patterns of renal injury in niddm patients with ...diabetic nephropathy is the single most frequent...

Diabetic nephropathy is the single most frequentcause of end-stage renal disease in western countries,and the proportion of uraemic patients who are dia-betic has been increasing in recent years [1, 2]. TheUnited States Renal Data System does not yet sepa-rate insulin-dependent (IDDM) from non-insulin-

dependent (NIDDM) diabetes mellitus. The Euro-pean Dialysis and Transplant Association Registryreported that 35% of diabetic patients requiring re-nal replacement therapy have NIDDM [2]; however,lack of precision in diabetes classification, originatingfrom insulin-treated (non-insulin-dependent) pa-tients, often classified as having IDDM, may havecaused an underestimation of the percent of uraemicdiabetic patients affected by NIDDM [3]. Indeed, re-gional surveys have shown a higher proportion ofNIDDM patients on renal replacement therapy, bothin the United States [4] and in Europe [3, 5, 6]. InItaly, when diabetes was classified by a specific

Diabetologia (1996) 39: 1569–1576

Patterns of renal injury in NIDDM patients with microalbuminuriaP. Fioretto1, M. Mauer2, E. Brocco1, M. Velussi3, F. Frigato3, B. Muollo3, M. Sambataro3, C. Abaterusso1, B. Baggio1,G. Crepaldi1, R. Nosadini1, 4

1 Department of Internal Medicine and Center for the Study of Aging of the National Research Council,University of Padova Medical School, Padova, Italy

2 Pediatric Nephrology, University of Minnesota Medical School, Minneapolis, Minnesota, USA3 Diabetes Centers of Monfalcone, Mestre, Este and Contarina, Italy4 Division of Endocrinology, University of Sassari Medical School, Sassari, Italy

Springer-Verlag 1996

Summary Microalbuminuria predicts overt nephrop-athy in non-insulin-dependent diabetic (NIDDM)patients; however, the structural basis for this func-tional abnormality is unknown. In this study we eval-uated renal structure and function in a cohort of 34unselected microalbuminuric NIDDM patients(26 male/8 female, age: 58 ± 7 years, known diabetesduration: 11 ± 6 years, HbA1 c: 8.5 ± 1.6%). Systemichypertension was present in all but 3. Glomerular fil-tration rate (GFR) was 101 ± 27 ml ⋅ min–1 ⋅ 1.73 m–2

and albumin excretion rate (AER) 44 (20–199) mg/min. Light microscopic slides were categorized as: CI) normal or near normal renal structure; C II) chan-ges “typical” of diabetic nephropathology in insulin-dependent diabetes (IDDM) (glomerular, tubulo-in-terstitial and arteriolar changes occurring in parallel);C III) “atypical” patterns of injury, with absent oronly mild diabetic glomerular changes associatedwith disproportionately severe renal structural chan-ges including: important tubulo-interstitial with orwithout arteriolar hyalinosis with or without globalglomerular sclerosis. Ten patients (29.4%) were

classified as C I, 10 as C II (29.4%) and 14 as C III(41.2%); none of these patients had any definablenon-diabetic renal disease. GFR, AER and bloodpressure were similar in the three groups, whileHbA1 c was higher in C II and C III than in C I pa-tients. Diabetic retinopathy was present in all C II pa-tients (background in 50% and proliferative in 50%).None of the patients in C I and C III had proliferativeretinopathy, while background retinopathy was ob-served in 50% of C I and 57% of C III patients. Insummary, microalbuminuric NIDDM patients arestructurally heterogeneous with less than one thirdhaving “typical” diabetic nephropathology. The pres-ence of both “typical” and “atypical” patterns of re-nal pathology was associated with worse metaboliccontrol, suggesting that hyperglycaemia may causedifferent patterns of renal injury in older NIDDMcompared to younger IDDM patients. [Diabetologia(1996) 39: 1569–1576]

Keywords NIDDM, renal structure, microalbumin-uria, glomerular filtration rate.

