Diabetes-induced damage to the retina of the eye
Medical condition
Diabetic retinopathy (also known as diabetic eye disease) is a medical condition in which damage occurs to the retina due to diabetes. It is a leading cause of blindness in developed countries and one of the lead causes of sight loss in the world, even though there are many new therapies and improved treatments for helping people live with diabetes.
Diabetic retinopathy affects up to 80 percent of those who have had both type 1 and type 2 diabetes for 20 years or more. In at least 90% of new cases, progression to more aggressive forms of sight threatening retinopathy and maculopathy could be reduced with proper treatment and monitoring of the eyes. The longer a person has diabetes, the higher his or her chances of developing diabetic retinopathy. Each year in the United States, diabetic retinopathy accounts for 12% of all new cases of blindness. It is also the leading cause of blindness in people aged 20 to 64.
Signs and symptoms.
Nearly all people with diabetes develop some degree of retina damage ("retinopathy") over several decades with the disease. For many, that damage can only be detected by a retinal exam, and has no noticeable effect on vision. Over time, progressive retinal damage may appear on a retinal exam, first with small bulges in retinal blood vessels called microaneurysms. Then larger abnormalities in retinal vessels: cotton wool spots, hemorrhages, lipid deposits called "hard exudates", intraretinal microvascular abnormalities, and abnormal-looking retinal veins. Eventually, many progress to a stage where new blood vessels grow throughout the retina. These new blood vessels often break and bleed. Minor bleeding can cause dark floating spots obstructing vision; major bleeding can completely block vision.
Around half of people with diabetic retinopathy develop swelling of the macula, called macular edema, which can begin at any time. If the swelling occurs near the center of the macula, it can cause vision disruptions ranging from mildly blurred vision to severe loss of the center of an affected person's visual field. Left untreated, around 30% of those with such swelling experience vision disruption over the next 3–5 years. Macular edema is the most common cause of vision loss in people with diabetic retinopathy.
The repeated processes of blood vessel growth, swelling, and scarring can eventually cause retinal detachment, which manifests as the sudden appearance of dark floating spots, flashes of light, or blurred vision.
Diagnosis and classification.
Diabetic retinopathy is typically diagnosed by retinal exam observations using ophthalmoscopy. The American Academy of Ophthalmology divides diabetic retinopathy into five categories of progressive severity. The first category, "no apparent retinopathy", describes those with a healthy retinal exam. The next three categories: mild, moderate, and severe nonproliferative diabetic retinopathy (NPDR) describe increasing levels of damage to the retina. People with mild NPDR have microaneurysms in the retina, but no other damage. Those with severe NPDR have 20+ retinal hemorrhages in each quadrant of the retina, a distinctive pattern of damage on the veins of the retina called "venous beading" in at least two retinal quadrants, and obvious intraretinal microvascular abnormalities anywhere on the retina. Moderate NPDR is defined as more severe than mild NPDR, but not yet meeting the criteria for severe NPDR. The fifth stage, proliferative diabetic retinopathy, is for those with new blood vessels forming throughout the retina ("retinal neovascularization"), or blood leaking into the vitreous humor ("vitreous hemorrhage") or between the vitreous membrane and retina ("preretinal hemorrhage").
The same guidelines separately divide macular edema into two categories: "macular edema apparently absent" and "macular edema apparently present". The latter is further subdivided into "mild"—retinal thickening or lipid deposits far from the macula's center; "moderate"— thickening or deposits near the center; and "severe"—thickening or deposits on the macula center. Optical coherence tomography is frequently used to assess macular edema.
Fluorescein angiography is used by retina specialists to further assess diabetic retinopathy severity and to determine sites of macular damage.
Screening.
