This section is from the book "A Treatise On The Materia Medica And Therapeutics Of The Skin", by Henry G. Piffard. Also available from Amazon: A Treatise On The Materia Medica And Therapeutics Of The Skin.
By diathesis in dermatology, we understand a general constitutional condition or dyscrasia, which is accompanied by, or predisposes to the development of certain forms of cutaneous disease. The diathesis may be hereditary or acquired. The important diatheses to be considered in this connection are the Scrofulous, the Rheumic, the Syphilitic, the Leprous, and the Ichthyotic. The last three will be considered in connection with the diseases Ichthyosis, Leprosy, and Syphilis. The Scrofulous diathesis is so thoroughly recognized and considered in works on general medicine and surgery, that I do not deem it expedient to occupy space for its special consideration. Its interest to the dermatologist resides in its influence as an etiological factor, in connection with lupus, lichen scrofulosorum and a variety of chronic dermic abscess, sometimes called "phlegmonous scrofulide." The presence of this diathesis, however, does not exclude the occurrence of some other forms of cutaneous disease; and in many cases it exerts a modifying or complicating influence on them.
The rheumic, or dartrous diathesis, as it is called in France, is the predisposing cause, I believe, of eczema, psoriasis, and pityriasis. As these three affections include fully one-fourth of all cases of skin disease that will present themselves for treatment, it is obvious that the constitutional condition which underlies them should be very carefully and fully considered. My own views on the subject were first detailed in a paper presented to the New York Academy of Medicine, in 1875, and are here reproduced, as increased observation and experience, and the results of treatment confirm me in the belief that they are substantially correct.