Corresponding author: Dr. P. Fioretto, Department of InternalMedicine, Via Giustiniani 2, I-35128 Padova, ItalyAbbreviations: IDDM, Insulin-dependent diabetes mellitus;NIDDM, non-insulin-dependent diabetes mellitus; DN, dia-betic nephropathy; GFR, glomerular filtration rate; AER, al-bumin excretion rate; PAS, periodic acid-Schiff.

questionnaire, the proportion of uraemic NIDDMpatients rose to 67% [6]. Although a large proportionof diabetic patients receiving renal replacement ther-apy have NIDDM, the pathology and natural historyof diabetic nephropathy (DN) in NIDDM is still un-clear. The clinical manifestations of DN, proteinuria,declining glomerular filtration rate (GFR) and in-creasing blood pressure, are similar in IDDM andNIDDM [7, 8]; nevertheless, whether these clinicalfeatures are consequences of similar underlying renallesions is unknown. In IDDM, glomerular, arteriolar,tubular and interstitial structural lesions occur largelyin parallel, although glomerulopathy is functionallythe most important change [9–14]. Quantitative mor-phometric studies have demonstrated that the lesionmost closely related to the decline in renal functionin IDDM is mesangial expansion, especially mesan-gial matrix accumulation [13, 15]. Also, we have re-cently reported, in a longitudinal study with sequen-tial renal biopsies performed 5 years apart in IDDMpatients, that the only morphometric parameter asso-ciated with increasing albuminuria was mesangialfractional volume [14]. The structural changes occur-ring in the arterioles, tubules and interstitium areusual concomitants, typically in proportion to the de-gree of glomerulopathy, and become severe onlywhen glomerulopathy is advanced; in fact advancedglobal glomerular sclerosis, interstitial fibrosis and ar-teriolopathy in the absence of abvanced glomerulo-pathy are uncommon in IDDM.

Non-diabetic renal disease is uncommon in pa-tients who have had IDDM for 10 or more yearswith overt nephropathy (personal observations),while the prevalence of non-diabetic renal lesionshas been reported to be high in proteinuric NIDDMpatients (approximately 30%). Parving et al. [16] re-ported that 8 of 35 (23%) NIDDM patients with pro-teinuria had non-diabetic glomerulopathies whichthese authors classified as minimal lesion nephropa-thy, mesangio-proliferative glomerulonephritis andits sequelae. Heterogeneity in renal lesions has alsobeen reported by Gambara et al. [17] who found thatonly 19 of 52 (37%) of proteinuric NIDDM patientshad typical changes of DN. An autopsy study did notconfirm these finding [18].

Microalbuminuria antedates clinical proteinuria inboth IDDM [19–21] and NIDDM [22, 23]; however,only approximately 20% of NIDDM patients withmicroalbuminuria progress to overt nephropathyover a decade of follow-up in contrast to over 80%of IDDM patients [23]. Indeed microalbuminuria inNIDDM is a better predictor of cardiovascular mor-tality than uraemia [22, 24, 25]. The low predictive va-lue of microalbuminuria for overt proteinuria inNIDDM may in part be explained by the high mortal-ity for cardiovascular disease, which prevents theprogression to clinical nephropathy. However, it hasalso been hypothesized that microalbuminuria in

NIDDM patients may be either consequent to dia-betic glomerulopathy, as in IDDM, or reflect general-ized endothelial dysfunction [8, 26]. This latter hy-pothesis remains speculative, since to date no infor-mation is available on renal structure in NIDDM pa-tients with microalbuminuria.

The present study investigates renal function andstructure in 34 microalbuminuric NIDDM patientsin order to describe the renal structural concomitantsof this functional disturbance.

