The anterior segment of the eye is supplied by the anterior ciliary arteries (branches from various recti) and the long posterior ciliary arteries (see Chap. 6). Apart from the factors discussed in Chap. 20, which can produce ischemia of the anterior segment, it is a universal finding in OIS. In the anterior segment, OIS produces the following circulatory changes.

Fluorescein iris angiography shows poor filling of the iris (Figs. 21.2, 21.3, and 21.4). Iris neovascularization is usually the earliest manifestation of OIS (Figs. 21.2, 21.3, 21.4, and 21.5); that, with the development of neovascularization of the angle of the anterior chamber and peripheral anterior synechiae, results in neovascular glaucoma and a rise of intraocular pressure. The latter can play an important role in the development of various other manifestations of OIS discussed below and shown in Fig. 21.1. However, if there is ciliary body ischemia, that can result in ciliary body atrophy [43] and consequently decreased aqueous humor production and hypotony. So it is not uncommon to find no notable rise of intraocular pressure because of the associated chronic ischemia of the ciliary body. Bietti and Neuschuler [44], in 13 patients with internal carotid artery occlusion or severe stenosis, found 3–8 mmHg lower intraocular pressure (IOP) on the involved side than on the other side with a normal internal carotid artery. Some of these patients may be seen for the first time weeks or months after the development of neovascular glaucoma, and the intraocular pressure may be within normal limits or even lower, because of the ciliary body ischemic degeneration after a prolonged initial period of high intraocular pressure, thus misleading one into believing that the eye did not have glaucoma. Other findings of anterior segment ischemia seen in OIS are discussed below.

Internal carotid artery occlusion or severe stenosis, in the absence of adequate collateral circulation, must obviously produce an extreme fall in the blood pressure of the ophthalmic artery, central retinal artery and posterior ciliary arteries. Ophthalmic artery stenosis or occlusion alone can produce the same hemodynamic effects as stenosis or occlusion of the internal carotid artery. The ophthalmic artery stenosis or occlusion alone has particular importance because in these cases of OIS Doppler evaluation of the carotid arteries may show no stenosis.

The following formula is used to calculate the ocular blood flow:

Perfusion pressure = mean arterial blood pressure in the retinal/choroidal vessels minus intraocular pressure.

Thus, in OIS the blood flow in the ocular arteries depends upon three factors: (a) resistance to blood flow, (b) blood pressure, and (c) intraocular pressure.

In OIS, even if the intraocular pressure is “normal” or only moderately elevated, in the presence of a very low mean blood pressure in the central retinal artery and posterior ciliary arteries due to occlusion or marked stenosis of the internal carotid artery or ophthalmic artery, that is more than sufficient to lower the perfusion pressure far enough to impair the retinal, choroidal, and optic nerve head blood flow. Moreover, most likely there are atherosclerotic changes in the ophthalmic and ocular arteries, which would increase vascular resistance. If the intraocular pressure is rising gradually with the onset of neovascular glaucoma, the blood flow in the central retinal artery and posterior ciliary artery may be accordingly compromised gradually, producing gradually progressive visual loss or recurrent episodes of amaurosis fugax rather than sudden blindness. This can be caused even by squeezing of the eyelids on blinking or stooping down, with the consequent rise of intraocular pressure, or by orthostatic hypotension. That may be enough to interfere momentarily with the circulation in the retina, optic nerve head, and choroid; this has been reported in the literature and was also seen in some of my patients with OIS [45]. A sudden and prolonged fall of systemic blood pressure for any reason, e.g., during sleep, can produce a sudden stoppage of the circulation in the central retinal artery and development of central retinal artery occlusion (see Chap. 13). A similar fall in the perfusion pressure in the posterior ciliary arteries would produce choroidal and optic disc ischemia; this was clearly demonstrated on fluorescein angiography. My case report below is a good example of that. Raitta [46] also reported a similar case. Ophthalmoscopically, choroidal infarcts and optic disc ischemia (optic disc cupping or anterior ischemic optic neuropathy) have been seen by me [45] and other workers (see below). Ruckman and Haas [47] and Russell [48] attributed the ocular ischemia in internal carotid artery occlusion to the “steal” phenomenon produced by a retrograde flow of blood in the ophthalmic artery from the external carotid artery to the internal carotid artery, as has also been reported by others [49, 50].

Thus, a patient with complete occlusion or severe stenosis of the internal carotid artery should be considered a potential candidate for anterior segment neovascularization, neovascular glaucoma, central retinal artery occlusion, and anterior ischemic optic neuropathy (Fig. 21.1); the reverse must also be borne in mind. These ocular manifestations may be the earliest clinical signs of a serious carotid artery disease, requiring urgent attention to prevent cerebral ischemic lesions, myocardial ischemia, or other disabling complications.

