These may be congenital or acquired. In the former case and in some of the latter the kidney is fixed in its unusual situation. In Congenital malposition it is generally the left kidney which is concerned. It may be depressed so as to lie as low as the brim or even the cavity of the pelvis. It is not infrequently seated opposite the sacro-iliac synchondrosis. Such kidneys have usually the hilum presenting forward, and they are flattened; their vessels are branches of the lower end of the aorta and of the iliac veins, or even entirely of the iliac vessels. The kidney may also lie nearer the middle line than normal or in the middle line.
These terms designate two conditions, both of them characterized by undue mobility of the organ. The kidney normally lies behind the peritoneum, which covers it only on its anterior surface. The organ, being in the loose retro-peritoneal tissue, is surrounded by a fatty capsule as well as by its more immediate connective-tissue capsule. It is fixed to the posterior wall of the abdomen by its vessels, partly by the peritoneum binding it down, and partly by the pad of adipose tissue which forms its fatty capsule.
In the Movable kidney the organ is unduly mobile behind the peritoneum; it may be movable within its fatty capsule or may carry this with it in its displacement. The organ is sometimes capable of great displacement up under the ribs, down into the pelvis, and. for a short distance across the middle line, although usually the mobility is limited.
The undue mobility occurs in the great majority of cases in females, and it is usually on the right side (in the proportion of 152: 12; see Newman). Its frequency in women is ascribed to the disturbances produced by pregnancy, and also to the wearing of stays. The relations of the right kidney to the liver and ascending colon probably account for the greater frequency on this side. The heavy liver, especially when pushed downwards by stays, may dislocate the kidney, and the ascending colon is more loosely attached on the right side of the abdomen than is the descending colon on the left. Rapid emaciation, by diminishing the pad formed by the fatty capsule, is not an infrequent cause. It is stated that there may be a local diminution of the fat in the capsule.
Floating kidney is, strictly, a kidney with a meso-nephron. The peritoneum covers both surfaces of the organ, and forma a mesentery which contains the vessels. This form is excessively rare, and its existence has been denied by some. It is of congenital origin. The degree of mobility is not greater than in many cases of movable kidney, and the two conditions are scarcely distinguishable during life.
The movable kidney is not very liable to secondary changes, although sometimes the abnormal position interferes with the flow through the ureter, and leads to hydronephrosis, or even inflammation of the pelvis. Of more importance is the fact that by dragging there may be serious nervous disturbances in the form of excruciating cramps. The kidney may, however, be movable without any such nervous symptoms presenting themselves.