Subjects and methods

Patients. The patients were participating in an ongoing multi-center study of renal structural-functional relationships inNIDDM. Patients were referred to the University of Padovafrom diabetes centres in the north-east of Italy for researchevaluation of renal structure and function, based upon the fol-lowing criteria.1. Age 70 years or less.2. Age at diagnosis of NIDDM 40 years or more. Diabetic pa-tients were diagnosed as having NIDDM when onset was afterage 40 years, when they were treated with diet alone or in asso-ciation with oral hypoglycaemic agents and/or insulin andwhen they were not receiving insulin in the first 2 years afterdiagnosis. Insulin-treated patients with normal body weighthad a glucagon test performed to confirm the diagnosis ofNIDDM (when C-peptide levels were normal).3. Known duration of NIDDM 2 years or more.4. Serum creatinine < 180 mmol/l.5. Absence of other obvious renal diseases including seriousstone disease, and presence of single kidney.6. Secondary causes of hypertension including known renal ar-tery stenosis and endocrinopathy.7. Requirement for systemic anticoagulation (heparin or cou-madin).

Otherwise all patients willing to participate were accepted re-gardless of renal function. In no case were renal biopsies per-formed for clinically indicated diagnostic purposes.

To date light microscopic slides from 34 microalbuminuricpatients have been evaluated and are presented here. Patientswere defined as microalbuminuric when AER was 20 mg ormore/min but 200 mg or less/min in at least two of three consec-utive sterile 24-h urine collections.

These studies were approved by the ethical committee ofthe University of Padova. Patients gave written informed con-sent before each study. The patients were admitted to the De-partment of Internal Medicine at the University of PadovaHospital where percutaneous renal biopsy and renal functionalstudies were performed. In patients on antihypertensive treat-ment, the therapy was withdrawn 3–5 days before and duringadmission; after the studies were completed, patients were re-turned to their previous therapy. During admission patientsunderwent at least three 24-h urine collections for measure-ments of AER after urine cultures were determined to be ster-ile. If bacterial growth was present, treatment was initiated andurine collections were repeated for determination of AER onsterile urine samples. Microscopic examination of the urinarysediment was performed. Glomerular filtration rate was deter-mined by the plasma clearance of Cr-EDTA, as described indetail elsewhere [27]. Blood pressure was measured at least10 times with the patient in the supine position. Body mass in-dex (BMI) was calculated as weight (kg)/height (m2). Patients

P. Fioretto et al.: Renal structure in microalbuminuric NIDDM1570

were defined hypertensive when the blood pressure values ex-ceeded 140/85 mm Hg [28], or when on antihypertensive ther-apy regardless of BP levels. HbA1 c was measured to assessmetabolic control. All patients had funduscopy performedthrough dilated pupils in the Department of Ophthalmologyof the University of Padova and, when required, fluoroangiog-raphic studies. Patients were classified as follows: 1) absence ofdiabetic retinopathy; 2) background diabetic retinopathy; 3)proliferative diabetic retinopathy.

Normal control subjects. Kidney biopsies were obtained from36 (17 male /19 female) kidney donors at the time of renaltransplantation at the University of Minnesota. Thirty-threeof these donors were living related donors and three were ca-davers; these control subjects were matched for age with thediabetic patients (age: 55.7 ± 7 years, range: 45.1–69).

Analytical procedures. HbA1 c values were measured by high-performance liquid chromatography (DIAMAT Analyzer,BIO-RAD, Calif., USA). AER was measured by radioimmu-noassay. 51 Cr radioactivity was measured in duplicate 1-ml ali-quots of plasma in a gamma counter (Cobra-5002 CPM, Camb-erra Packard, Milan, Italy) with the energy window set to 240–400 KeV [27].

Renal structure. Kidney biopsies were performed under ultra-sound guidance by experienced investigators. Safety require-ments included normal blood pressure levels ( ≤ 140/85 mmHg) at the time of the biopsy as well as normal coagulationstudies and platelet counts. Aspirin or other antiplatelet agentswere withdrawn 2 weeks before the biopsy. After kidney bi-opsy, tissue was immediately examined under a dissecting mi-croscope to ensure adequate numbers of glomeruli.

