MICROANEURYSM LEAKAGE
Microaneurysms can leak blood (haemorrhage) and plasma.
Dot and Blot Haemorrhages
Capillary wall breakdown results in the leakage of red blood cells and various forms of haemorrhage depending on the nature and site of the vessel involved. Dot haemorrhages occur following rupture of capillaries in the outer plexiform layer. More extensive haemorrhaging from capillaries leads to extravasation of blood throughout the full thickness of the retina (blot haemorrhage).
The anatomy of the retina determines the appearance of the blood. If the blood is in the bipolar layer it will be vertical in shape. Looking from above it will be appear to be a dot. If it is in the nerve fibre layer it will appear to be horizontal in shape. It will appear to be like a flame because of the way the nerve fibres track towards the optic nerve and covers the retinal blood vessels, which are contained within the nerve fibre layer.
Hard Exudate Formation
Leakage of plasma leads to thickening of the retina. This can occur anywhere in the retina; however, at the macula it is termed macular oedema. At the edge of the leakage a “tide-mark” may occur. This consists of precipitated fat (lipid) and is called exudate. It is sometimes called hard exudate to differentiate it from soft exudates; a term no longer used for cotton wool spots (see later). The lipid lies within macrophages, which are cells that act like the body’s refuse collectors. The exudate is semi-solid, giving it a “hard” appearance.
The fluid leaks out from the microaneurysm in a radial or circular fashion. The tide-mark will therefore often appear to be circular in shape and this pattern of exudate formation is termed circinate. In real life these rings are often incomplete.
INTRA-RETINAL MICROVASCULAR ABNORMALITIES
Intra-retinal microvascular anomalies (IRMA) are believed to be either flat new vessels growing within the layers of the retina rather than between the retina and the vitreous or the dilated remnants of capillaries within a large area of capillary occlusion. Fluorescein angiography would tend to favour the latter theory. Also new vessels tend to grow at the border between healthy perfused retina and areas of capillary occlusion, whereas IRMA tend to be found within areas of capillary occlusion.
Although arterioles and venules may be seen on photography, capillaries are not visible except with fluorescein angiography
Where the capillary network becomes occluded only remnants of perfused capillaries remain. These become dilated (and visible) and tortuous in shape. Their presence may indicate the presence of a significant area of capillary closure and consequent retinal ischaemia. IRMA are also often seen abutting cotton-wool spots. Their significance in this setting is less certain.
VENOUS CHANGES
Venous abnormalities include venous beading (VB), reduplication (VR) and the formation of omega loops.
Venous beading occurs where there is widespread capillary ischaemia adjacent to a vein and may represent foci of endothelial cell proliferation that have failed to develop into new vessels. Venous beading and IRMA are often found adjacent to one another.