British Joiurnial of Ophlthalmology, 1978, 62, 351-355

Pathophysiology of diabetic retinopathyJ. G. CUNHA-VAZFrom the Department of Ophthalmology, Ultiversity oJ Coimbra, Coimbra, Portiugal

SUMMARY After a brief analysis of the pathological picture of diabetic retinopathy, of which onlythe topographical distribution of the vascular lesions appears to be specific, the results obtainedwith 2 new methods of study of the retinal circulation, are presented. These methods are vitreousfluorophotometry and fluorometric determination of segmental retinal blood flow. Vitreousfluorophotometry has shown that a disturbance of the blood-retinal barrier, possibly functional,appears in diabetic eyes before any lesion is clinically visible in the fundus, and that there is a closecorrelation between the severity of the vascular lesions and higher vitreous fluorophotometryreadings. Blood flow studies have shown that in diabetes the retinal blood flow increases markedlywith progress of background retinopathy, decreasing finally where proliferative retinopathy, withmarked arteriolar narrowing, is present.

On the basis of these findings a working hypothesis for the pathogenesis of diabetic retinopathyis presented.

Diabetic retinopathy is onie of the most clhallengingproblems facing ophthalmological research. Its inci-dence continues to increase all over the world, sothat today it is one of the most frequent causes ofblindness. The incidence of blind registrations dueto diabetes is now more than 15 million a year. Thissituation is particularly serious because, althoughmuch is known about the pathology of diabeticretinal disease, practically nothing is known aboutits cause.

Ashton, who has contributed so extensively toour knowledge of diabetic retinopatlhy, remarkedin 1974 that 'we must continue to look for morefundamental scientific investigations and at thesame time develop new ways of examining thediabetic retina in an effort to unravel the stillunsolved mysteries of diabetic retinopathy'.

In this presentation an effort will be made toreview the present state of knowledge on thepathophysiology of diabetic retinopathy, withparticular emphasis on personal studies using newmethods of examining the retinal circulation andblood-retinal barrier.

Pathology

Diabetic retinopathy is basically a microangiopathy,the usual, initial presenting sign of which is the

Address for reprints: Professor Jose Cunha-Vaz, ClinicaOftalmol6gica, Hospitais da Universidade de Coimbra,Celas, Coimbra, Portugal

appearance of retinal microaneurysms at the pos-terior pole. In 1972 1 examined the pathology ofdiabetic retinopathy and other vascular retino-pathies using injection methods, a variety of stains,and the digestion technique. Light microscopicexamination of retinal 'digests' is particularlyappropriate to study alterations in the retinalvascular bed. From this study a clear pattern ofdisease was observed to occur in diabetic retino-pathy. The results suggested that changes are con-fined to the small vessels in the form of endothelialproliferation, rare microaneurysms, and signs ofimpending cellular degeneration in a few vascularbranches. These initial lesions are focal and locatedpreferentially at the posterior pole of the retina.At first the endothelial proliferation and micro-

aneurysms appear to be confined to the venousside of the retinal circulation, whereas at this stageendothelial degeneration changes appear to belimited to capillaries on the arterial side of thecirculation (Fig. 1).The pericytes, although showing clear signs of

disease, are affected in a highly irregular manner.With progression of the disease the capillaries on

the arterial side of the retinal circulation showincreased cell loss and closure. Simultaneously, onthe venous side of the circulation, there is anincrease in the number of microaneurysms. As theareas of capillary closure enlarge, they are seen tobe traversed by a few enlarged capillaries, whichappear to act as arteriovenous shunts, receiving the

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blood diverted from the surrounding closed capil-lary net. These observations are summarised inTable 1.Are these lesions specific for diabetes? Examina-

tion of other vascular retinopathies emphasises theprobable importance of local factors and that thelesions described in diabetic retinopathy are sharedby a wide variety of apparently unrelated diseases.

Fig. I Diabetic retinopathy, showing capillary closureint the arterial side and the presence of endothelialprolifrration an1d nmicroaneurysmns in the vencous side.Digest pr-epar-ation. (PAS-IS 85)

Endothelial proliferation, prevailing on the venousside of the circulation, is a frequent finding inpolycythaemia, leukaemia, myelomatosis, circinateretinopathy, central retinal vein thrombosis, macro-globulinaemia and Eales's disease.

Microaneurysms are probably the commonestretinal lesion in diabetes (Ashton, 1963) but theyare also seen, frequently in large numbers, in circi-nate retinopathy, Eales's disease, leukaemia, myelo-matosis, macroglobulinaemia, central retinal veinthrombosis, hypertensive retinopathy, glaucoma,and retinoblastoma.

