The Retina: Vascular Diseases I

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14 The Retina: Vascular Diseases I

RETINAL VASCULAR OCCLUSIONS

The posterior ciliary circulation supplies the optic disc and choroid through the short posterior ciliary arteries and the circle of Zinn (see Ch. 17); the central retinal artery leaves the ophthalmic artery in the orbit and only supplies the capillaries on the surface of the optic disc and the retina (see Ch. 20). Retinal vascular occlusions are a common cause of visual loss especially in elderly, hypertensive, diabetic or arteriosclerotic patients. Patients usually present with sudden unilateral visual loss although this might be noticed only coincidentally some time after the initial event particularly in elderly people. All patients need a careful assessment for associated underlying systemic hypertension, diabetes or other atherogenic diseases. Subclinical cardiac and cerebrovascular disease is frequently present and patients should be investigated with these in mind as prophylactic medical or surgical treatment might be indicated to limit or prevent further systemic vascular damage.

ARTERIAL OCCLUSIONS

CENTRAL RETINAL ARTERY OCCLUSION

This is usually a disease of the elderly and is most commonly caused either by thrombosis in the retrobulbar portion of the central retinal artery or blockage of the artery by an embolus which usually originates in the heart or carotid artery. Complete occlusion may be preceded by attacks of amaurosis fugax. All patients should be screened for hypertension, diabetes, cardiac valvular and coronary artery disease and carotid atheromatous disease. Temporal arteritis usually presents as an anterior ischaemic optic neuropathy but occasionally causes a central retinal artery occlusion (CRAO) and needs to be excluded (see Ch. 17). In younger patients, the underlying aetiology is occasionally found to be an inflammatory arteritis of which polyarteritis nodosa, systemic lupus erythematosus and syphilis are perhaps the commonest causes. Spasm of the central retinal artery is an ill-defined and controversial phenomenon that occurs only in exceptional circumstances such as drug toxicity with ergot overdosage or pre-eclamptic toxaemia of pregnancy. CRAO is a rare feature of migraine.

Experimentally retinal infarction takes 1½ hours of complete arterial occlusion. Occasionally some eyes with CRAO achieve some visual recovery probably because the embolus fragments and disperses. If a patient is seen within a few hours of the initial loss of vision it is worthwhile to presume that the occlusion is embolic in nature. Working with this hypothesis attempts should be made to dislodge the embolus by massaging the globe to fluctuate intraocular pressure, giving intravenous acetazolamide or performing paracentesis of the anterior chamber to lower intraocular pressure. However, visual recovery is rare and the prognosis is generally very poor. Patients have a reduced life expectancy from vascular disease; cardiac infarction is the most common cause of death.

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Fig. 14.7 Cilioretinal infarction with central retinal artery sparing is the converse situation to that shown in Fig. 14.5. Here, a small cilioretinal branch is occluded, thus infarcting the retina temporal to the optic disc. A frill of axoplasmic material from the viable retina is seen surrounding the cilioretinal infarct. Cilioretinal infarcts have a similar aetiology to anterior ischaemic optic neuropathy, temporal arteritis, which although it usually presents with more extensive disc infarction, must be excluded as a potential cause.

RETINAL EMBOLI

Embolization of the retinal circulation usually occurs with cholesterol or fibrin and platelet emboli from the carotid arteries, or calcific fragments from a stenosed aortic valve. Occasionally more exotic material such as talcum powder in drug addicts or fat emboli in patients with multiple fractures can be seen. Emboli may produce permanent or transient visual loss; cholesterol emboli are also frequently seen coincidentally on routine examination of an asymptomatic eye. Transient uniocular visual loss is known as amaurosis fugax and is virtually always due to embolization, whether or not emboli are actually seen on examination. Patients typically notice uniocular visual loss starting as a concentric peripheral dimming of vision or a vertical curtain coming over the eye, depending on whether the central retinal artery or a branch retinal artery is affected. Most attacks last for periods of a few minutes but may persist for 2–3 hours before vision quite rapidly returns to normal. Attacks may happen singly or in groups, sometimes occurring in clusters or showers during a day. Patients with retinal embolization have an increased risk of developing a permanent stroke over the next few months and the risk is substantially increased if the amaurosis fugax is accompanied by signs of transient cerebral ischaemia or an embolus is visible in the retina. This means that the ophthalmologist plays an important role in the management of these patients by referring them for prophylactic medical or surgical treatment to forestall the development of permanent neurological sequelae. In young patients, mitral valve prolapse or cardiac septal defects can present as amaurosis fugax. Migraine is another rare cause in young adults and may produce transient visual loss by affecting either the retinal or the choroidal circulation. The features are often atypical and more common causes need to be excluded before a diagnosis is made. Choroidal migraine produces patchy visual loss, rather like pieces missing from a jigsaw puzzle, due to involvement of the choroidal lobular circulation (see Ch. 9).