Light microscopy. Most of the core was placed in Zenker’s fix-ative, embedded in paraffin and processed for light microscopy[29]. Light microscopy sections (2-mm thick) were stained withhaematoxylin and eosin and periodic acid Schiff (PAS). PASsections from all patients and normal control subjects wereconcomitantly and blindly evaluated by two observers, in orderto develop a qualitative overview of the patterns of injury. Thetwo observers evaluated the light microscopy slides simulta-neously, and therefore an agreement on the categorizationwas discussed and made at the time of the reading. Based onthe resultant overview of the biopsy material the followingclassification was developed:

Category C I): Normal or near normal renal structure. Thesepatients had biopsies which were normal or showed very mildmesangial expansion, tubulo-interstitial changes or arteriolarhyalinosis in any combination (Fig. 1A).

Category C II): Typical diabetic nephropathology. These pa-tients had established diabetic lesions with balanced severityof glomerular, tubulo-interstitial and arteriolar changes. Thispicture is typical of that seen in IDDM patients with obviouslight microscopic DN changes (Fig. 1B).

The severity of lesions in the different compartments, aswell as the possible heterogeneity among different glomerularprofiles, was evaluated as described elsewhere [13]. Quantita-tive morphometric measures of mesangial (r = 0.87, p <0.0001) [13] and interstitial expansion (r = 0.90, p < 0.0001)[30] are closely correlated with the semiquantitative light mi-croscopic analysis. Our previous studies have shown highly sig-nificant correlations between glomerular lesions, interstitialexpansion, arteriolar hyalinosis and global glomerular sclerosis[11, 13, 31].

Category C III): Atypical patterns of renal injury. These pa-tients had absent or only mild glomerular diabetic changeswith disproportionately severe renal structural lesions includ-ing:(a) Tubular atrophy, tubular basement membrane thickeningand reduplication and interstitial fibrosis (tubulo-interstitiallesions) (Fig. 1C).(b) Advanced glomerular arteriolar hyalinosis commonly asso-ciated with atherosclerosis of larger vessels (Fig. 1D).(c) Global glomerular sclerosis ( > 25 %) in the presence of ab-sent or mild mesangial expansion (index of mesangial expan-sion ≤ 0.5 according to our score system [13]) (Fig. 1E).

These patterns were present in any possible combination. Con-sidering the complexity of the patterns of injury and the lim-ited number of patients, we pooled these three groups (a, b, c)into one single category (C III).

It should be noted that patients in categories III and IIcould have similar levels of diabetic glomerulopathy, expressedas index of mesangial expansion, but those in C III had dispro-portionately severe tubulointerstitial, vascular or glomerulo-sclerotic lesions.

Immunofluorescent microscopy. One 2–3 mm cortical core wassnap frozen for immunofluorescence studies. The tissue wasblindly examined using rabbit or goat antisera specifically re-active to human IgA, IgG, IgM, fibrinogen, C3, C4, C1q.

Electron microscopy. Three 1-mm cortical cores were placed in2.5 % glutaraldehyde in Millonig’s buffer and processed forelectron microscopy [29]. Ultrathin sections were obtained onan LKB ultramicrotome (LKB, Bromma, Sweden) and ob-served on a Hitachi H/600 elecron microscope (Hitachi, Tokyo,Japan). Three glomeruli per biopsy were examined and pic-tures taken at final magnifications of 3900 × and 12 000 × , aspreviously described [32]. Electron microscopic morphometricanalysis is in progress. Photomicrographs obtained at 12000 × were analysed for diagnostic purposes.

None of the patients in the three groups described abovehad light immunofluorescent or electron microscopy findingsof any definable renal disease other than typical diabetic ne-phropathy or the patterns described above.