Areas of capillary closure are seen in scleroderma,hiypertensive retinopathy, pernicious anaemia, andcentral retinal vein thrombosis (Fig. 2).

Arteriovenous shunts appear in circinate retino-pathy and central retinal vein thrombosis. Basementmembrane thickening and pericyte degeneration aresimilarly found in other retinopathies, but becauseof their especial prevalence in diabetic retinopathythey merit further consideration.

BASEMENT MEMBRANE THICKENINGThis is believed to play an important role not onlyin diabetic retinopathy but in diabetic microangio-pathy as a whole. Ashton reviewed the subject indetail in 1974, commenting on basement membranethickening in several non-diabetic conditions, itsinconstancy, and its presence in all types of diabetes.Electron microscopical studies of diabetic retino-pathy show that basement membrane thickening isassociated with a variety of haematogeneous ele-ments, which could have arrived at this situation

Table I Evolution oj retinal vascular lesionts in diabetes

Stages

0.

1. Initial

Vitreous*fluoroplhotoss:etry( X /0 ' g1isil)

5.7

12 8

Blood flosv

4.3

Ophthmalmmoscopy

PathologySmytall vessels

Ve',ous sile

4 8 Rare aneurysm Endoth. prol.

Aneurysm + t-

Large vsresels

Arterial sile

Endoth. deg. Vein

2. Intermediate

3. Advanced 36 7

4. Final 300

Numerousaneurysmshaemorrhagesexudate

6 4 Same lesions as instage 2

4-2 Same lesions plusretinitis proliferans

Endoth. prol. - -, Endoth. deg.

Aneurysm

Endotlh. deg. -,

Aneurysm i,:

A-V shunts

Large area cap. clos.

Endoth. deg. - V -,-

Aneurysm -4- -1-

Generalised cap. clos.

Vein -!t

Focal cap. clos.

Vein

Artery A

Vein

Artery j- F-

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DISIlRIBUTION OF RETINAL VASCULAR LESIONSl It has been shown that there is no one lesion which

is absolutely specific for diabetic retinopathy. Themost characteristic features of diabetic retinopathy,microaneurysms, capillary closure, basement mem-brane thickening, and pericyte damage appear todiffer from other vascular retinopathies only intheir frequency and widespread distribution. Whilethe vascular involvement in other retinopathies is

v*. localised initially either to the arterial or to thevenous side of the circulation (Cunha-Vaz, 1967;Wise et al., 1971), there are indications that in diabeticretinopathy the entire vascular tree at the posterior

t' pole of the retina is involved from the beginning.

I

Fig. 2 Central retinal vein thrombosis, showing a

marked degree of capillary closure. The capillary niet hasbeen replaced by a few dilated anastomotic vesselspresenting endothelial proliferation and microaneurJysnis.Digest preparation. (PAS-H x 77)

only through leakage due to breakdown of the endo-thelial barrier. Leakage of plasma components isespecially evident in microaneurysms and is appar-

ently responsible for the greatly thickened wall. Itis also important to recall that leakage of this kindnecessarily implies a preceding endothelial injury.

In his electron micrographs of diabetic basementmembranes Ashton observed a characteristic multi-layered appearance with interposed cellular debris,a finding which Vracko and Benditt (1970) believeto be due to repeated endothelial cell death withregeneration and replacement, each new generationof cells contributing their own basement laminaeand entrapping the cellular debris of the dead cells.This may explain the comparatively healthy appear-

ance of the endothelium even in the presence ofgross periendothelial pathology, including degener-ate pericytes. Basement membrane thickeningappears, therefore, to be a secondary feature of thedisease, due in part, possibly, to glycoproteininsudation from an initial endothelial dysfunction,as considered by Williamson and Kilo (1976).

PERICYTE DAMAGEThere is abundant evidence that these cells are

selectively involved in the diabetic process (Coganand Kuwabara, 1963), the damage taking the formof a peculiar eosinophilic degeneration of the nucleusbefore the cell disintegrates altogether. Althoughsimilar changes can be observed in other retino-pathies, they are by no means so common as indiabetes.

Recent clinical resear0h

The advent of fluorescein angiography and plhoto-coagulation has had an enormous impact on thestudy of diabetic retinopathy, the former teachingus much about the natural history of retinopathyalthough without giving much new informationabout the underlying defect (Ashton, 1976).

Recently, however, two new clinical methods ofstudy of the retinal circulation, vitreous fluoro-photometry and fluorometric determination ofsegmental retinal blood flow, have been developedin our department. These methods have givenvaluable information on the physiopathogenesis ofdiabetic retinopathy.

