—  SPECIALTY CONFERENCE  —

Renal Pathology

Case 2 - Acute Tubular Necrosis

Seymour Rosen
Department of Pathology
Beth Israel Deaconess Medical Center and Harvard Medical School
Boston, Massachusetts, USA


Click on each slide thumbnail image for an enlarged view
Clinical History
19-year old college freshman running in a 400 m race (no prior training) fell just before the end, sustained abrasions, but completed the race. He had not eaten all day, drank minimally on the nine-hour bus ride home, and vomited once. In the Emergency Room, he vomited again, and physical examination was significant for mild hypotension and abrasions. Urinalysis revealed proteinuria (3+), many RBC's, and no casts. He was admitted 3/28; biopsied 3/29.

  CREATININE CREATINE KINASE
3/28 2: 41 AM 3.0 320
3/28 7:10 AM 3.9  
3/28 3:00 PM 4.3  
3/29 7:00 AM 5.1 468
3/30 8:00 AM 3.6 1104
3/31 6:35 AM 2.4 1135
4/07 3:42 PM 1.2 118



Case 2 - Figure 1 - Cortex. Labyrinth containing an unremarkable glomerular tuft. The proximal tubular brush borders are fairly well maintained as documented by electron microscopic studies (inset). The latter shows vacuolar change likely related to endocytosis. PAS
Case 2 - Figure 2 - Cortex. Labyrinth containing two unremarkable somewhat distorted glomerular tufts. A medullary ray is seen in the upper left. The proximal tubular brush borders seemed to be at least in part maintained, but are not clearly delineated as in figure 1. PAS
Case 2 - Figure 3 - Inner stripe of outer medulla. The photograph depicts vasa recta , the vessels of which contain cells which seem to be, at least in part, of hematopoietic origin, i.e. erythroblasts and granulocytic precursors. H&E


Case 2 - Figure 4 - Inner medulla, towards the papillary tip. The collecting ducts show necrosis and regenerative changes. Very focal acute inflammation was noted in the stroma (lower right). H&E
Case 2 - Figure 5 - Inner medulla, towards the papillary tip. The collecting ducts show regenerative changes. PAS


Case 2 - Figure 6 - Inner medulla, towards the papillary tip. There appears to be stromal edema and collecting duct injury. Masson trichrome
Case 2 - Figure 7 - Inner medulla, towards the papillary tip. The collecting ducts show regeneration with focal denudation of basement membranes which is best seen in the right hand photograph. Limited stromal acute inflammation is noted as well (left) . Masson trichrome

Renal Biopsy Findings
The renal biopsy contained fragments of renal tissue including cortex, a small portion of inner stripe of outer medulla and medullary tissue near or at the papillary tip. The cortex included approximately 8 glomeruli, one of which was globally sclerotic. The remainder of the glomeruli were unremarkable. Minimal interstitial fibrosis and tubular atrophy were recognized; vessels showed no abnormalities. Specifically, the proximal tubules seemed intact with limited brush border reduction. The small fragment of inner stripe included vasa recta which contained cells that seemed to be hematopoietic in nature suggesting immature granulopoietic and erythroblastic elements. The papillary tissue was remarkable in that there was collecting duct necrosis and regeneration as well as focal acute inflammation in the interstitial tissues. Immunofluorescence studies were negative and electron microscopic studies of cortical tissue showed unremarkable glomeruli and proximal tubular vacuolization with general brush border maintenance.

Renal Biopsy Diagnosis
Consistent with "Acute Tubular Necrosis". This biopsy is unusual because apparent hematopoietic elements were present in the vasa recta, findings which usually are seen in the autopsy studies of ATN. Furthermore, remarkably, biopsy sampling included papillary tip in which tubular necrosis and regeneration was identified. Given the physiological data documenting the limited oxygen available in this area, this observation is not surprising and is in concert with experimental studies showing the singular vulnerability of the medulla to hypoxic injury.

"Acute Tubular Necrosis"
The term "acute tubular necrosis" (ATN) is used to designate a clinical situation in which kidneys fail to function, but are morphologically basically intact [1]. The circumstances/etiologies include hemorrhage, sepsis, trauma and nephrotoxins. Commonly, there is sufficient renal perfusion such that there is adequate blood flow to largely maintain tubular integrity but not to sustain adequate glomerular filtration. Hence, the term, "acute tubular necrosis" does not truly reflect the histologic changes in this condition.

