Identification of non-diabetic glomerular disease in renal biopsies from diabetics—a dilemma

Steen Olsen

Institute of Pathology, Herlev Hospital, Copenhagen, Denmark

Correspondence and offprint requests to: Professor Steen Olsen, Jaegersborg Alle 163, DK-2820 Gentofte, Denmark.

Diabetic nephropathy is one of the most frequent and clinically important complications to diabetes mellitus (DM), affecting approximately 40% of patients who have had diabetes for more than 20 years and contributing to a substantial part of patients entering into end-stage renal failure programmes. Our knowledge of the clinical development and morphological basis for diabetic renal disease has augmented tremendously during the last half century [1,2] and during the last decade we have been able to understand at least some elements of its pathogenesis. Nephropathy in a patient suffering from DM may, however, not be related directly to the diabetic disease but may be a complicating renal disease, not due to the diabetic metabolic abnormality. The situation is further complicated since logically both lesions may be present in the same patient. While it was formerly held that non-diabetic renal disease was of minor importance in DM, this seems not so obvious to-day, and opinions of investigators about prevalence of non-diabetic renal disease in DM, as revealed by renal biopsies, have been conflicting. The correct diagnosis is important for the patient since prognosis and treatment may vary according to the underlying cause. Furthermore: if a substantial part of renal disease in diabetics had other causes than the diabetic metabolic derangement, this must be a confounding factor in the interpretation of intervention trials.

The classical diabetic nephropathy is morphologically primarily a glomerulopathy, characterized clinically by proteinuria and progressive decrease of glomerular filtration. Tubular atrophy and interstitial fibrosis is a common final pathway for many renal diseases including diabetic nephropathy. These lesions have formerly been interpreted as ischaemic sequels to vascular and glomerular lesions but they may also occur in diabetics with only mild vascular and glomerular lesions and at an early stage of diabetic renal disease. In some reports this was even rather frequent [3,4] and it is today not at all obvious that tubulointerstitial lesions are of ischaemic origin. A competing hypothesis relates them to the diabetic state, just as glomerulopathy [57]. This problem will not be addressed here, I have been asked to provide a brief presentation of problems related to the differentiation of non-diabetic from diabetic glomerular lesions in renal biopsies. Since our problem is one of differential diagnosis it is first necessary to give a brief review of glomerular lesions, which can be regarded with confidence as being directly caused by the diabetic metabolic abnormalities.

Glomerular lesions of diabetic nephropathy

Following the first seminal report by Kimmelstiel and Wilson [8] and later on worked out in great detail by others, we have now a clear idea of the histopathology of diabetic renal disease and its progress with time. We know now, that at the onset of insulin dependent DM (IDDM) (in this type the zero time is well defined) there is enlargement of the glomeruli indicating increased glomerular function, but no other structural lesions by light microscopy (LM) or electron microscopy (EM). The first ultrastructural lesions (widening of the mesangial matrix and thickening of the peripheral glomerular basement membrane) can be morphometrically identified 2–3 years after onset [9]. Changes can usually not be seen by LM until several years of diabetic disease. They are described in two forms. In the diffuse form there is a widening of all mesangial regions as well as thickening of the peripheral capillary basement membranes, corresponding to what is already apparent at an earlier time from morphometry on EM pictures. The diffuse lesion is most conspicuous in PAS-stained sections. In the nodular type some of the mesangial regions take on the appearance of round nodules which are acellular in their centres but surrounded by a rim of mesangial nuclei. While these types are sometimes described as separate histologic entities, there are good reasons to regard the nodular type as a further development of the diffuse type. Missing a formal proof of this (e.g. by the study of sequential biopsies from the same patients), it is clear that nodules are formed from mesangial matrix, that they occur later in the disease than the diffuse lesion and that nodules never occur without associated marked diffuse lesions. To this account of what is regarded as the most important glomerular lesions may be added that hyaline (exudative) lesions sometimes occur in the form of capsular drops and hyaline caps.

