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.

Fig. 14.1 The fundus of a recent CRAO shows a cloudy, opaque and swollen infarcted retina particularly noticeable in the posterior pole where the retina is thickest. The fovea is the thinnest area of the posterior retina and there is little tissue here to infarct so that the underlying redness of the choroid is seen contrasting with the whiteness of the surrounding infarcted retina to give the classical ‘cherry red spot’ appearance. Within a few hours mild swelling of the optic disc often appears due to products of retrograde axoplasmic transport accumulating in the optic nerve head. In the acute stage blood in the retinal arteries is dark from stagnation and the arteries themselves are often attenuated. These acute appearances subside over a period of 2–4 weeks, usually leaving the eye with perception of light. Complete blindness should suggest more extensive vascular occlusion involving the choroidal or optic disc circulation or ophthalmic artery.

Fig. 14.2 Fluorescein angiography is not usually performed but if it is done in the acute stages there is a considerable delay in filling of the central retinal artery circulation following injection of dye; this is seen as black retinal arteries against the normal background choroidal fluorescence which, of course, is unaffected by the occlusion. The artery fills eventually either directly or by retrograde flow from the venous circulation. The central vessels in the posterior pole fill poorly, possibly due to the retinal oedema increasing external tissue pressure.

Fig. 14.3 Late changes following CRAO in the right eye. The optic disc is pale and the branches of the central retinal artery are attenuated in contrast to those of the normal left eye. Note the irregular whitish-yellowish deposition in the arterial walls which is probably a mixture of gliosis and lipid deposition from vascular endothelial damage.

Fig. 14.4 Histology shows atrophy of the inner two-thirds of the retina, (supplied by the retinal circulation), following CRAO. The outer retina is maintained by the choroid (see Ch. 13).

Fig. 14.5 Cilioretinal arteries of varying size are found in about 20 per cent of eyes. These are part of the posterior ciliary circulation and are therefore spared in a CRAO. The patch of retina supplied by the artery is left viable; if the patient is fortunate this will include the macula. A small cilioretinal vessel is seen clearly in this patient supplying an area of retina temporal to the optic disc. Minute cholesterol emboli can be seen in the inferior and superior temporal arteries and also in a small branch artery adjacent to the macula, indicating the aetiology of the occlusion. Fluorescein angiography demonstrates the slow rate of filling of the retinal circulation and the normal filling of the cilioretinal artery and choroid.

Fig. 14.6 This example of cilioretinal artery sparing in the presence of a CRAO in a pigmented eye demonstrates clumping of the blood column in the inferior temporal branch artery. This characteristic sign of a stagnant retinal circulation is known as ‘cattle trucking’.

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.