Diabetes – Introduction To Retinopathy

Description

Diabetic retinopathy (DR) is a microvascular complication of diabetes mellitus which can result in severe loss of vision.

• Type 1, or insulin-dependent diabetes mellitus (IDDM), is due to insulin deficiency.
• Type 2, or non-insulin-dependent diabetes mellitus (NIDDM), is due to insulin resistance. These patients may still require insulin treatment, if diet and other medical treatments fail.

The elevated blood glucose levels (hyperglycaemia) in diabetic patients adversely affect retinal capillaries. The consequences include capillary leakage and occlusion, retinal and macular edema and the formation of abnormal new vessels and fibrous tissue (proliferative diabetic retinopathy, PDR). Several theories have been proposed to account for the development of DR. For example:

• Aldose reductase is an intracellular enzyme that converts sugars into alcohols. Since these alcohols diffuse across cell membranes less readily than sugars, they create intracellular osmotic pressure, resulting in cellular swelling and damage. Aldose reductase is abundant within the retina and lens, and is increasingly active during hyperglycaemia.
• The retinal capillaries in diabetic patients contain a reduced number of pericytes, which normally support the capillary wall and maintain the blood-retinal barrier. This pericyte loss may allow capillary distension and the formation of microaneurysms, which are permeable to water and large molecules.
• Capillary occlusion may result from a combination of vascular changes observed in diabetic patients, including capillary wall thickening and damage, platelet dysfunction and red blood cell deformities. This occlusion results in hypoxic retinal tissue, which may release ‘vasoproliferative factors’, such as vascular endothelial growth factor, leading to new vessel formation (neovascularisation) and fibrosis.

Symptoms

Early DR is usually asymptomatic. Macular edema may result in perceived distortion of shapes (metamorphopsia) and reduced central vision. Retinal ischaemia, macular oedema, vitreous haemorrhage or retinal detachment may cause severe loss of vision.

Signs

Pupillary dilatation is essential for examination. Although direct ophthalmoscopy can reveal many of the signs of DR, slit-lamp examination with an accessory lens enables three-dimensional appreciation of retinal thickening, neovascularisation and retinal detachment.

• Microaneurysms appear as small, round, red dots.
• Intraretinal haemorrhages result from rupture of retinal capillaries or microaneurysms. Dot or blot haemorrhages occur deep within the retina; flame-shaped haemorrhages are more superficial.
• Cotton wool spots are white, fluffy lesions within the retinal nerve fibre layer. They signify axoplasmic stasis, which may be secondary to retinal ischaemia.
• Hard exudates have a well-defined, yellow, waxy appearance. Their presence indicates previous or current retinal oedema. The exudates form as pigment epithelial cells preferentially remove water from the oedematous fluid, leaving residues of supersaturated lipoproteins and lipids.
• Macular edema is visible (with slit-lamp biomicroscopy) as retinal thickening at the macula. Occasionally, visible pockets of fluid are visible (cystoid macular oedema).
• Venous beading and looping usually signify retinal ischaemia.
• Intraretinal microvascular abnormalities (IRMAs) are dilated capillaries that seem to function as arteriovenous shunts through ischaemic retina. They may be difficult to distinguish from areas of flat retinal neovascularisation.
• Neovascularisation is visible as numerous, fine, tortuous vessels which may extend towards the vitreous, to which they are attached. Fibrous tissue strands may also exist.
• Preretinal (subhyaloid) haemorrhage is often oval or crescent-shaped, while vitreous haemorrhage may obscure the fundus. (These are both complications of PDR.)
• Other signs – tractional and rhegmatogenous retinal detachments, iris neovascularisation, cataracts and optic neuropathy.

Numerous scales exist to grade DR. The International Clinical Diabetic Retinopathy Disease Severity Scale was designed in 2002 to facilitate communication between health care professionals, and allows grading with a direct ophthalmoscope (see following chapters).

Incidence, Significance, Differentlal Diagnosis, Management– see next condition