Statistical analysis

Data are expressed as mean ± 1SD. Values for AER, not nor-mally distributed, were logarithmically transformed beforeanalysis and are expressed as median and range. Comparisonsamong the diabetic groups first used a one-way analysis of vari-ance (ANOVA) and then unpaired Student’s t-test for parame-ters shown to be different by ANOVA. Values of p < 0.05 wereconsidered significant.

Results

Clinical features of patients. Twenty-six of the 34 pa-tients (all Caucasian) were males. Age was 58.5 ±7.3 years (mean ± 1SD) and known NIDDM durationwas 10.8 ± 6.3 years. Body mass index (BMI) was28.9 ± 3.9 kg/m2 (normal values in a group of age-matched normal control subjects: 24 ± 2) and HbA1cwas 8.5 ± 1.6 % (normal range: 4.1–6.1%). GFR was101 ± 27 ml ⋅ min–1 ⋅ 1.73 m–2 (normal range in a

P. Fioretto et al.: Renal structure in microalbuminuric NIDDM 1571

group of 19 age and sex-matched normal control sub-jects: 85–135) and AER was 44 (20–199) mg/min (me-dian, range) (normal values: 5 [0–14]).

All but 6 patients were receiving antihypertensivetherapy, and the majority of them were on ACE inhi-bition. Overall, only 3 patients were normotensive,according to the criteria described above.

Based on the light microscopic findings 10 patients(5 male/5 female) were allocated to category I(29.4%), 10 patients (9 male/1 female) to category II(29.4%) and 14 (10 male/2 female) to category III(41.2%). Important tubulo-interstitial changes wereobserved in all but 1 patient, who had very severe

arteriolar hyalinosis lesions, in category III; thesetubulo-interstitial lesions were often associated witharteriolar changes and in some (n = 3) patients to se-vere (> 25%) global glomerular sclerosis.

The frequency of global glomerular sclerosis was 0(0–20) (median, range) in CI, 0 (0–36) in CII and 4(0–46) in CIII.

The clinical features of the patients as divided intothe three structural groups are summarized in Tables1 and 2.

CI patients tended to be younger than patients ingroups CII and CIII, although the difference was notsignificant by ANOVA (p = 0.13); 50% of patients in


A C

B D

E

Fig. 1. A Glomeruli from a patient in category C I. Glomerularstructure is near normal with minimal mesangial expansion(PAS). B Glomerulus from a patient in category C II, withwell-established diabetic nephropathology. Diffuse mesangialexpansion, advanced arteriolar hyalinosis and mild interstitialfibrosis are present (PAS). C Glomerulus from a patient incategory C III (a), with near normal glomerular structure andtubular basement membrane thickening, tubular atrophy andsevere interstitial fibrosis (PAS). D Glomerulus from a patientin category C III (b), with mild mesangial expansion and se-vere arteriolar hyalinosis, affecting both afferent and efferentglomerular arterioles (PAS). E Glomeruli from a patient incategory C III (c). Glomerular structure is near normal in oneglomerulus, while the adjacent shows global glomerular sclero-sis (PAS)

CI were females, while in groups CII and CIII therewas a clear preponderance of males (Table 1). Knownduration of NIDDM tended to be different amonggroups (ANOVA, p = 0.08), with group CII patientshaving the longest duration (t-tests, p < 0.05 vs CIand CIII). HbA1 c levels were significantly differentamong groups (ANOVA, p < 0.01); subsequent anal-ysis showed that CII and CIII patients had higherHbA1 c values than CI patients (t-tests, p < 0.005 andp < 0.05, respectively). BMI was also different amonggroups (ANOVA, p < 0.02); BMI was only mildly in-creased in CII patients (range: 18.1–31.5) and was sig-nificantly higher in CI (range: 27.7–40.1) and CIII pa-tients (24.5–35.1) compared to CII (t-tests, p < 0.05for both). AER levels were superimposable in thethree groups (Table 2). GFR was not different in thethree groups of patients; however, 5 of the 10 patientsin CII had GFR values under 85 ml ⋅ min–1 ⋅ 1.73 m–2

compared to 1 of 10 in CI and 2 of 14 in CIII. Systolicand diastolic blood pressure values were similar inthe three groups; two of the 3 normotensive patientswere in group CI and one in group CIII. Seven of 10patients in group CI, 10 of 10 in group CII and 11 of14 in group CIII were receiving antihypertensivetherapy.

