Inflammation of the pelvis of the kidney is not always suppurative, as it may be produced by other causes besides the septic agents. A catarrhal pyelitis occurs in the course of some fevers, or it may be present to some extent in acute Bright's disease.

Calculi, by their mechanical irritation, will induce an inflammation of the pelvis, often spoken of as Calculous pyelitis. It is sometimes said that calculi produce suppurative inflammations, but this can scarcely occur unless there be an extension of the septic process from the bladder upwards. The irritation, however, may lead to such inflammation as to cause a considerable abundance of leucocytes in the urine, but not a true pus. On the other hand, septic inflammations, being accompanied by decomposition of the urine, often result in the formation of calculi.

The presence of calculi in the pelvis of the kidney has frequently an irritative effect on the kidney itself. There thus arises in some cases a regular interstitial nephritis, the condition of the kidney coinciding with that in chronic Bright's disease. As already mentioned the contracted kidney so produced may give rise to hypertrophy of the left ventricle and the other associated phenomena.

Suppurative pyelitis is the result of the propagation of septic processes from the bladder by the ureter. It usually occurs in cases where there is some obstruction at the neck of the bladder or in the urethra, and a cystitis with decomposition of the urine has occurred. There is frequently some coincident dilatation of the pelvis, and there may be a proper pyonephrosis.

The mucous membrane is thickened and frequently of a bluish colour. It is not infrequently incrusted with phosphates. The contained matter is a thick brownish ill-smelling pus.