closely resemble psoriasis; but here the history will aid us greatly if we bear in mind a few fundamental facts. In psoriatic cases of long standing we will have the history of repeated outbreaks of eruption, but they will all have presented the same general type - that is to say, a repetition of the same kind of eruption - a squamous syphilide will probably have been preceded by other eruptive attacks; but these have been in all probability a different sort of eruption - papular, pustular, or what not. Syphilis rarely repeats itself in its manifestations. If in addition we learn from the patient the prior existence of the primary lesion, or if we find other coexisting lesions, as alopecia, mucous patches, throat trouble, etc., we should not long remain in doubt as to the nature of the eruption about which we have been consulted. The existence of squamous lesions on the palms and soles in connection with squamous patches on the general surface is evidence positive of syphilis. In psoriasis the epidermic proliferation or desquamation is much greater than in syphilis. We have known a case of exfoliative dermatitis to be mistaken for psoriasis; but if we recollect that the characteristic feature of the former disease is the exfoliation of quite extensive laminae, of not very greatly thick­ened epidermis, sometimes several square inches in extent, there is no excuse for mistaking the one disease for the other. Psoriasis and syphilis may of course coexist. There will be but little difficulty in differentiating the respective lesions.


Psoriasis may coexist with eczema, both presenting typical lesions, or we may have lesions of mixed character, in which it would be hard to say which disease pre­dominated. Certain diseases of other organs appear to bear a close relationship to psoriasis. This is notably true of arthritic affections and also of asthma. As a rule, these do not coexist with the psoriasis, but manifest themselves during the time that the skin is free from eruption, alternating as it were with the cutaneous lesion.