Results of the funduscopic or fluoroangiographicevaluations are given in Table 3. In several patientsvarying degrees of hypertensive retinopathy werepresent with or without diabetic retinopathy.

Normal control subjects. Based on the scores for in-terstitial fibrosis, 3 of 36 (8%) subjects had importanttubulo-interstitial changes. Several normal controlsubjects had mild arteriolar hyalinosis lesions; 6(16%) had more advanced arteriolar lesion scores,comparable to those observed in patients in catego-ries II and III (scores ≥ 1.0 according to our previ-ously described scoring system [13]). However, noneof the control subjects had arteriolar hyalinosis le-sions of comparable severity to those observed in thepatients categorized in group III because of vascularchanges.

Discussion

In IDDM patients with long disease duration( ≥ 10 years) and overt nephropathy, non-diabetic re-nal disease is rare (Mauer M, unpublished data).Thus, in the vast majority of long-term IDDM pa-tients, the loss of kidney function is related to a well-defined pattern of diabetic nephropathology, includ-ing glomerular basement membrane and tubularbasement membrane thickening and mesangial ex-pansion, especially matrix accumulation, but also ar-teriolar hyalinosis affecting both afferent and effer-ent glomerular arterioles [9–15]. Interstitial fibrosisis frequently present, especially in patients with ad-vanced glomerulopathy and typically in areas withglobal glomerular sclerosis and tubular atrophy [11,13]. This pattern of renal lesions is quite monotonousand predictable in IDDM patients with clinical neph-ropathy.

Three studies have reported that non-diabetic re-nal disease is quite frequent in NIDDM patientswith overt nephropathy. Parving et al. [16] foundthat 1 of 4 of such patients had renal diseases otherthan diabetes; Gambara et al. [17] confirmed these


Table 1. Clinical features of patients divided into three renal structural categories

Category male/female % Age(years)

Known NIDDM duration(years)

BMI(kg/m2)

HbA1c(%)

C I 5/5 23.4 54 ± 9 8 ± 3 31 ± 4b 7.5 ± 0.8C II 9/1 29.4 60 ± 6 14 ± 6a 26 ± 4 9.6 ± 1.8d

C III 12/2 41.2 61 ± 6 10 ± 8 30 ± 3b 8.5 ± 1.3c

p values

Data are mean ± SDa p < 0.05 vs C I and C III; b p < 0.03 vs C II; c p < 0.05 vs C I; d p < 0.005 vs C I

Table 2. Renal function and blood pressure of patients divided into three renal structural categories

Category AER(mg/min)

GFR(ml ⋅ min−1 ⋅ 1.73 m−2)

Blood pressure

Systolic (mmHg) Diastolic (mmHg)

C I 45 (20–198) 111 ± 20 142 ± 19 91 ± 10C II 50 (22–190) 91 ± 36 154 ± 13 92 ± 7C III 39 (20–198) 101 ± 21 155 ± 16 90 ± 10

Data are mean ± SD (AER is median and range) NS for all comparisons

Table 3. Diabetic retinopathy in relation to patterns of renalinjury

Category Diabetic retinopathy

Absent Background Proliferative

C I 5 5 0C II 0 5 5C III 6 8 0

data reporting that only 1 of 3 of NIDDM patientswith overt nephropathy had typical patterns of DN.Khan et al. [33] recently observed the presence ofnon-diabetic renal disease in 42% of 153 NIDDM pa-tients with overt nephropathy; the occurrence of non-diabetic renal disease was much lower (12%) in theseries of 33 proteinuric patients studied by Olsenand Mogensen [34]. In all these studies, however, pa-tients were referred to the nephrologist and kidneybiopsies were not performed on the basis of researchprotocols, but for clinical indications. Thus, thesestudies do not describe the usual NIDDM patientswith nephropathy, but those with an unusual clinicalcourse; also the different results may reflect differ-ences in the criteria for kidney biopsy. A large au-topsy study on NIDDM patients did not confirm ahigh incidence of non-diabetic renal diseases [18].Thus, the available data on renal structure in NIDDMpatients with proteinuria are still inconclusive.