VENOUS OCCLUSIONS

Retinal venous occlusions are the commonest retinopathy after diabetes and age-related macular disease. They are classified according to whether the occlusion affects the central retinal vein or its tributaries. The early physical signs are dilatation of the affected vein, haemorrhage in the territory of the retina drained by the vein, retinal oedema, varying amounts of capillary closure and retinal ischaemia with cotton-wool spots. Venous occlusions vary considerably in their severity and the visual disability depends on the part of the retina affected, the severity of the occlusion and the amount of retinal ischaemia. Mild venous occlusions cause visual disability from retinal haemorrhage or vascular leakage and macular oedema whereas more severe lesions also produce ischaemia and retinal capillary closure and carry the risk of neovascularization.

CENTRAL RETINAL VEIN OCCLUSION

Mild central retinal vein occlusions (CRVOs) can be asymptomatic but more severe occlusions present with loss of vision due to macular oedema, ischaemia or haemorrhage, which varies from minor to severe. Features of CRVO are retinal haemorrhages which characteristically extend out to the peripheral retina, dilated retinal veins, retinal oedema and variable amounts of ischaemia. The appearance of the fundus can vary from a grossly haemorrhagic fundus with optic disc oedema, to mild forms with only a few fundal haemorrhages and mild vascular changes; this range is reflected in the initial visual symptoms and subsequent clinical sequelae.

Patients may present with nonischaemic or ischaemic occlusions but about 30 per cent of patients presenting with a nonischaemic CRVO progress to develop retinal ischaemia. Patients with the ischaemic type are more likely to have poor presenting vision (worse than 20/200), a relative afferent pupillary defect, cotton-wool spots, large areas of retinal ischaemia and extensive deep haemorrhages in all four quadrants. Significant ischaemia is defined on fluorescein angiography as more than 10 disc diameters of nonperfusion. Optic disc neovascularization is rare but ischaemic CRVO can progress to neovascularization on the iris and in the angle and eventually rubeotic glaucoma, typically about 3–4 months after the acute episode. Panretinal photocoagulation can halt iris and angle neovascularization and prevent rubeotic glaucoma. The Central Vein Occlusion Study recommended waiting until new vessels developed on the iris or angle before performing panretinal photocoagulation but some retinal specialists believe that early treatment with panretinal photocoagulation in eyes with marked ischaemia is advisable, particularly when follow-up may be difficult. Predictors of neovascularization include a relative afferent papillary defect, vision worse than counting fingers and more than 30 disc diameters of nonperfusion on fluorescein angiography. Macular grid laser photocoagulation has been shown to be ineffective for macular oedema in patients with nonischaemic CRVO although some specialists believe that young patients (aged less than 55 years) may benefit from this treatment. No treatment has yet been shown to improve the ocular prognosis.

Elderly patients with central retinal vein occlusion have an increased incidence of arteriosclerosis, smoking, hypertension and diabetes. The role of associated arterial disease in the pathogenesis remains controversial but, as the central retinal artery and vein share a common fascial sheath within the optic nerve, thickening and hypertrophy of the artery can compromise the vein’s diameter leading to obstruction. Patients often notice visual loss on waking with vision having been normal the night before and it has been postulated that lower ocular perfusion due to lower blood pressure and pulse rate and increased intraocular pressure during sleep contribute to this (see Ch. 7). CRVO is also a well recognized association of raised intraocular pressure; this is particularly important to recognize as lowering the IOP in the fellow eye may prevent a subsequent occlusion in this eye.

Hyperviscosity states can produce a similar picture to central vein occlusion from venous stasis, although a hyperviscosity retinopathy is bilateral and usually less severe. Venous occlusion must also be differentiated from low arterial pressure retinopathy (ocular ischaemic syndrome or slow flow retinopathy).

Although CRVOs are most common in those aged over 60 years, younger patients may also be affected.

In younger patients an ocular or systemic vasculitis, hyperlipidaemia and clotting abnormalities need to be excluded in addition to the other risk factors such as contraceptive pill use. CRVO can occur from inflammatory phlebitis in association with sarcoidosis or Behçet’s disease. However, most younger patients are generally well and have no associated systemic disease; the probable aetiology in these patients is a congenital vascular anomaly. In these patients involvement of the fellow eye, later development of occult systemic disease or reduced life expectancy from vascular disease is unusual.

Patients with retinal vein occlusion have an increased incidence of death from cardiac or cerebral causes. Furthermore, there is a 15 per cent risk of recurrence in the same or fellow eye over 5 years. All patients require medical investigation (Table 14.1).

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