Postoperative Ischemic Optic Neuropathy

Although uncommon, alteration in vision or blindness after anesthesia, particularly after open heart surgery, is well documented in the nonanesthesiology literature [1-17]Table 1. In some series, the incidence of postoperative visual loss varies between 0.1% and 1% [1,10,14]. Recent retrospective reviews demonstrate one case in 56,000 surgical procedures at one university hospital [18] and six cases over a 10-yr period at another [3]. The recent rate of reporting may be low due to anesthesiologists' concerns regarding litigation, and ophthalmologists' opinion that the occurrence of visual defect after general anesthesia is well established and unremarkable.Table 1: Case Reports of Postoperative BlindnessAnesthesia texts limit their discussion of postoperative visual loss to pressure-related eye injuries associated with positioning [19,20]. However, in recent years, the most commonly reported cause of postoperative visual loss is, in fact, ischemic optic neuropathy. Apart from two case reports [3,17], this devastating complication has not been addressed in the anesthesiology literature. Since the anesthesiologist must maintain the patients' safety during surgery, a better understanding of ischemic optic neuropathy is warranted. This paper focuses on the neurovascular-ophthalmologic mechanisms responsible for postoperative loss of vision. We will review neurovascular-ophthalmic anatomy and the etiology of postoperative ischemic optic neuropathy. Although the visual prognosis of ischemic optic neuropathy is usually grim, we will discuss possible treatment modalities and preventative measures, which in some cases are straightforward, such as ensuring physical protection of the orbit. Others involve more controversial issues, specifically the safe level of hematocrit and arterial blood pressure for the maintenance of perfusion to the visual apparatus [3]. Anatomy Blood Supply to the Optic Nerve The optic nerve Figure 1 may be divided into four parts: anterior (or intraocular), posterior (or intraorbital), intracanalicular, and intracranial. Each part has a unique blood supply which may account for differences in the means by which ischemic damage occurs.Figure 1: Diagram of the visual pathway at the base of the brain. The sites of the more common causes of postoperative visual loss are shown on the left side. AION = anterior ischemic optic neuropathy; PION = posterior ischemic optic neuropathy.The anterior portion of the optic nerve, which includes the optic disk and the portion of the optic nerve within the scleral canal Figure 1 and Figure 2, is supplied mainly by the short posterior ciliary arteries via the choriocapillaris around the optic disk or via branches to form an anastomotic microvascular ring around the optic nerve [21]. There may also be a double arterial supply from pial branches of the ophthalmic artery [21-23], but the central retinal artery does not furnish branches to the optic nerve in this region.Figure 2: Diagram of the anterior optic nerve showing the arterial and small vessel supply to the choroid and optic nerve as it passes through the lamina cribrosa. Short post a = short posterior ciliary artery; a = artery; v = vein; n = nerve.The posterior portion Figure 3 of the optic nerve has a peripheral vascular supply from pial vessels derived from neighboring branches of the ophthalmic artery. The central retinal artery often supplies branches to the central fibers but the blood flow in the posterior optic nerve is significantly less than that in the anterior portion [24].Figure 3: Diagram of the arterial blood supply to the visual pathway. a = artery.The intracanalicular and intracranial portions of the optic nerve Figure 1 and Figure 3 are supplied by the pial network from branches of the internal carotid and anterior cerebral arteries whereas the optic chiasm is supplied by the internal carotid or anterior cerebral artery [25]. The retrogeniculate optic radiation and occipital cortex Figure 1 are supplied by the posterior and middle cerebral arteries [8]. Blood Supply to the Retina There are two sources of blood supply to the retina: terminal branches of the central retinal artery to the inner layers and the choriocapillaris to the outer layers Figure 2. However, both circulations must be intact to maintain retinal activity [26,27]. The central retinal artery enters the optic nerve Figure 3 and divides at the optic disk to form arteriolar branches which form capillary networks in two inner retinal layers except over the fovea which is supplied solely by the choriocapillaris Figure 4. The choriocapillaris is the inner capillary layer of the choroid. Its lobular structure has a feeding arteriole in the center and draining venules at the periphery with