Central Retinal Artery Occlusion
Central retinal artery occlusion is a relatively rare emergent condition of the eye resulting in sudden painless vision loss. This vision loss is usually dramatic and permanent and the prognosis is poor. Patients particularly at risk include those with giant cell arteritis, atherosclerosis, and thromboembolic disease, a wide variety of treatment modalities have been tried over the last one hundred years with little to no success, with the exception of hyperbaric oxygen therapy.
Rationale for Hyperbaric Oxygen Therapy in the Management of Central Retinal Artery Occlusion
The arterial blood supply to the eye is provided by the ophthalmic artery, one of the branches of cavernous portion of the internal carotid artery. Some of the branches of the ophthalmic artery (lacrimal, supraorbital, ethmoidals, medial palpebral, frontal, dorsal nasal) supply orbital structures, while others (central artery of the retina, short and long posterior ciliaries, anterior ciliaries) supply the tissues of the globe. The central retinal artery enters the globe within the substance of the optic nerve and serves the inner layers of the retina through its many branches. The long posterior ciliary arteries provide blood to the choroid and the outer layers of the retina. There are approximately twenty short posterior ciliary arteries and usually two long posterior ciliary arteries. The posterior ciliary vessels originate from the ophthalmic artery and supply the entire uveal tract, cilio-retinal arteries, the sclera, the margin of the cornea, and the adjacent conjunctiva. The anterior ciliary arteries also arise from the ophthalmic artery, supply the extra-ocular muscles, and anastamose with the posterior ciliary vessels to form the major arterial circle of the iris, which supplies the iris and ciliary body.
The visual signs and symptoms of vascular occlusive diseases of the retina are dependent on both the particular vessel occluded, the degree of occlusion, the location of the occlusion, and the presence or absence of a cilio-retinal artery. In approximately 15%-30% of individuals, a cilio-retinal artery is present. This artery is part of the ciliary (not retinal) arterial supply but supplies the area of the retina around the macula (central vision area.) If a cilio-retinal artery is present, central vision may be preserved in central retinal artery occlusion. The outcome of these disorders also depends on the vessel occluded and the degree of occlusion, but also on the time interval until therapy is initiated and the presence of alternate sources of oxygen to the ocular tissues.
In central retinal artery occlusion, the inner retinal layers (ganglion cell layer and inner nuclear layer), which are normally served by the retinal circulation, may obtain enough oxygen via diffusion from the choroidal circulation to function normally if the individual is exposed to elevated partial pressures of oxygen. Animal models have shown the choroidal supply of oxygen to the inner layers of the retina may be sufficient to maintain ganglion cell viability even when the retinal vessels have been completely obliterated. Normally, the choroidal circulation supplies the majority of the oxygen to the retina. Under normoxic conditions, approximately 60% of the retina’s oxygen comes from the choroidal circulation. Under hyperoxic conditions, the choroid is capable of supplying 100% of the oxygen needed by the retina.
In considering the effect of treating central retinal artery occlusion with supplemental oxygen, four key factors determine success:
Therapy must be initiated before the retinal tissue is irreparably damaged;
The degree of occlusion of the blocked vessel may vary - this may account for why some patients respond to oxygen at lower partial pressures than others;
Some patients may not respond to oxygen therapy, even if it is initiated promptly, if the level of occlusion is at the ophthalmic artery because in this event, the blood supply to the posterior ciliary vessels is blocked as well and there is no alternate choroidal blood supply to provide oxygenation of the inner layers of the retina; and
An adequate partial pressure of oxygen must be maintained to keep the retina viable until circulation is restored.
The aetiology of the arterial occlusion (thrombosis, embolus, arteritis, vasospasm) has also been described as affecting outcome. Careful classification of the factors involved in an individual case of central retinal artery occlusion is crucial to understanding the natural outcome and results of therapy. In the largest published series of central retinal artery occlusion patients, Hayreh describes the natural progression of this condition without hyperbaric oxygen therapy. He found that patients with transient central retinal artery occlusion (resolution of symptoms in minutes to hours) and those with cilio-retinal arteries had much better outcomes than those who did not. In those patients without cilio-retinal arteries, 80% had a final outcome of counting fingers or less and only 1.5% of them obtained a final vision of 20/40 or better.
Recanalisation occurs in retinal vessels after central retinal artery occlusion. In relatively few cases, however, does this angiographic reperfusion lead to an improvement of vision. The retina has the highest rate of oxygen consumption of any organ in the body at 13ml/100g/min. Therefore, it is very sensitive to ischaemia. In order to be effective, the administration of supplemental oxygen must be continued until such time as flow through the retinal artery has resumed to a level sufficient to maintain inner retinal viability under normoxic conditions.