Microalbuminuria in NIDDM has been shown topredict mortality, mainly cardiovascular [22, 24, 25]and, in approximately 20 % of patients, the develop-ment of overt proteinuria [23]. However, whetherthe raised urinary albumin excretion is an expressionof underlying diabetic renal lesions is still unknown.Only one study to date evaluated renal structure inmicroalbuminuric NIDDM patients [35]; surprisinglythese authors, who reported diagnostic heterogeneityin proteinuric NIDDM patients [17], found that all16 microalbuminuric NIDDM patients had classic le-sions of diabetic glomerulopathy.

Since 1992 we have been performing kidney biop-sies on the basis of a research protocol rather thanclinical indications. The present paper summarizesthe preliminary analysis in the microalbuminuric pa-tients studied to date.

The initial reading of the light microscopy tissue ofthese NIDDM patients made apparent the inade-quacy of current descriptive formulations, largelybased on observations of research biopsies in IDDM(of which the authors have reviewed several hun-dreds). Virtually all IDDM patients with at least10 years of diabetes and overt nephropathy have ob-vious diabetic glomerulopathy. Diabetic glomerulo-pathy, although less severe, is usually quite advancedin microalbuminuric IDDM patients. We recently re-ported that all IDDM patients with AER over 30 mg/min had electron microscopic morphometric mea-sures of diabetic glomerulopathy above the normalrange [32]. Since electron microscopic morphometricanalysis has not been completed in these patients, itis currently impossible to make any precise compari-son between diabetic glomerulopathy in IDDM andNIDDM microalbuminuric patients. Nevertheless,many NIDDM patients with microalbuminuria didnot have glomerulopathy, or they had very mild me-sangial expansion by light microscopy. Thus, 70% ofthe microalbuminuric NIDDM patients had normal

or near normal glomerular structure by light micros-copy, with or without tubulo-interstitial and arteriolarchanges. The remaining 30% had renal changes typi-cal of DN in IDDM, with glomerular, tubulo-intersti-tial and vascular lesions occurring in parallel.

Thus, we developed a new classification systemwhich included three major groups. Category I wasdefined as normal or near-normal renal structure, cat-egory II as “typical” patterns of renal injury (similarto the changes in IDDM). Category III was definedas “atypical” patterns of renal injury, including severetubulo-interstitial and/or arteriolar hyalinosis and/orglobal glomerular sclerosis lesions in the presence ofabsent or mild glomerular changes.

Thus, despite comparable renal function, NIDDMpatients with microalbuminuria are structurally het-erogeneous: only 29 % had “typical” DN, 29% hadnear normal renal structure and 42% severe tubulo-interstitial and/or vascular lesions disproportionateto the mild glomerular involvement. In this series ofpatients we did not find cases of any definable non-di-abetic renal disease. The difference between our find-ings and those in previous reports in proteinuricNIDDM patients (see above) may be explained bythe study design, in that patients in the present studyhad kidney biopsies performed on the basis of a re-search protocol as opposed to clinical indication foratypical course.