Due to the lack of symptoms, most people with diabetic retinopathy are unaware that they have the condition until they visit an eye doctor. Both the American Diabetes Association (ADA) and the International Council of Ophthalmology (ICO) recommend regular eye exams for those with diabetes to screen for diabetic retinopathy (except those with gestational diabetes). The ADA recommends a comprehensive eye examination at the time of type 2 diabetes diagnosis, and within five years of the onset of type 1 diabetes. For women with diabetes who become pregnant, the ADA recommends an eye examination before pregnancy, in each trimester, and for a year post partum. The ICO recommends eye examinations for those with diabetes include a visual acuity examination and a retinal examination via ophthalmoscopy or retinal photography.
Iceland, Ireland, and the United Kingdom are the only countries with full national diabetic retinopathy screening programs, while substantial regional screening programs have been implemented in parts of mainland Europe, parts of Asia, and Botswana. In the UK, screening for diabetic retinopathy is part of the standard of care for people with diabetes. After one normal screening in people with diabetes, further screening is recommended every year. Teleophthalmology has been employed in these programs.
Causes.
Diabetic retinopathy is caused by prolonged high blood glucose damaging the small blood vessels of the retina, though the mechanism by which this occurs is unknown. Progression of diabetic retinopathy is accompanied by loss of capillary cells, increased blood vessel permeability in the retina, and altered retinal blood flow, all of which can reduce the amount of blood oxygen that gets delivered to the retina. Poor oxygenation of tissues drives the formation of new blood vessels throughout the retina, resulting in the proliferative stage of disease. These new blood vessels tend to rupture easily, causing bleeding within the eye, scarring, and damage to the retina or macula. Recent evidences have found a strong association between diabetic retinopathy and inflammation.
Risk factors.
The major risk factors for developing diabetic retinopathy are duration of diabetes, poor blood sugar control, and to a lesser extent high blood pressure. Five years after diabetes diagnosis, around 25% of those with type 1 diabetes have some degree of diabetic retinopathy, while 2% have proliferative diabetic retinopathy. By 15 years after diagnosis, that increases to 80% with some retinopathy, and 25% with proliferative disease. Children are an exception—regardless of duration of diabetes, children rarely experience sight-threatening retinopathy; however, puberty can accelerate retinopathy progression. Pregnancy can also accelerate the progression of diabetic retinopathy (although women with gestational diabetes are not at risk).
Both chronically high blood sugar (measured by a high HbA1c) and highly variable blood sugar are associated with developing diabetic retinopathy. Several more minor risk factors also exacerbate diabetic retinopathy, namely kidney disease, abnormal blood lipids, high body mass index, and smoking.
Genetic predisposition to diabetic retinopathy in type 2 diabetes consists of many genetic variants across the genome that are collectively associated with diabetic retinopathy (polygenic risk) and overlaps with genetic risk for glucose, low-density lipoprotein cholesterol, and systolic blood pressure. Several variations in the "VEGFC" gene have also been associated with an increased risk of developing macular edema.
People with Down syndrome, who have extra chromosome 21 material, almost never acquire diabetic retinopathy. This protection appears to be due to the elevated levels of endostatin, an anti-angiogenic protein, derived from collagen XVIII. The collagen XVIII gene is located on chromosome 21.
Incidence of Retinitis Pigmentosa is observed to result in fewer and less severe microvascular lesions in both humans and mouse models. Retinitis Pigmentosa results in loss of rod receptors in the mid peripheral field, reducing the oxygen consumption that is linked with release of VEGF and growth of unwanted blood vessels in the retina.
Obstructive sleep apnea (OSA) has been associated with a higher incidence of diabetic eye disease due to blood desaturation caused by intermittent upper airway obstructions. Treatment for OSA can help reduce the risk of diabetic complications.
Pathogenesis.
Diabetic retinopathy is the result of damage to the small blood vessels and neurons of the retina. The earliest changes leading to diabetic retinopathy include narrowing of the retinal arteries associated with reduced retinal blood flow; dysfunction of the neurons of the inner retina, followed in later stages by changes in the function of the outer retina, associated with subtle changes in visual function; dysfunction of the blood-retinal barrier, which protects the retina from many substances in the blood (including toxins and immune cells), leading to the leaking of blood constituents into the retinal neuropile. Later, the basement membrane of the retinal blood vessels thickens, capillaries degenerate and lose cells, particularly pericytes and vascular smooth muscle cells. This leads to loss of blood flow and progressive ischemia, and microscopic aneurysms which appear as balloon-like structures jutting out from the capillary walls, which recruit inflammatory cells; and advanced dysfunction and degeneration of the neurons and glial cells of the retina. The condition typically develops about 10–15 years after receiving the diagnosis of diabetes mellitus.
