Naturally this suggests the further question, Are there any distinctive histological changes in the gouty kidney? On this point Walker Hall has some apposite reflections. Taking Folin’s figures as a basis, it transpires that in acute and chronic nephritis, also in arterio-sclerosis, there is an average content of 2·5 mg. uric acid per 100 grams of blood. Now, notwithstanding the fact that in these conditions an appreciable quantity of the renal tissues is, functionally speaking, temporarily or permanently out of action, nevertheless “the extraction of uric acid from the blood and its subsequent excretion are practically normal.” The inference is that a relatively small moiety of renal tissue suffices for the excretion of the daily quantum of uric acid in the urine.
Now in contrast thereto, the blood content in gout and lead poisoning is about 4·5 mg. uric acid per 100 grams of blood, or “an increase of about 50 mg. in the total blood-stream at any one moment (an increase from the normal 70 up to 120 mg.).” Continuing, Walker Hall observes that “the gouty kidney per se, even when arterio-sclerotic conditions prevail, does not show anything like the amount of cellular damage which occurs in acute or chronic diffuse nephritis.” Thence he argues if histological changes be taken as a criterion of functional efficiency, then the gouty kidney should be more capable of excreting freely than the diffuse nephritic organ.
How does this work out in actual daily life? he asks. “0·5 gram, in a normal adult, represents the average daily endogenous uric acid excretion in the urine, while that of a gouty subject is about 0·45 gram. Now, assuming that the type and extent of the endogenous metabolism is identical in each instance, then the balance, i.e., 0·05 gram, is distributed between the uric acid pent up in the tissues and the uratic deposits, i.e., tophi.” Walker Hall tells us it has been stated that about 0·01 gram suffices to cover the amount deposited as tophi every twenty-four hours. The residual 0·04 gram runs to swell the amount in the blood and lymph-streams. “The increase is 0·0114 to 0·0118 gram per litre of blood; in other words, the actual increase of uric acid circulating through the kidneys is about 0·00047 per hour,” which, as Walker Hall contends, “seems to be a very trifling difference, especially as it is one of amount and not a type.” In other words, it is quantitative, not qualitative. But, trivial as the disparity is, to what may it be referred? To Walker Hall’s mind, if we are to appreciate the standpoint of those who maintain that the gouty uricæmia is referable to renal inadequacy, it is necessary to postulate the presence of a poison acting upon the renal tubules specifically.
In the gouty uric acid excretion is maintained at a “low physiological level to the very end,” and it is easier, he thinks, to adopt the above hypothesis as to its cause than “to conceive of a poison acting upon the nuclear processes in such a way as to induce a persistently low uniform level” of purin excretion.
This view, viz., of a toxin acting specifically upon the uric acid excreting cells of the kidney, seems to be the only reasonable assumption available. But even this is difficult of adherence when we recall the fact that the effect of the toxin is so readily neutralised by a few grains of atophan. Always we have to recollect, too, that under normal conditions, even given a constant diet, the elimination of uric acid displays wide variations. Also the uric acid output in the subjects of chronic gout, when placed on a fixed diet, differs but little from that of normal individuals on a like dietary. At most the excretion but tends to fall to, or slightly below, the lower normal limits of uric acid elimination.
From the foregoing considerations it is but too obvious that those who render obeisance to the primary renal origin of gout have not only yet to prove that the functions of the kidney are defective, but also upon them lies the onus probandi why gouty subjects should exhibit, or acquire, such a disability.
Uratic Deposits in Nephritis
Here, again, we light upon another point of contact between gout and nephritis, for an interesting feature of the latter disorder is that the retained uric acid, purins, and other excretory products are deposited in cartilage and serous membranes. At these sites they are frequently detected post mortem, though they fail of ante-mortem recognition.
Impressed by the fact that, at post-mortems, uratic incrustation of the articular cartilages was frequently observed in persons who had never suffered from overt gout, Ord and Greenfield sought to ascertain the frequency with which such uratic deposits were associated with renal disease. Out of ninety-six cases presenting renal lesions, no less than eighteen exhibited uratic deposits in the joints.