Autopsy studies of ATN in the modern era are very limited. In 1962, Finckh and co-workers could find no correlation between changes in the kidneys of patients and the occurrence of ATN [2]. This material was re-examined 20 years later by Solez & Finckh [3] who, indeed, found interesting correlations. Distal tubular necrosis and regeneration were significant findings in patients with ATN. There was a negative correlation between distal tubular necrosis and urine volume. Finally, only proximal tubular regeneration, rather than necrosis, was a significant finding in such kidneys. Overall, inflammatory changes are mild but, "leukocytes" within the vasa recta do correlate with the presence of clinical ATN [4]. When first described, they were considered to be of hematopoietic origin [5] and, indeed, immunoperoxidase studies seem to confirm that observation [6], showing these cells to be erythroblasts (hemoglobin +), immature granulocytes /hematopoietic/endothelial progenitor cells (CD-34 +) and megakaryocytes (Factor VIII+). Indeed, one animal model of ATN induced by methemoglobin administration is characterized by granulopoiesis within the vasa recta [7]. Recent studies, however, have put an entirely new perspective on the meaning of these cells, suggesting that they may originate from the bone marrow [8] and be reparative in nature for both the vasculature and renal epithelial elements.

The first renal biopsies of this condition were done in the 1950's by Brun and Munk [9] who, in fact, commented on the limited parenchymal compromise in spite of the severe organ failure. Much of the interesting progress made thereafter rested on the electron microscopic findings from 25 biopsies of patients with ATN [10, 11, 12] . These biopsies were from patients with established and recovering acute renal failure. There was great variation in the patients' ages, when the biopsies were obtained and the etiology of the acute renal failure. Etiologies included shock following surgical operation, trauma, or postpartum hemorrhage, and sepsis. In half of the patients, poisoning or nephrotoxicity was considered to have contributed to the acute renal failure.

Electron microscopic studies of cortical tubules showed single cell necrosis/desquamation with defects (predominantly in distal tubules and correlated with sustained renal failure), reduction of proximal tubular brush border and diminishment of basolateral infoldings [10, 11, 12] . Fine structural studies of medullary tubules revealed reduction of brush border and basolateral infoldings in pars recta (S3). Reduction of basolateral infoldings was also seen in the medullary thick ascending limb; single cell loss was much more prominent in the thick ascending limb and collecting duct than S3. In terms of exact percentages, in the cortex, the proximal tubular loss was 0.8% versus the distal tubular loss, 5.2%. In the medulla, tubular cell loss was somewhat greater, approaching 10%, especially from the medullary thick ascending limb and collecting duct. Amazingly, in spite of heterogeneity of clinical circumstances, the findings were very similar in the biopsies of patients with ATN. In essence, cellular injury in native ATN results in minimal literal cell necrosis, cellular simplification, and cell loss as illustrated by cellular defects which were much more pronounced in the distal nephron.

Transplant ATN provides a far more homogeneous situation than native ATN and, theoretically, should provide an important insight into the pathophysiology of native ATN. By light microscopy, again, there is limited tubular necrosis but, such changes were significantly higher in ATN versus groups with stable function [14]. Electron microscopic studies, however, showed a poor correlation between transplant ATN and groups with stable renal function [14]. The medulla has not been examined in this situation. One interesting study was that of Kwon, et al [15] in which biopsies were done shortly after graft reperfusion and again on post transplant day 7. The findings were restricted to the cortex and outer stripe of outer medulla. Transport ATPase was poorly retained in the basolateral membranes without redistribution to apical membranes, such as seen in the animal models of ischemia reflow [16]. Importantly, the changes were comparable in sustained and recovering ATN.

The sharp division of nephrotoxic and ischemic etiology as causes of "acute tubular necrosis" is not correct [17, 18] . There is a continuum in which overtly nephrotoxic and ischemic causes are at the ends of the spectrum with the majority of patients falling into a group of multiple etiologies. It is generally the animal models, with some exceptions, in which there is a single basis for renal failure. Gentamicin is a classic example of the complexities of these issues [19]. To produce renal failure in the rat, super-normal doses of gentamicin are necessary. If there is, as Zager has shown, brief ischemia as well, pharmacological doses of gentamicin will produce renal failure [19]. Thus, ischemia and gentamicin are synergistic in the production of renal injury.

Myeloid bodies are typical of gentamicin toxicity in experimental animals [20]. The presence of myeloid bodies in renal epithelium of man indicates gentamicin exposure but is not correlated with total dose of gentamicin, duration of therapy, or serum concentration. Of 15 patients with these myeloid bodies, clinical evidence of gentamicin toxicity was identified in only one case [20]. Thus, tubular changes typical of nephrotoxicity in the rat do not necessarily reflect clinical nephrotoxicity in man. This paradox is also seen with cisplatin toxicity. In rats, lesions produced by cisplatin are characterized by extensive S3 necrosis with disagreement regarding the involvement of the distal tubule (in my experience, I found it to be unaffected). However, one study of human kidneys with cisplatin toxicity [21] has shown focal tubular epithelial coagulative necrosis, affecting primarily the distal and collecting tubules, with dilatation of the convoluted tubules and formation of casts. Another study found more even involvement of proximal and distal tubules [22].

