Diabetic retinopathy: a clinical overview

A study of physiologic angiogenesis in the human using the dental pulp as an in vivo model

With the commonly used in vivo animal models of angiogenesis, direct extrapolation of results to the human is not possible. The results presented from this study exemplify various phases of angiogenesis, from cell migration to apoptosis. This supports the use of the dental pulp of the developing human tooth as a viable model of in vivo physiologic angiogenesis.

Association of plasma fibrinogen level and blood pressure with diabetic retinopathy, and renal complications associated with proliferative diabetic retinopathy, in Type 2 diabetes mellitus

AIM

To clarify the association of several clinical parameters, including plasma fibrinogen level, with diabetic retinopathy in patients with Type 2 diabetes mellitus (DM).

METHODS

A total of 294 Japanese patients with Type 2DM were studied; 53 patients with no diabetic retinopathy (NDR), 90 with background diabetic retinopathy (BDR), and 151 with proliferative diabetic retinopathy (PDR). Multiple logistic regression analysis was performed to assess variables independently associated with diabetic retinopathy in two settings: presence of retinopathy of any severity and presence of advanced retinopathy.

RESULTS

The following parameters were identified as independent factors associated with the presence of diabetic retinopathy (NDR vs. BDR + PDR): type of therapy (P<0.0005), log-transformed plasma fibrinogen level (P < 0.05), mean blood pressure (P < 0.05), and duration of diabetes (P < 0.05). The independent variables associated with advanced retinopathy were type of therapy (P<0.00005), age (P<0.0005) and nephropathy (P<0.05). Body mass index, smoking and hypertensive status, HbA1c and total cholesterol levels were not independently associated.

CONCLUSIONS

These data suggest that in patients with Type 2 DM, an increased blood viscosity due to high fibrinogen level as well as an elevated intravessel pressure play a role in the development of diabetic retinopathy, and that the progression to PDR is influenced or accompanied by the deterioration of renal status.

Immunohistochemical and ultrastructural study of microvessels in diabetic veins

The aim of this study was to examine the localization and features of the intramural microvessels in the wall of inferior limb veins in diabetic patients. The study was conducted in a group of 10 insulin-dependent diabetic patients, who had suffered inferior limb amputation as a consequence of chronic limb ischemia. Sections of long saphenous and posterior tibial veins were investigated by light and transmission electron microscope. The von Willebrand factor and albumin were detected by immunohistochemical techniques. The results show the presence of numerous microvessels in the intima and inner third of the media layer of the venous wall of the sections studied. These microvessels present morphological and functional modifications in relation to those observed in the control veins. They also showed the presence of endothelial cells migrating from the lumen. The authors conclude that the venous wall of inferior limbs in amputated diabetic patients presents a neovascularization process. Some of the endothelial cells of the venous lumen can also have a role in the angiogenesis.

Characterization of [2-3H]deoxy-D-glucose uptake in retina and retinal pigment epithelium of normal and diabetic rats

The outer blood-retinal barrier which results from the tight junctions between retinal pigment epithelial cells (RPE) restricts the flow of nutrients reaching the retina. We characterize the transport of [2-3H]deoxy-D-glucose (2-DG) across isolated mammalian neural retina and RPE in terms of their kinetics constants. In addition, the effect of insulin on glucose transport was studied by using streptozotocin-induced diabetic rats. RPE accumulates 2-DG by a temperature-sensitive and energy-dependent complex kinetics mechanism. The retina takes up 2-DG by an energy and Na(+)-dependent saturable system with an apparent Km of 2 mM. Insulin induced an increase of 2-DG uptake by normal retina. The retina of diabetic rats shows lower levels of 2-DG accumulation. These levels can be returned to the normal ones by exposure to insulin. Although insulin does not affect, significantly, 2-DG accumulation by RPE, 2-DG uptake of RPE from diabetic rats shows a normal saturable kinetics with an apparent Km of 20 mM. Those findings suggest the presence of different types of glucose transporters in retina and RPE. Insulin-sensitive glucose transport in retina might be involved in the manifestation of diabetic retinopathy.

