ETIOLOGY - ACUTE PRODUCTIVE OR DIFFUSE NEPHRITIS.
This is the most serious and important of the forms of acute ne phritis for the reason that its lesions are from the first of a permanent character. It does not follow exudative nephritis, nor is it merely a modification of it; from the very outset it is a different form of in flammation. In the kidneys of persons who have been sick only a few days, the characteristic lesions are already evident. Productive nephritis is governed by the same law as that which belongs to pro ductive inflammation in other parts of the body—the disposition of the inflammation to continue as a subacute and chronic condition. It is of importance to recognize that in exudative nephritis the lesions are temporary, and after their subsidence the kidneys return to their normal condition, just as the lungs do after a lobar pneumonia. In productive nephritis, on the other hand, some of the lesions are per manent, the kidneys can never return to their normal condition, just as in an interstitial pneumonia the lung never gets rid of the new connective tissue.
Post-scarlatinal nephritis is nearly always of the productive form. Nephritis complicating diphtheria or developed during pregnancy is very frequently of this type. A primary nephritis in a person over twelve years old, if of subacute form, is almost invariably a produc tive nephritis. On the other hand, this form of nephritis very seldom complicates any of the infectious diseases except scarlatina and diphtheria.
These facts assist very much in making the diagnosis between the two forms of acute nephritis. It is easy to remember that post-scar latinal nephritis and primary nephritis of subacute type are nearly always of the productive form ; and that nephritis with diphtheria and pregnancy is often of the productive form; while acute nephritis under all other conditions is regularly of the exudative form.