each short posterior ciliary artery supplying a distinct area [28] as an end-artery [29], and little cross-circulation between adjacent areas, so that watershed zones may form at their boundaries where blood supply can be tenuous. The watershed zones between short posterior ciliary arteries, which may be situated anywhere between the fovea and the nasal border of the optic disk, play an important role in determining the site and extent of optic nerve head ischemia. The short posterior ciliary arteries and central retinal artery are end-arteries but proximal to these branches; the ophthalmic artery has anastomotic connections with branches of the external carotid arteries [30] although these may be insufficient to protect the optic nerve from ischemia [31]. Although there is both alpha-adrenergic and cholinergic innervation of the choroid, the role of autonomic nerves in control of blood flow is uncertain [27].Figure 4: Diagram of the fovea showing the layers of the sclera, choroid, and retina. The arterioles and capillaries from the retinal artery are absent at the fovea which relies solely on the choroid for its blood supply.Posterior Scleral Foramen The optic disk overlies the posterior scleral foramen which is bridged by the lamina cribrosa, a perforated, sieve-like structure composed of collagen and elastic fibers, through which the optic nerve and central retinal vein and artery pass [26]Figure 2. Small posterior foramina, which may constrict the structures passing through them, can be identified ophthalmoscopically as flattening of the normal cup shape of optic disks [32,33]. Postoperative Loss of Vision Lesions resulting from an ischemic event in the visual pathway and causing postoperative visual loss are associated with various causes of decreased oxygen delivery Table 2 and can be classified by the main locus of injury Figure 3. Visual losses due to ischemic insult of the optic nerve are separated into anterior ischemic optic neuropathy and posterior ischemic optic neuropathy because these parts of the optic nerve have different blood supplies, different predisposing factors for infarction, and variably different clinical pictures [6,34]. Visual loss associated with the optic radiation and occipital cortex is considered cortical blindness. Rarer etiologies of postoperative visual loss, such as central retinal artery occlusion and obstruction of venous drainage from the eye, will also be discussed.Table 2: Causes of Decreased Oxygen Delivery to the Visual PathwayAnterior Ischemic Optic Neuropathy In the elderly nonsurgical population, anterior ischemic optic neuropathy and optic neuritis are equally prevalent disorders of the optic nerve second only to glaucoma [35]. The visual loss of anterior ischemic optic neuropathy is caused by infarction at watershed zones between the areas of distribution of the small branches of the short posterior ciliary arteries in the choriocapillaris [24,36]Figure 2. Individual variation in blood supply to the optic nerve can affect the pattern and severity of visual loss of anterior ischemic optic neuropathy. When considering the effects of ischemia on the optic nerve, it should be stressed that damage does not follow an "all or none" law: differing degrees of ischemia produce differing effects [37,38]. Mild ischemia in the anterior optic nerve produces stasis of axoplasmic flow without alteration in visual impulse transmission. Local accumulation of axoplasm, at the site of damage where active axoplasmic transport is interrupted, causes the axons to swell [39]. Asymptomatic optic disk swelling may be the earliest sign of anterior ischemic optic neuropathy [11,37] and may resolve spontaneously without developing into overt ischemic infarction in some juvenile diabetics [40]. Moderate ischemia interferes with both axoplasmic flow and visual impulse transmission resulting in visual loss which is initially reversible, while severe ischemia results in immediate irreversible damage. Although there is considerable variability, the most common visual field defect in anterior ischemic optic neuropathy is loss of function in the lower hemifield (altitudinal hemianopia) with sparing of the central vision [36,41,42]. This pattern is reflected in the ischemic damage found in postmortem studies of optic nerves. Histologic cross-sections demonstrate complete loss of fibers in the superior half of the nerve, partial peripheral loss in the lower half, while the densely packed, small diameter axons, arising from the center of the visual field are often spared [36]. Anterior ischemic optic neuropathy is classified, depending on its etiology, into two groups: "nonarteritic," the main culprit in the postoperative period, and "arteritic," which is rarer and inflammatory in origin. Nonarteritic. This