Vitreous fluorophotometry is a clinical method forthe quantitative study of the blood-retinal barrier(Cunha-Vaz et al., 1975). The examination of anumber of diabetic patients presenting differentdegrees of retinopathy showed a significant break-down of the blood-retinal barrier in every eyeexamined. Furthermore, it was found that distur-bance of the blood-retinal barrier, as shown byvitreous fluorophotometry, appears before anylesions such as capillary closure and microaneurysmsare clinically visible in the fundus. This studyshowed that a breakdown of the blood-retinalbarrier, possibly functional, is undoubtedly one ofthe earliest changes to occur in the retina in diabetes.Using vitreous fluorophotometry we have nowexamined 45 patients with confirmed diabetes, butan apparently normal retina as shown by ophthal-moscopy and fundus fluorescein angiography andhaving 10/10 vision. In spite of the 'normal' fluores-cein angiograms, the vitreous fluorophotometryreadings were constantly above normal, with averagevalues of 57 x 10-8 g/ml of fluorescein in the pos-terior vitreous and 4.7 x 10-8 giml in the middlevitreous 1 hour after an intravenous injection of1 g of sodium fluorescein (Table 2).Another point that emerged from our studies

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and which deserves further attention is the probable

role of alteration of the blood-retinal barrier in theprogress of diabetic retinopathy, as testified by theclose correlation between the severity of the vascular

lesions and the vitreous fluorophotometry readings.Vitreous fluorophotometry of 24 diabetic patientspresenting simple background retinopathy charac-terised by moderate numbers of microaneurysmsbut no signs of macular involvement (Fig. 3),showed average values of 12-8 x 10-8 g/ml of fluores-cein in the posterior vitreous and 107 x 10-8 g/mlin the middle vitreous, whereas average values of36-7 x 10-8 g/ml of fluorescein were recorded in theposterior vitreous of diabetic patients showing

numerous microaneurysms, haemorrhages, dilatedvascular channels, and areas of capillary closurewith signs of macular involvement, includingcoalescing hard exudates (Fig. 4).As regards the pathophysiology of diabetic

retinopathy, vitreous fluorophotometry has, there-fore, contributed by demonstrating that a breakdown Fig. 3 Fluroescein angiograof the blood-retinal barrier is apparently the earliest background retinopathy withcchange to occur in the retinae of diabetic patients.This observation has since been confirmed by elec-tron microscopy in alloxan-diabetic dogs and shownto be associated with opening of the interendothelialjunctions (Wallow and Engerman, 1977). Similarly,

the fact that a close correlation between progressionof the vascular disease and increasing vitreousfluorophotometry readings was observed indicatesthat the altered permeability of the blood-retinalbarrier is likely to play an important role in thedevelopment of the entire pathological picture ofdiabetic retinopathy.The development of a new direct method for

estimation of human retinal blood flow by slit-lampfluorophotometry has also proved useful (Cunha-Vaz and Lima, 1977). Examination of a group ofdiabetic patients showed close correlation betweenincreased retinal blood flow and the clinical stagingof the retinopathy (Table 1). Retinal blood flow is

within normal limits in the initial stages of retinal

involvement in diabetes, when the only alteration Fig. 4 Fluorescein angiogrcis a breakdown of the blood-retinal barrier. background retinopathy with

Table 2 Vitreousfluorophotometry and superior temporal bloodflow in diabetic patients

im ofa case classified as

out maculopathy

am of a case classified as

maculopathy

Vitreous fluorophotometry Segmental blood flow( x 10-8 g/ml) (Sup. tenmporal)

Retinal involvement in diabetesPosterior Middle Bloodflow Transit time Arterial

(litre/min) (second) diameter (sunw)

'Normal retina' 5-7±1-9 4-7±1-7 4-3±0 5 0 36±0 05 188±15

Simple background retinopathy 12 8±6 3 10 7±4-9 4 8±0 6 0 29±0 05 179 ±18

Background retinopathy with maculopathy 36-7+14-2 28-5±11-1 6-4±1 1 0-23±0-06 183±18

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Pathophysiology of diabetic retinopathy

Patients with minimal background retinopathyshow a slight increase in retinal blood flow, whereasmarked increase of segmental blood flow is observedin the more advanced stages of background retino-pathy with maculopathy.