It is unclear whether mesangial hypercellularity is a feature of glomerular diabetic lesions. Using quantitative technique, Wehner and Anders [10] as well as Kimmelstiels group [11] found a significant increase of mesangial nuclei in nodular and diffuse diabetic glomerulopathy. On the other hand, Østerby, counting on EM montages, did not find hypercellularity in the early stage of juvenile DM (0–5 years) [12]. In experimental diabetic nephropathy there is early mesangial proliferation and increased expression of growth factors [13]. Using immunofluorescence, a diffuse, linear localization of IgG in glomerular capillary walls, and sometimes also of small quantities of other plasma components, is frequent. There is general agreement that this is due to non-specific trapping of plasma components along the glomerular capillary wall, and they are not associated with presence of deposits on EM.

In parallel with the progress of glomerulopathy, arteriolar sclerosis and hyalinosis develop. The hyaline arteriolosclerosis affects the afferent as well as the efferent arteriole, which is not the case with non-diabetic arteriolosclerosis. Otherwise there are no differences between the structure of arteriosclerosis and arteriolosclerosis in diabetics and non-diabetics. Arteriosclerosis occur earlier and in more severe degree in diabetics than in the non-diabetic population. It is therefore not unexpected that arteriosclerosis and focal scars are frequent in biopsies from patients with DM, especially non-insulin dependent DM (NIDDM) which occurs in older patients.

Glomerulopathy not related to the diabetic disease

Glomerulonephritis (GN)
It is generally held that GN complicating IDDM is comparatively rare, probably around 2–3% in unselected cases with proteinuria and duration of diabetes of more than 10 years. Some reports have shown an impressively high prevalence of GN in NIDDM. The widely diverging results of the reports indicate, however, that something must have gone wrong.

We have recently analysed 10 renal biopsy studies (including our own) specifically dealing with NIDDM [3,4,1421]. The rates of GN in these series vary between 0 and 66% and other complicating renal diseases were reported to be present in between 0 and 20%. These diverging results can of course be due to selection criteria (only one was a population based cross-sectional study [15]) or geographical differences, but the criteria for diagnosis of GN and interobserver variation in the pathologists interpretation of structural lesions may also be responsible.

As most types of glomerulonephritis are due to deposition of antibodies in the glomerular capillary walls or the mesangium, immunohistochemical investigation will often, together with the LM characteristics of the glomerular lesions, provide an unambigous diagnosis. This applies to the most common GN form, IgA nephropathy, as well as several other such as membranous GN, membranoproliferative GN, postinfectious, complex mediated GN, and GN associated with systemic lupus erythematosus. GN in systemic vasculitis may be without immune deposits, but the LM picture (crescentic or focal, segmental proliferative GN) is easily distinguished from the lesions seen in diabetes.

While the identification of these types in the reports of complicating renal disease in NIDDM must be regarded as unproblematic, this is certainly not the case for two other types: mesangial proliferative GN and minimal change disease. There is therefore reason to discuss these entities specifically.

Mesangial proliferative GN
This is a descriptive term indicating diseases with increased mesangial cell number (more than three mesangial cells per mesangial region in at least 80% of all glomeruli). This light microscopical picture (as defined by a WHO committee [22]) can be seen in several types of GN such as IgA nephropathy, in the resolution phase of postinfectious GN, in some cases of GN associated with SLE etc. These types have all heavy immune deposits and their composition (immunoglobulin classes) pattern and location makes them easy to distinguish from diabetic glomerulopathy. There is, however, a comparatively rare mesangioproliferative GN without immune deposits. We meet two problems with this type if it is detected in a patient with diabetes. One is that mesangial hypercellularity (as discussed above) may be a feature of diabetic glomerulopathy, at least in some stages of its development. The other is that the usual evaluation of mesangial cell number is subjective, and as such dubious, especially in cases with only weak or moderate increase of the number of cells. A biopsy diagnosis in a diabetic patient of slight or moderate mesangioproliferative GN without immune deposits must therefore be regarded as questionable.

