الفهرس 
Anatomy of the retinaOnly 14 pages are availabe for public view
 |  | from | 133 |  |  |
| | | from | 133 | | |
Abstract
Diabetic macular edema is the most common cause of moderate visual loss in patients with diabetes. Macular edema may be the first symptom of diabetic retinopathy and may be associated with proliferative or non-proliferative retinopathy. This problem presents a significant form of impairment for working patients with diabetes since it may affect their ability to read and to drive and to maintain a productive career.
Macular edema results from the breakdown of the blood-retinal barrier in the retinal capillaries. The tight endothelial cell junctions break down resulting in increased vascular permeability and increased fluid accumulation in the outer layers of the retina. Microaneurysms are believed to play a significant role by acting as sources for fluid and lipid transudation. Factors that are believed to cause the formation of microaneurysms are loss of pericytes and supporting astrocytes in the retina, increased capillary transmural pressure, and local production of vasoproliferative factors such as vascular endothelia growth factor (VEGF). Hyperglycemia is believed to be the main factor that causes increased oxidative stress, the accumulation of advanced glycation endproducts, and generation of diacylglycerol. The substance activates protein kinase C which in turn increases VEGF expression.
Clinically the findings in patients with diabetic macular edema are micraneurysms, dot and blot hemorrhages, and lipid (hard) exudates. These result in areas of retinal thickening around the macula and cystic changes in the macula. In advanced stages, there may be atrophy of the pigment epithelium or fibrous changes within the central foveal area. Sometimes lipid deposits surround a group of actively leaking microaneurysms in a circinate pattern (circinate rings). When exudates occupy the foveal area it is believed that permanent visual loss will occur even if the exudates reabsorb after treatment. In other cases, diffuse macular edema may also occur. In this condition there is diffuse leakage from capillaries surrounding the macula, without identifying specific focal areas of leakage.This type of edema is more suitable to treatment by laser photocoagulation.
Fluorescein angiography is used in the diagnosis and treatment of diabetic macular edema. It is used to guide laser therapy when indicated. A new diagnostic modality most useful in the diagnosis and treatment of macular edema is optical coherence tomography (OCT). Images of the retina are obtained that appear similar to a histologic section of the macula. In diabetic macular edema, the macular thickness can be several times as normal and the cystic spaces of fluid accumulation are easily demonstrated. This tool is gradually replacing most methods of assessing the treatment response to any treatment modality for diabetic macular edema.
The classic form of treatment for diabetic macular edema is laser photocoagulation. The ETDRS showed that focal/grid photocoagulation reduced the rate of visual acuity loss in patients with CSME.Also, the degree of visual gain following laser is moderate, and may take months to occur.These observations have led to the search for other new approaches for the treatment of diabetic macular edema, both primary cases and those not responding to laser photocoagulation.
In some cases it is believed that vitreous traction plays a role in the development of DME. A thin epiretinal membrane forms on the retinal surface resulting in tangential traction. Surgical removal of the membrane has resulted in reduction of edema and a modest improvement of visual acuity.
More promising is the pharmacologic treatment of diabetic retinopathy. Currently clinical trials are in progress to study the efficacy and safety of PKC inhibitors and somatostatin analoges administered systemically. Early reports appear promising.
In addition, recent trials are starting for local treatment of diabetic macular edema using intravitreal injection of triamcinolone. These new treatment modalities offer potential for medical treatment and possibly better visual outcomes.
Furthermore, some studies showed that the vascular endothelial growth factor (VEGF) is the major angiogenic stimulus responsible for increase of vasopermeability, cell proliferation and angiogenesis in diabetic retinopathy (DR). So several types of anti VEGF drugs adminsterered by intravitreal injections were found to be effective in reducing macular edema secondary to diabetic retinopathy such as Bevacizumab, Ranibizumab, Pegaptanib sodium.
Fluocinolone acetonide intravitreal implant although resulted in reduced DME and a trend toward better acuity, there were significant side effects which have limited its widespread use. These included requiring a filtering procedure and needing to be explanted to manage intraocular pressure