is the most common form of anterior ischemic optic neuropathy and is usually the result of transient decrease in oxygen delivery in the choroid surrounding the optic disk [43]. The proposed etiology is multifactorial: decreased perfusion pressure, increased resistance to blood flow, and decreased oxygen-carrying capacity can each result in decreased oxygen delivery. Perfusion pressure to the optic nerve head is the difference between arterial pressure and venous pressure or intraocular pressure if that exceeds venous pressure. To maintain the perfusion pressure above a critical level to prevent ischemia, both systemic and local factors are involved. Anterior ischemic optic neuropathy has been reported after various causes of systemic hypotension including cardiac arrest [44], hemodialysis [45], and even sleep in susceptible patients [37]. It has been reported bilaterally after intraoperative hypotension [3] and after acute severe hemorrhage, usually involving the gastrointestinal tract [11,40,46,47]. It would seem that intraoperative hypotension is only rarely associated with anterior ischemic optic neuropathy as indicated by the American Society of Anesthesiologists' Closed Claims Project which, out of a database of 3000 claims, has only one alleged anterior ischemic optic neuropathy (personal communication, F. W. Cheney, MD, 1994). A combined epidural/general anesthetic technique was used for radical prostatectomy, during which the lowest blood pressure during surgery was 70/40 associated with an episode of acute hemorrhage. However, in other reports of higher incidence of anterior ischemic optic neuropathy after cardiac surgery, hypotension was considered [1,10,14]. Small areas of anterior optic nerve infarction after of hypotension may be because the associated visual patients may at a with glaucoma and areas of optic carotid artery with resulting decreased perfusion pressure in the ophthalmic artery and posterior ciliary arteries, has been associated with anterior ischemic optic neuropathy flow from venous pressure is as a in postoperative anterior ischemic optic neuropathy after head and surgery due to local obstruction of venous in central venous pressure with in result in decreased perfusion pressure as well as increased blood resulting in increased intraocular pressure resistance to flow in the short posterior ciliary arteries may be increased due to arterial Anterior ischemic optic neuropathy is more common in and has been reported in patients with and vascular vascular and from and The role of is In case such as and have been in the of anterior ischemic optic neuropathy after visual loss has been reported after which, was and in of the ophthalmic and short posterior ciliary arteries However, visual loss from anterior ischemic optic neuropathy has been by of to perfusion pressure with Although some have that anterior ischemic optic neuropathy with increased intraocular pressure, which causes a decrease in blood flow the anterior optic nerve to ischemia. pressure is on from sleep a which may to the incidence of anterior ischemic optic neuropathy at that intraocular pressure are to in blood which is to in both central venous pressure and with anterior ischemic optic neuropathy in intraocular pressure with in which may a critical decrease of perfusion pressure at the optic nerve head with degrees has been to intraocular pressure for procedures in the optic and may be for the maintenance of blood flow to the optic nerve head during other surgical there is arterial pressure Anterior ischemic optic neuropathy can with both and glaucoma after and after in patients with increased intraocular pressure A case of intraoperative external due to eye which in postoperative anterior ischemic optic neuropathy was associated with intraocular pressure cardiac surgery, is with there is an in intraocular pressure which may to the of postoperative anterior ischemic optic neuropathy However, these intraocular pressure during are controversial some studies not an in intraocular pressure with Anterior ischemic optic neuropathy more often in small optic disks The small area of the posterior scleral foramen has little for of nerve fibers in to of the optic nerve A may with increased pressure from fibers the blood ischemia, and the swelling of the nerve can blood Anterior ischemic optic neuropathy has been reported in patients with increased blood to blood such as or Anterior ischemic optic neuropathy after has been in to by the of This is controversial studies of have shown that in not flow or studies demonstrate that anterior ischemic optic neuropathy is not usually caused by It does not more in patients with carotid in the central retinal artery than the short posterior ciliary However, reports of anterior