Minimal change nephropathy
This term indicates, according to the WHO classification of glomerular disease [22], conditions with normal glomerular structure or minor glomerular abnormalities such as slight mesangial hypercellularity. Clinically it comprises minimal change nephrotic syndrome as well as mild or moderate persistent isolated proteinuria. Significant immune deposits detected by immune histochemistry or on EM are not present. Podocyte foot processes are more or less effaced, but this is unspecific and can be seen in all renal diseases with proteinuria [23]. The pathogenesis is unknown.

The application of this diagnosis in a patient with DM presents serious problems. Normal glomeruli by LM and no deposits by immunofluorescence or electron microscopy is completely compatible with early diabetic glomerulopathy with slight or even moderate albuminuria. On the other hand, patients suffering from DM may of course also be affected by other glomerular diseases, possibly even with a higher incidence that the non-diabetic population. Among these complicating conditions are the `classical', well defined glomerular immune disorders mentioned above, but also the less well characterized renal diseases collected under the term minimal change nephropathy. To this may be added that it is well known that a collection of biopsies with normal glomeruli may comprise cases of the more serious renal disease, focal, segmental glomerulosclerosis and hyalinosis, due to missing representation of the abnormal glomeruli in a biopsy with a restricted number of glomeruli. The correct diagnosis may in such cases be made in a later biopsy from the patient.

It will be clear from the above discussion that it is hazardous to make the diagnosis of glomerulonephritis based upon light microscopy alone, especially in the situation dealt with here. This may underestimate the prevalence of GN in DM [24], but probably also overestimate if slight or moderate mesangial hypercellularity alone is used as a criterium. Immunofluorescence microscopy should always be added to the investigation of such cases, and optimally also EM.

The clinical context is of course also significant. If it is strongly different from diabetic nephropathy, for example, if nephrotic syndrome appears suddenly early in the course of the diabetic disease, the diagnosis of minimal change nephrotic syndrome becomes convincing. Haematuria, which is not characteristic for diabetic nephropathy may indicate GN.

Other glomerular diseases
In the ten reports discussed above, many other renal diseases than GN was found, among these amyloidosis and myeloma which can also involve glomeruli. In amyloidosis there is deposition of the abnormal protein in the mesangial matrix and along the capillary walls. The morphology is usually different from diffuse and nodular diabetic glomerulopathy, but nodules may nevertheless be seen. Mesangial nodules can also be seen in myeloma and related dysproteinaemias [25].

Conclusion

The conclusion from these considerations must be the following. If a patient suffering from DM develops clinical nephropathy (proteinuria) there may logically be three reasons. Diabetic glomerulopathy may be in progress, non-diabetic renal disease alone may be responsible for the nephropathy, or both may be present. Non-diabetic nephropathy is most often glomerulonephritis, but also other renal diseases, not related to the diabetic disease (amyloidosis etc). A renal biopsy studied by light microscopy and immune fluoroscence may provide the key to correct diagnosis, either presenting characteristic diabetic glomerulopathy alone or unequivocal GN with glomerular immune deposits. There are, however, situations where a biopsy cannot lead to a reliable diagnosis. Renal diseases associated with normal glomerular structure (minimal change nephropathy) or with slight or moderate alterations which cannot with certainty be differentiated from diabetic renal disease in early stages (mesangial proliferative GN without immune deposits) may of course also occur in diabetics and proteinuria may be related to one of these diseases. Such a patient may belong to the category which never develop diabetic renal disease or he or she may have diabetic glomerulopathy at an early stage where there is proteinuria but no structural changes on LM. Even if the biopsy is investigated with EM the situation is not necessarily unambigous: slight or moderate thickening of the glomerular basement membrane or mesangial matrix may be present in patients with longstanding diabetes without proteinuria.

We have expressed the opinion that complicating GN in NIDDM is present in about 10% [14], other investigators have reported 25% [15] or even more. Actually it may be impossible at the moment to gain reliable knowledge of the prevalence due to the problems discussed above. The group of patients with DM and normal glomeruli or slight or moderate mesangial proliferation should be investigated carefully in order to solve this important problem.

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