The efficacy of aldose reductase inhibitors in the management of diabetic complications. Comparison with intensive insulin treatment and pancreatic transplantation

Recently, aldose reductase inhibitors (ARIs) have been registered in several countries for the improvement of glycaemic control. However, their efficacy is still controversial. ARIs inhibit the enhanced flux of glucose through the polyol pathway. As such, they can never be more effective than normoglycaemia, and so their potential benefits and limitations should be considered relative to the effects of prolonged euglycaemia. The clinical effects of ARIs can be put into perspective by assessing the effects of improved glycaemic control attained in randomised trials of intensive insulin treatment [such as the Diabetes Control and Complications Trial (DCCT)] and after pancreatic transplantation. Although direct comparison of these 3 interventions is hampered by differences in patient populations, duration and methods of follow-up and in the potency of ARIs, the effects of these 3 metabolic interventions and their course in time appear remarkably similar. For neuropathy, all 3 interventions induce an increase in average motor nerve conduction velocity of approximately 1 m/sec during the first months of treatment. At the same time, improvement of painful symptoms may occur. These changes probably largely represent a metabolic amelioration of the condition of the nerves. Around the second year of treatment with all 3 forms of metabolic improvement, an acceleration of nerve conduction of a similar magnitude occurs, with signs of structural nerve regeneration and some sensory recuperation. Experience with ARIs in nephropathy is still limited, but similar improvements in glomerular filtration rate and, less consistently, in urinary albumin excretion were found during short term normoglycaemia produced by all 3 forms of treatment. Comparison of a small number of studies, however, shows differences between intensive insulin regimens, pancreatic transplantation and ARIs in effects on retinopathy. Retinopathy often temporarily deteriorates in the early phases of improved glycaemic control, but this is not noted with ARIs. New microaneurysm formation was slightly reduced in a single long term study with the ARI sorbinil, but the preventive effects on the overall levels of retinopathy seemed less strong than in normoglycaemia trials of similar duration. However, the pharmacodynamic effects on inhibiting the polyol pathway differ among ARIs, and the half-life of the inhibiting effect of sorbinil may have been too short for a complete reduction of polyol pathway activity. The trials of prolonged intensive insulin therapy and pancreatic transplantation have demonstrated that very strict metabolic control must be maintained continuously for many years before a significant reduction of complications can be demonstrated.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of aldose reductase inhibition with tolrestat on diabetic retinopathy in a six months double blind trial

To study the effects of the new aldose reductase inhibitor tolrestat on diabetic retinopathy, 31 diabetic patients with various degrees of retinopathy were randomly assigned to either tolrestat (200 mg once daily) or placebo treatment for six months. Separate morphological features of diabetic retinopathy were assessed by fundus photography and fluorescein angiography before and at the end of the study. The results showed some amelioration of clinical signs of diabetic retinopathy during aldose reductase treatment. Hard exudates, intraretinal hemorrhages and focal fluorescein leakage increased on average in the placebo and decreased in the tolrestat group. The difference was statistically significant for focal fluorescein leakage only. The permeability of the blood retinal barrier was determined by vitreous fluorophotometry before and at the end of the study. No change in permeability values was found.

Early selective neuroretinal disorder in prepubertal type 1 (insulin-dependent) diabetic children without microvascular abnormalities

The duration of diabetes before puberty is not considered relevant to the future development of complications. To evaluate the effects of diabetes on the neural retina, we analysed macular function by steady-state focal electroretinography in 20 prepubescent diabetic children without vascular retinopathy and in 39 sex- and age-matched normal children. The mean (+/- SD) response related to retinal cellular elements between the photoreceptors and ganglion cells was significantly lower in diabetic children than in the control group (0.38 +/- 0.12 vs. 0.51 +/- 0.13 microV; unpaired t-test = 3; P = 0.005). Similarly, ganglion cell function showed a significant impairment in diabetic children with respect to the control group (0.4 +/- 0.13 vs. 0.53 +/- 0.09 microV; unpaired t-test = 5.4; P = 0.0001), whereas the photoreceptors appeared unaffected. Metabolic control and disease duration were not correlated with functional deficits. Our results suggest that before puberty, early diabetes may have a selective effect on the neural retina prior to the appearance of microvascular changes. A focal electroretinogram could identify diabetic children with neurosensory disorders who may have a higher risk of developing microvascular retinopathy.