ischemic optic neuropathy have been associated with from a from carotid after of the or in with cardiac anterior ischemic optic neuropathy has been reported with retinal artery and with after artery surgery Anterior ischemic optic neuropathy has been associated with due to or with or without hypotension to a hematocrit of in in an in retinal blood flow but a decrease in blood flow This may a decrease in oxygen delivery to the outer layers of the and the anterior optic nerve, areas more to ischemia anterior ischemic neuropathy are most commonly over the with a incidence of the peripheral such as or However, anterior ischemic optic neuropathy also rarely in The of anterior ischemic optic neuropathy is visual in severity from a decrease in visual to [43]. However, it can in small an that glaucoma When associated with acute hemorrhage, the may or be for to [40]. There is also a form of anterior ischemic optic by of visual function over The is often on in the and the may be only of the visual defect there is loss of the visual may but rarely central visual loss In to severe a defect is initially optic disk which may or part of the disk and is by at the disk The of visual loss may not to the severity of disk during the in the disk, the surrounding choroid, and watershed zones is but the of visual is decreased in patients with anterior ischemic optic ischemic damage to the and to the axons of the optic pathway in both The disk usually in 2 and is by optic the most common is little of visual function the of normal by after However, there have been case reports of of the visual field defect after a and of visual in to of patients disk In the of anterior ischemic optic there is loss of after because pressure in the lamina is as optic nerve axons Anterior ischemic optic neuropathy usually but after a the other eye usually Ischemic in visual and can be found bilaterally in patients with anterior ischemic optic neuropathy even if the visual defect is a of damage in the the eye often an optic disk with less than normal and of of the optic nerve fibers in the scleral foramen Postoperative anterior ischemic optic neuropathy associated with hypotension usually bilaterally with a visual [40]. of have been including and blood but have been [43]. to decrease intraocular pressure and to decrease swelling have been within of the of visual loss to anterior optic nerve and have with A recent small that has been associated with of central vision optic nerve is a for anterior ischemic optic a clinical It blood flow in the central retinal artery and vascular resistance in the posterior ciliary arteries reports have been in patients with and anterior ischemic optic neuropathy while have been less and out that the rate is the as the rate of the optic nerve the vision may if it has been by anterior ischemic optic neuropathy However, anterior ischemic optic neuropathy has been reported in the eye 1 to 2 after the and associated with at the sites of The incidence of vascular and is and there is of visual loss anterior ischemic optic neuropathy is less common than the [43]. It in patients than with severe loss of vision and of and other systemic There is of the blood vessels and the is by artery anterior ischemic optic the of the short posterior ciliary arteries is occlusion due to disk with retinal than in the form and [43]. Although anterior ischemic optic neuropathy has been reported in the postoperative period It is important to anterior ischemic optic neuropathy if it to blindness. The for anterior ischemic optic neuropathy is at to protect the eye and to arrest of the Although the treatment is usually most patients Ischemic Optic Neuropathy ischemic optic neuropathy as acute loss of vision and visual field to anterior ischemic optic neuropathy. It is to be caused by decreased oxygen delivery to the posterior portion of the optic nerve Figure 1 between the optic foramen at the and the central retinal of where the optic nerve is most to ischemia. This is from an arterial only by pial vessels which can be In the intracanalicular and the intracranial portions of the optic nerve have a more blood ischemic optic neuropathy spontaneously less than anterior ischemic optic neuropathy whereas the is reported with a in postoperative patients associated with hypotension and in patients after severe hypotension ischemic optic neuropathy is not associated with vascular as often as anterior ischemic optic because the less pressure to the vessels passing through it than the more lamina cribrosa. the other the is the blood supply to the anterior optic nerve may protect it from the of and hypotension better than the posterior The posterior optic nerve has less than the anterior so that a insult results in a of ischemic and a period often the loss of