The “atypical” patterns of renal injury observed inmany of our patients are probably related to hyper-glycaemia, since HbA1c levels were higher both in pa-tients with “typical” and with “atypical” patterns oflesions compared to patients without lesions. Thus,hyperglycaemia may cause different patterns of renalinjury in older NIDDM compared to youngerIDDM patients. The tubulo-interstitial and vascularchanges could also be related to aging and systemichypertension. However, hypertension was present inalmost all patients (except for 3) in all 3 structuralcategories, and “per se” cannot account for the differ-ent lesions observed in category III. Further, meanage was similar in category II and III patients, despitethe different patterns of renal injury in the twogroups. Also, we examined a large number of age-matched normal control subjects and found that se-vere lesions, as observed in NIDDM patients, wereuncommon in the non-diabetic subjects. Neverthe-less, aging and blood pressure may have varying im-pact in different patients, and therefore may contrib-ute, in presence of other factors, to renal injury inthis subset of patients.

The reasons why the kidney may react differentlyto hyperglycaemia in different patients with NIDDMare not clear. It can be hypothesized that the hetero-geneity in renal structure might reflect the heteroge-neous nature of NIDDM “per se”. Patients with“typical” DN lesions had the longest known diabetesduration, worse metabolic control and they all had


diabetic retinopathy; interestingly their BMI onlyslightly exceeded normal values, as opposed to clearlyincreased BMI values in categories I and III.

The nature of this interrelationship of diabetic mi-lieu and renal structural changes remains enigmatic;however, these relationships also extend to diabeticretinopathy. Diabetic retinopathy was present in allC II patients, background in 50% and proliferativein 50 %. None of the patients in C I and C III had pro-liferative retinopathy, while background diabetic ret-inopathy was observed in 50% of C I and 57% of CIII patients. Thus, all C II patients had diabetic retin-opathy and all patients with proliferative retinopathyhad “typical” DN.

A high proportion of microalbuminuric NIDDMpatients (29%) had normal or near-normal renalstructure. They tended to be younger and had shorterdiabetes duration and better metabolic control thanpatients with renal lesions (categories II and III). Al-though we do not have an explanation for the abnor-mal AER in these patients, it is possible that microal-buminuria in this subset is a clinical manifestation ofgeneralized endothelial dysfunction rather than of re-nal damage “per se”. Parving et al. [16] observedwhat they termed “minimal lesion” nephropathy in 4of 36 NIDDM patients with clinical proteinuria. It ispossible that the patients diagnosed as having “mini-mal lesion” nephropathy in this latter study are similarto our category I patients, however this will requirelongitudinal studies to be addressed. Suffice it to sayhere that a substantial proportion of NIDDM patientswith microalbuminuria or proteinuria may have in-creased glomerular capillary wall permeability to pro-tein for reasons not currently understood.

In summary, in this unselected series of 34 casesbiopsied for research reasons, NIDDM patients withmicroalbuminuria do not have non-diabetic renal dis-eases; however they may have different patterns ofrenal injury compared to IDDM patients. More“typical” diabetic nephropathy patterns are seenamong microalbuminuric NIDDM patients with pro-liferative retinopathy and normal BMI, while “aty-pical” patterns of renal injury are more commonamong those with increased BMI and background orno retinopathy. Finally, a subset of microalbuminuricNIDDM patients have near-normal renal structureby light microscopy; in these patients the increasedrenal permeability to protein might be expression ofgeneralized endothelial dysfunction.

Long-term longitudinal studies are needed to de-termine the course of renal function in these patientswith different patterns of renal injury.

Acknowledgements. This work was supported by grants fromthe Italian National Research Council (PF 40# 9300428,9 400464), the Italian Association LIONS for Diabetes andBayer Company, Milan, Italy. Dr. P. Fioretto is the recipientof the Albert Renold Fellowship award from the EASD. The

method for the mathematical analysis of GFR was suppliedby the LADSEB-CNR, Padova (Dr. K. Thomaseth). We thankMisses B. Mazzucato, I. Barzon, L. Trivellato and L. Samadellofor expert technical help. We are indebted to all the patientswho participated in these studies and to the nursing staff ofthe Department of Internal Medicine of the University of Pad-ova Hospitals.

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patterns of renal injury in niddm patients with ...diabetic nephropathy is the single most frequent...

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