Patients presenting with proliferative retinopathy,however, have lower near-normal retinal blood flowvalues. The results obtained in late-onset diabeticproliferative retinopathy point clearly to the 'occlu-sive' nature of the retinopathy.A number of matters relevant to the patho-

physiology of diabetic retinopathy are highlightedby this study. In the first place it appears that changesin retinal blood flow are not the earliest finding indiabetes, and the question arises as to why progres-sion of the disease should give rise to increasedflow. Probably capillary and venous dilatation isresponsible for the initial increase in blood flow insimple background retinopathy. Later on, endothe-lial damage and progressive thickening of thebasement membrane lead to progressive capillaryclosure and the development of arteriovenousshunts which short-circuit the capillary bed andshorten its length. This factor, associated withprogressive loss of autoregulation on the part of theretinal arteries, is the most probable cause ofincreased blood flow in advanced backgroundretinopathy.

Also to be considered is the possibility that theobserved increase in retinal blood flow may itself bedeleterious to the retina and responsible for furtheradvancement of the disease. In this context it maybe relevant to note that conditions which are asso-ciated with decreased blood flow, for example, caro-tid insufficiency, high myopia, and optic nerveatrophy, have a beneficial effect on the evolution ofdiabetic retinopathy.The recent clinical studies outlined above provide

the basis for a working hypothesis of the evolutionof diabetic retinopathy. It appears to start at thelevel of the endothelial membrane of the smallretinal vessels with reversible breakdown of theinner blood-retinal barrier. The initial changes inthe capillary cell population, which include endo-thelial proliferation on the venous side and genera-lised pericyte damage, are possibly related to thecapillary-venous dilatation linked with the initialincrease in blood flow. Basement membrane thicken-ing associated with endothelial swelling or prolifera-tion might be responsible for the capillary closureand subsequent shunt formation. Shunt formationand loss of arteriolar autoregulation appear to beinvolved in the increase of retinal blood flow at thislater stage.

While local factors appear to be responsible forthe entire evolution, once the process has been

started by changes in the blood-retinal barrier, theabsence of any clear relation between diabeticcontrol and progression or regression of the retino-pathy suggests that subsequent development islargely autonomous. There is, however, evidenceof a direct relationship between the initial break-down of the blood-retinal barrier and diabeticcontrol (Cunha-Vaz et al., 1977).

I think that there is good support for the viewthat processes of two kinds take part in the patho-genesis of diabetic retinopathy. There is, in thefirst place, a generalised disorder of small bloodvessels of unknown cause which is specific to dia-betes, metabolically determined, and at least in partrelated to the degree and duration of departurefrom metabolic normality. And in the second placethere are local responses specific to the retina,conditioned by its vascular peculiarities and thepatient's age, which, once initiated, are largely self-perpetuating.

This study was supported by Centro de Oftalmologiado Instituto Nacional de Investigaqao Cientifica,Portugal.

References

Ashton, N. (1963). Studies of the retinal capillaries inrelation to diabetic and other retinopathies. British Journalof Ophthalmology, 47, 521-538.

Ashton, N. (1974). Vascular basement membrane changesin diabetic retinopathy. British Journal of Ophthalmology,58, 344-366.

Ashton, N. (1976). Pathology of diabetic retinopathy. InActa XXII Concilium Ophthalmologicum, Vol. I, pp.175-181. Masson: Paris.

Cogan, D. G., and Kuwabara, T. (1963). Capillary shunts inthe pathogenesis of diabetic retinopathy. Diabetes, 12,293-300.

Cunha-Vaz, J. G. (1967). In Aspectos da FisiopatologiaVacular da Retina. Imprensa de Coimbra: Coimbra.

Cunha-Vaz, J. G. (1972). Diabetic retinopathy: human andexperimental studies. Transactions of the OphthalmologicalSocieties of the United Kingdom, 92, 111-124.

Cunha-Vaz, J. G., Abreu, J. R., Campos, A. J., and Figo, G.(1975). Early breakdown of the blood-retinal barrier indiabetes. British Journal of Ophthalmology, 59, 649-656.

Cunha-Vaz, J. G., Fonseca, J. R., Abreu, J. R., and Ruas, M.(1977). (Unpublished observations.)

Cunha-Vaz, J. G., and Lima, J. P. (1977). Archives ofOphthalmology. 00, 000-000.

Vracko, R., and Benditt, E. P. (1970). Capillary basallamina thickening: its relationship to endothelial celldeath and replacement. Journal of Cell Biology, 47, 281.

Wallow, I. H., and Engerman, R. L. (1977). Permeability andpatency of retinal blood vessels in experimental diabetes.Investigative Ophthalmology and Visual Science, 16,447-461.

Williamson, J. R., and Kilo, C. (1976). Basement-membranethickening and diabetic microangiopathy. Diabetes, 25,925-927.

Wise, G. N., Dollery, C. T., and Henkind, P. (1971). TheRetinal Circulation. Harper & Row: New York.

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