Comments on the clinical impact of hypertension in type I diabetes

In type I diabetes, the quality of life and, in essence, the long-term prognosis or life expectancy of the patient are invariably related to the manifestation of untoward complications. Increased arterial blood pressure (hypertension) has a great influence in these complications. Cumulative evidence has shown that proteinuric type I diabetic patients are easily susceptible to hypertension and its accompanying sequelae. The debilitating effects of hypertension on the progressive development of diabetic nephropathy leading to renal dysfunction and mortality in renal transplant patients have been documented. Proliferative retinopathy and cardiovascular lesions are also frequent devastating complications in hypertensive-diabetic patients. The mechanism of sodium/lithium countertransport activity and the genetic predisposition to hypertension require further elucidation.

Glucose-induced microvascular functional changes in nondiabetic rats are stereospecific and are prevented by an aldose reductase inhibitor

Exposure of skin chamber granulation tissue vessels in nondiabetic rats to 11 or 15 mM D-glucose (but not L-glucose or 3-O-methylglucose) twice daily for 10 d induces vascular functional changes (increased albumin permeation and blood flow) identical to those in animals with mild or severe streptozotocin diabetes, respectively. These vascular changes are strongly linked to increased metabolism of glucose via the sorbitol pathway and are independent of nonenzymatic glycosylation as well as systemic metabolic and hormonal imbalances associated with the diabetic milieu. (J. Clin. Invest. 1990. 85:1167-1172.)

Effects of very mild versus overt diabetes on vascular haemodynamics and barrier function in rats

Rats injected i.p. with a single dose of nicotinamide (250 mg/kg) 15 min prior to i.v. injection of streptozotocin (65 mg/kg) develop a very mild form of diabetes characterized by slight elevations of plasma glucose, increased levels of HbA1, and reduced insulin secretion in response to an i.v. glucose tolerance test. These rats gain weight normally and they are not hyperphagic, glycosuric, or polyuric. The effects of this very mild form of diabetes vs overt streptozotocin diabetes of three months duration on regional vascular 131I-albumin clearance, blood flow (assessed by 15 microns 85Sr-microspheres), and renal filtration function were examined in male Sprague-Dawley rats. Plasma glucose levels of rats with mild diabetes were 7.4 +/- 0.9 (mean +/- SD) (mmol/l) vs 6.5 +/- 0.6 for control rats and 31.3 +/- 6.0 for overtly diabetic rats. HbA1 levels were increased 1.4 fold in mildly diabetic and 2.3 fold in overtly diabetic rats. Vascular clearance of 131I-albumin was markedly increased in ocular tissues (anterior uvea, retina, and choroid), sciatic nerve, aorta, new (subcutaneous) granulation tissue, and kidney of both diabetic groups, although increases in overtly diabetic rats exceeded those in the mildly diabetic group (2.2-4.6 times control animals vs 1.6-3.3 times, respectively). Likewise, both overt and very mild diabetes markedly increased glomerular filtration rate (approximately 1.8 times and 1.2 times control animals, respectively), urinary excretion of endogenous albumin (approximately 9 times and 4 times) and IgG (approximately 15 times and 4 times), as well as regional blood flow in the anterior uvea, choroid, and sciatic nerve. Increases in tissue sorbitol levels were much larger in overtly diabetic rats (generally 10-20 times control animals) than in mildly diabetic rats (1.5-3 times controls). myo-Inositol levels were significantly decreased only in lens and sciatic nerve of overtly diabetic rats. These observations indicate that even very mild diabetes is associated with vascular functional changes which develop more slowly than in overtly diabetic rats, but are disproportionately large (in view of the minimal increases in glycaemia and tissue polyol levels) compared to those in overtly diabetic rats.