Craig Blackwell, MD

Santa Cruz, CA
Diplomate: American Board of Ophthalmology
Fellow: American Academy of Ophthalmology

Welcome to the Website of Craig Blackwell, MD

An Ophthalmology Practice in Santa Cruz, CA

Diabetes 1- Introduction

Complications of Diabetes, Introduction

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If you have diabetes or are close to someone who does, you may ask, … Why is it important to keep blood sugar and blood pressure under control? The answer is very simple, it comes from the relation between blood sugar and complications, like Retinopathy, Nephropathy, and Neuropathy, which we will explain below.

The basic concept: Elevated blood sugar damages the fine capillary blood vessels that deliver oxygen and nutrients to tissues throughout the body.

Video: Diabetes and the Eye

1. Retinopathy:

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The retina is a privileged location where we can look at the fine capillary blood vessels and directly observe damage so we will describe capillary changes as they are seen to develop in the retina.

Stage One. The earliest changes in retinopathy involve decreased function of nerve cells in the retina, before any visible vascular changes occur.

Stage Two. High blood sugar damages capillary vessel walls causing them to become leaky and develop bulges, called microaneurisms. Leakage first consists of the clear serum part of blood, then red blood cells. Leakage into any tissues is called edema. Edema in the center of the retina is called Macular Edema and will impair vision.

Stage Three. The capillaries shut down and stop delivering blood. The expected outcome of this is that the retina, deprived of its blood supply, doesn’t function well and eventually dies. The unexpected part is that the hypoxic retina sends out a chemical help messenger that stimulates growth of new vessels, which sounds like a good idea, but has bad consequences. The new vessels are fragile and have a strong tendency to bleed, bringing scar tissue into the eye. This is proliferative retinopathy, and signals a dangerous escalation in vision-threatening processes.

Photo. Diabetic Retinopathy. Shown here is a moderate level of leakage, with blood (arrowhead) and serum (arrow). More specifically, the arrow is pointing to a ring of “exudate,” in which there is clear fluid (edema) in the center with clumps of lipid around the edge.

2. Nephropathy:

Damage to the kidneys leads to system wide bad consequences. Your body is less able to clear waste products from the blood, regulation of blood chemistry is impaired, protein leaks out in urine, etc. This is the leading cause of end stage kidney disease requiring dialysis.

3. Neuropathy:

Damage to the nervous system leading to loss of sensation in peripheral structures, like feet, frequently leading to persistent pain.

4. Vasculopathy:

As the peripheral circulation shuts down structures at the far end of the pipeline don’t get adequate blood supply. Wounds don’t heal. Toes and feet are lost.

Now we are going to show you the motivation for paying attention to blood sugar and blood pressure.

Hemoglobin A1c

If you don’t already know what Hemoglobin A1c is it is time to learn.

There are several ways to measure blood sugar control. A random blood sugar is of limited use because it is subject to considerable variability depending on time since the last meal. Taking the blood sugar, either fasting or two hours after eating is more consistent, and because these are simple to do, are among the most frequent tests used to diagnose diabetes. We will skip glucose tolerance test, because it is more complicated and less commonly used.

Hemoglobin A1c is different. Contained within red blood cells, the hemoglobin molecule is responsible for carrying oxygen in the blood stream. When blood sugar is consistently high, extra sugar (glucose) molecules become attached to the hemoglobin molecules. The amount of extra sugar attached can be measured and gives a very useful measure of blood sugar control over a 3 month period of time. That is the tool used by almost all studies about diabetes and is likely the one your doctor uses to manage your diabetes.

Complications of Diabetes

1. The Diabetes Control and Complications Trial (DCCT)

The DCCT is probably the most important study demonstrating the beneficial effects of good blood sugar control on reducing complications. Only diabetics who require insulin for blood sugar control were included in this study whose purpose was to compare conventional blood sugar treatment versus intensive treatment.

The study and findings are discussed in detail in the DCCT page.

Video: Diabetes Control and Complications

For Retinopathy:

  • Comparing Conventional versus Intensive treatment, progression of Retinopathy was reduced 76% in the Primary Prevention group and 54% in the Secondary Intervention Group.
  • There was a very strong relationship between Retinopathy and A1c level. For every 10% decrease in A1c there was a 39% decrease in retinopathy.

For Nephropathy:

  • For every 10% decrease in A1c there was a 25% decrease in risk of Nephropathy (as measured by albuminuria).

2. United Kingdom Prospective Diabetes Study (UKPDS)

The companion study for diabetics who were managed without Insulin was the United Kingdom Prospective Diabetes Study (UKPDS). It showed a similar benefit of improved blood sugar control.

The study and findings are discussed further in the UKPDS page.

  • Retinopathy and Nephropathy rates were reduced by 25% in the Intensive Therapy group.
  • For every percentage point decrease in A1c (eg. from 9% to 8%) there was a 35% reduction in the risk of microvascular complications.

A secondary aim of the UKPDS was to investigate the effect of high blood pressure on complications. They found:

  • Lowering blood pressure to a mean of 144/82 significantly reduced strokes, diabetes-related deaths, heart failure, microvascular complications, and vision loss.


There is no doubt that improved blood sugar and blood pressure control are important in minimizing long term complications of diabetes.

That means the extra time and effort involved with diet, exercise and following a medication regimen pays off in preserving sight, kidney function, vascular health, etc.


Standards of Medical Care in Diabetes – 2008. American Diabetes Association. Diabetes Care. 2008; Vol.1, (Suppl.1) S12

Diabetes Control and Complications Trial. JAMA. 2002;287:2563

United Kingdom Prospective Diabetes Study. Diabetes Care. 2002;25;(Suppl.1) S28

The information contained here is for educational purposes and is not a replacement for the individual care of your physician.

C. Blackwell,MD Dec.2008