Ischemia, per se, is complicated. Ischemia can be viewed either as global (for example, renal artery occlusion) or regional (as the medullary hypoxia following indomethacin administration). Furthermore, the global ischemia following renal artery occlusion can be further divided into warm ischemia (typical of the ischemia reflow model of ATN) and cold ischemia (typical of what occurs with the transplant kidney). The morphological and physiological consequences of both these kinds of ischemia are very different [23]. Warm ischemia is renodestructive: 90 minutes of warm ischemia results in lethal cell death of proximal tubular cells as an immediate consequence. After shorter periods of warm ischemia and subsequent reflow, there is extensive destruction of S3 which may include, depending on the period of arterial occlusion, the proximal convoluted tubules. The distal nephron, on the other hand, shows relatively limited injury. In contrast, cold ischemia is renoprotective and many hours are needed to produce non-viability. Indeed, after 48 hours of cold ischemia, the first immediate lethal injury can be demonstrated in podocytes and peritubular capillaries. In kidneys subjected to 12 and 16 hours of cold ischemia and subsequently transplanted, Harvig and coworkers [24, 25, 26] found limited necrosis in the proximal tubules, mainly involving pars recta. On the other hand, in the inner stripe of the outer zone of the medulla, there was extensive necrosis involving the loops of Henle and collecting ducts. These cold ischemia studies contrast sharply with the findings of warm ischemia reflow model in the rat where there is severe necrosis of S3 and, with prolonged obstruction, S1 and S2 become necrotic as well [27]. This necrosis is time-dependent, highly variable in extent (S3 variability seen at periods <45 minutes and S1S2 variability at periods >45 minutes) and depends importantly on truly warm ischemia [28]. The inconsistency of proximal tubular injury in these models likely relates to the fact that even a few degrees of cooling can be very renoprotective.

The distal nephron model of ATN is another example of the interaction between ischemia and nephrotoxins and illustrates the effect of regional rather than global ischemia [29, 30] . The renal medulla operates in hypoxic environment because of arteriovenous diffusion in vasa recta, high oxygen consumption by the medullary thick ascending limbs, and a low hematocrit which is characteristic of medullary vasculature. In particular, Zhang's and Edwards' [31] theoretical model of medullary oxygenation predict extreme vulnerability of the papillary tip to small changes in medullary oxygen utilization, in hematocrit, or in capillary permeability to oxygen. This prediction is borne out by the papillary tip necrosis which is seen frequently in the distant nephron model and is particularly relevant to the present case. Furosemide administration inhibits sodium transport and raises medullary P02 in the rat to cortical levels [32]. This has also been documented in man, as well, using MRI imaging, because the magnetic qualities of deoxyhemoglobin differ from those of oxygenated hemoglobin [32].

The only way that the medulla can tolerate this hypoxic state is by efficient mechanisms that precisely regulate and match regional transport activity and blood supply. Two major participants that help maintain oxygen availability are prostaglandin and nitric oxide generation [29, 30] . The distal nephron model is based on utilization of inhibitors of prostaglandin (indomethacin) and nitric oxide synthesis (L-NAME ) in concert with a radiocontrast agent that enhances oxygen consumption for tubular reabsorption [29, 30] . Severe injury is not seen in proximal tubules, except for those situated immediately adjacent to the inner stripe. Proximal tubular vacuolar change does occur, just as seen with contrast nephropathy in man. These vacuoles are membrane outpouchings from basolateral membranes, but other organelles and brush border are unaffected. This vacuolar change in man and animal does not correlate with renal dysfunction. At 24 hours, with the complete protocol, MTAL injury is extensive and there is evident polarization of injury with the MTAL's preserved adjacent to the vas recta. The extent of this injury correlates with renal failure in the complete model (indomethacin, L-NAME, and contrast). However, L-NAME and indomethacin produce renal failure as well, with much more limited MTAL injury.

In summary, ATN (both native and transplant) is characterized by limited overt tubular injury in spite of severe organ failure. Attempts to create analogous animal models using single insults do not seem to recapitulate the multifactorial nature of human ATN and generally result in severe parenchymal injury. Furthermore, especially in the clinical context of ATN, clear separation of ischemic and nephrotoxic injury in the etiology of ATN does not seem appropriate. Finally, the ischemia reflow model of ATN, based on a single extensively destructive insult, has provided multiple agents which have been effective in rats to ameliorate renal failure [1], but have failed in extensive human trials. Other animal models should receive more attention.

References

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