Although
diabetes may affect multiple parts of the eye, the most common area
affected is the retina. Diabetic
retinopathy is divided into three classifications--background retinopathy,
pre-proliferative retinopathy, and proliferative retinopathy.
Background diabetic retinopathy (#22034, #9365) is characterised by
retinal capillary microaneurysms, some venous abnormalities, hemorrhages,
exudates, and edema.
1.
Capillary microangiopathy--the initial lesion of diabetic
retinopathy. --Microvascular
obstructions and permeability changes; non-profusion of capillaries.
The earliest changes occur in the capillary beds, then in larger
pre-capillary arterioles (leading to cotton-wool spots) and are caused by
deposition of PAS positive plasma derivitives onto defective endothelium.
--Retinocapillary microaneurysms(#22036)These develop adjacent to
areas of capillary non-profusion. --Basement
membrane thickening. This
also contributes to gradual closure of small arterioles. --Loss of pericytes. Ratio
of pericytes to endothelial cells is normally 1:1 (even greater with age).
This is reversed in diabetics.
Loss of pericytes creates a weaker vessel wall partially explaining
aneurysm formation.
2.
Intraretinal hemorrhages --Flame-shaped--blood deposited
superficially between fibers of nerve fiber layer.
--Dot and blot--focal deposits in deeper the inner nuclear and
outer plexiform layers.
3.
Exudates (#22038)
Hard,
yellow, waxy protein and lipid from serum exudate or from degenerating
neural elements. These are
deposited in the outer plexiform layer seen as an eosinophilic material on
histologic sections(#22039). They
can sometimes form a circinate pattern around the macula.
4.
Macular changes--intraretinal --Due to vascular
permeability--Macular edema (which can progress to macular retinoschisis
and hole formation) and hard exudates. --Due to retinal vascular occlusion--Cotton wool spots and
fluorescein angiography showing focal capillary dropout or enlargement of
foveal avascular zone with subsequent ischemia.