An experimental study suggests that pericyte death is caused by blood glucose persistently activating protein kinase C and mitogen-activated protein kinase (MAPK), which, through a series of intermediates, inhibits signaling through platelet-derived growth factor receptors—signaling that supports cellular survival, proliferation, and growth. The resulting withdrawal of this signaling leads to the programmed cell death (apoptosis) of the cells in this experimental model.
In addition, excessive sorbitol in diabetics is deposited on retina tissue and it is also proposed to play a role in diabetic retinopathy.
Recent studies have found a strong correlation between retinal inflammation and diabetic retinopathy progression.
A genetic study showed that diabetic retinopathy shares a similar genetic predisposition with levels of glucose, low-density lipoprotein cholesterol, and systolic blood pressure, indicating that glycemic control and cardiometabolic factors may be important in the development of diabetic retinopathy.
Lipid peroxidation plays a notable role in the progression of diabetic retinopathy. Free radicals such as hydroxyl and hydroperoxyl species with oxygen as functional group oxidize lipids and phospholipids, and at cellular level bring about membrane lipid peroxidation and in this way can trigger diabetic retinopathy.
Management.
There are four common treatments for diabetic retinopathy: anti-VEGF injections, steroid injections, panretinal laser photocoagulation, and vitrectomy. Current treatment regimens can prevent 90% of severe vision loss.
Although these treatments are very successful (in slowing or stopping further vision loss), they do not cure diabetic retinopathy. Caution should be exercised in treatment with laser surgery since it causes a loss of retinal tissue. It is often more prudent to inject triamcinolone or anti-VEGF drugs. In some patients it results in a marked increase of vision, especially if there is an edema of the macula.
In addition, standard treatment for diabetic retinopathy includes improving control of blood sugar, blood pressure, and blood cholesterol, all of which can reduce diabetic retinopathy progression.
Mild or moderate NPDR.
For those with mild to moderate non-proliferative diabetic retinopathy, the American Academy of Ophthalmology recommends only more frequent retinal exams—every six to twelve months—as these people are at an increased risk of developing proliferative retinopathy or macular edema. Injection of anti-VEGF drugs or steroids can reduce diabetic retinopathy progression in around half of eyes treated; however, whether this results in improved vision long term is not yet known. The lipid-lowering drug fenofibrate also reduces progression of disease in people with mild to moderate disease.
Diabetic macular edema.
Those at highest risk of vision loss – that is, with edema near the center of the macula – benefit most from eye injections of anti-VEGF therapies aflibercept, bevacizumab, or ranibizumab. There is no widely accepted dosing schedule, though people typically receive more frequent injections during the first year of treatment, with less frequent injections in subsequent years sufficient to maintain remission. Those whose eyes don't improve with anti-VEGF therapy may instead receive laser photocoagulation, typically in the form of short laser pulses.
Those with macular edema but no vision loss do not benefit from treatment; the American Academy of Ophthalmology recommends deferring treatment until visual acuity falls to at least 20/30. The diabetic macular edema manifestation is difficult to predict. Autoantibodies against hexokinase 1 are commonly associated with diabetic macular edema manifestation. Nearly one-third of patients with diabetic macular edema were found to be positive for anti-hexokinase 1 autoantibodies. Importantly, these autoantibodies were rare in patients with diabetic retinopathy only or diabetes mellitus only. However, these autoantibodies fail to predict disease onset. They likely manifest secondary to the tissue-damaging stimulus at diabetic macular edema onset and cannot be used to predict diabetic macular edema before its