A 61-year-old man was seen in my clinic complaining of intermittent blurring of vision in his right eye for 6 months. At his first visit, his visual acuity was 6/300 in that eye, with relative afferent pupillary defect. Examination of the anterior segment revealed extensive neovascularization of the iris and complete peripheral anterior synechiae of the angle of the anterior chamber and intraocular pressure of 26 mmHg (intraocular pressure in the fellow normal eye was 19 mmHg). Examination of the fundus revealed slightly engorged retinal veins (Fig. 21.7a) and a couple of very tiny retinal hemorrhages in the temporal periphery, with no other abnormality. Fluorescein fundus angiographic findings are shown in Figs. 21.7b–f. They revealed extremely slow filling of the retinal, choroidal, and optic disc circulation, with no retinal microvascular abnormality, except for an occasional microaneurysm. The central retinal artery at the disc did not start to fill till 22 s (Fig. 21.7b) (normally 10–12 s) after the injection of the dye and the retinal arteriole did not fill completely, even 60 s later (Fig. 21.7e) (normally within I second). Nasal choroid (i.e., in the distribution of the medial posterior ciliary artery) did not start to fill until 26 s after the injection of the dye (Fig. 21.7c) – normally it should start filling just before or simultaneously with the start of central retinal artery filling on the disc. The nasal choroid filled completely in about 20 s (normally less than a second), while the temporal choroid (i.e., in the distribution of the lateral posterior ciliary artery) showed no filling till about 43 s after the injection of the dye (normally it should fill synchronously with medial posterior ciliary artery). The optic disc started to fill with the filling of the medial posterior ciliary artery (Fig. 21.7c) which supplied the entire disc. The retinal vein had not filled even 52 s after the injection of the dye (Fig. 21.7e) (normal retinal arteriovenous circulation time is only 2–3 s). Thus, there was a gross delay in the entire circulation of the posterior segment. By the late phase (i.e., about 15 min after the injection of the dye – Fig. 21.7f), the retinal and choroidal vascular bed had filled completely, and the main retinal vessels leaked fluorescein and showed staining. Carotid arteriography revealed a total occlusion of the right internal carotid artery.

On follow-up, 3 months after the initial visit, the right eye had a visual acuity of bare hand motion, intraocular pressure of 25 mmHg, cupped optic disc, and pulsating central retinal artery on the disc. Three months later, the visual acuity in that eye was no light perception, intraocular pressure 19 mmHg, almost complete obliteration of the retinal vessels and evidence of old peripheral choroidal infarct over the 360^° of the fundus (fundus findings identical to those are seen in Figs. 21.8, 21.9, and 21.10).

There can be no disagreement that such fundus changes are due to OIS secondary to carotid artery occlusive disease. These eyes not only have sluggish circulation in the retinal vascular bed but also have as poor or poorer circulation in the choroid, optic disc, and iris. I have discussed this subject in detail elsewhere [45, 50].

A 70-year-old man, at his initial visit to my clinic, gave a history that 6 days previously, he woke up in the morning with blurred vision in the left eye and recurrent attacks of pain in and around the eye. Previously, he had had recurrent attacks of amaurosis fugax. On examination, the visual acuity was counting fingers in the left eye and 20/25 in the right eye. The anterior segment in the left eye showed neovascularization of the iris and angle all over (Fig. 21.2), with open angle, and in the right eye, there was a small patch of iris neovascularization without any angle neovascularization. The IOP was 34 mmHg in the left eye and 17 mmHg in the right eye. The left fundus showed evidence of cilioretinal artery occlusion, with the retinal infarct involving the macular region and pulsating and narrow retinal arteries, with a cup-disc ratio of 0.9. The right fundus was within normal limits except for a cup-disc ratio of 0.8.

A 59-year-old man was first seen in an emergency room with the following history. About 2 weeks previously, he had started to lose vision in his right eye, with right-sided headaches. The visual loss was not sudden but gradual, so that he lost vision completely over 3–4 days. On examination, visual acuity was no light perception in the right and 20/20 in the left eye. The anterior segment of the right eye showed the presence of severe neovascularization of the iris with a fibrovascular sheet in the angle and total peripheral anterior synechiae. The anterior segment of the left eye was normal. The IOP was 45 and 10 mmHg in the right and left eye, respectively. The right fundus showed an almost empty vascular bed with retinal opacity and a pale disc, with no other abnormality. The left fundus was within normal limits.