vision There are often the of the optic nerve, but disk a of the can of the optic nerve Postoperative posterior ischemic optic neuropathy has been reported after procedures such as open heart surgery, radical and The etiology is not well but to be multifactorial: severe hypotension and are combined with at one other of the central retinal or venous in which posterior ischemic optic neuropathy has cardiac arrest associated with for severe due to gastrointestinal and to the eye There are vascular etiologies of postoperative loss of vision other than ischemic optic neuropathy Table 3. retinal artery and ophthalmic venous obstruction should be 3: of Postoperative Visual blindness may follow or such as cardiac resulting in infarction of watershed areas in or occipital It has been reported after artery surgery and may also result from or during blindness is by loss of visual with of to and normal The may not be of the loss of which in most cases but may or in the or occipital the retinal due to during open heart surgery resolve within of the of but rarely some and as postoperative visual loss retinal artery occlusion as and occlusion of a of the artery results in a field defect or vision. The visual of retinal artery obstruction is often severe initially but it ischemic optic neuropathy which usually has a but of with retinal artery occlusion a at the and or in the retinal arterioles Optic the in ischemic optic in only of with central retinal artery occlusion ischemic optic central retinal artery occlusion often is caused by from an of the carotid artery It is important to retinal artery occlusion in case carotid artery surgery is indicated However, the clinical may not be because visual loss due to retinal may be by ischemic optic neuropathy or cerebral or may also cause central retinal artery occlusion after radical surgery by and hypotension and after of alpha-adrenergic has been in vision in some patients of venous drainage from the eye may positioning results in external pressure on the has to the of reports of this cause of postoperative visual loss in the in severe cases normal or retinal of the and of the and the surrounding the optic disk, which but without of vision. severe cases in patients in immediate loss of vision was and partial or complete of Postoperative Visual Loss and The of ischemic optic neuropathy should be if a of visual loss during the postoperative if it is is from and the lower peripheral sparing the central vision. will disk in anterior ischemic optic neuropathy but not usually in posterior ischemic optic neuropathy. However, the clinical varies and can be may be for various the of may not after surgery, patients may vision to be a normal part of from anesthesia the may be as or or the loss of vision may to a of the site of the is of the the prognosis retrogeniculate demonstrate visual whereas most cases of ischemic optic neuropathy there is an a should be such as carotid artery which may and anterior ischemic optic neuropathy to be these patients to of their ischemic optic neuropathy is in the postoperative period, should be to the and to on Ischemic optic neuropathy has a visual but and in the after the ischemic insult may decrease the nerve and the in the anterior optic nerve surgical optic nerve may be considered in patients with anterior ischemic optic neuropathy measures, such as the maintenance of normal to arterial pressure and normal hematocrit may have some and it to maintain these to prevent of the eye intraoperative external pressure on the and and during In review of the literature to a between postoperative visual loss and decreased blood pressure and of the in the visual pathway. Blood flow with a lower hematocrit due to decreased However, the that this results in increased oxygen delivery is open to A review of the that cerebral oxygen delivery is at a hematocrit of to the of There are two Although the of blood in vessels the it does not affect at the capillary level where oxygen is the critical for oxygen hematocrit results in increased cerebral blood flow by Ischemic areas are and would to the by oxygen delivery Since are both a be between the and the optic nerve with to their oxygen delivery. as is decreased with these it more that would than protect ischemic optic neuropathy in the period [3]. [3] out that recent in may to an in the incidence of postoperative ischemic optic neuropathy Although have not been there have been that blood can be the is or from have been after of that this was that patients a However, other such as hypotension or in the so it is that can be from these results to clinical Perfusion hypotension is in most reports of ischemic optic neuropathy but there is little discussion a safe level of arterial pressure. Although in the retinal arteries it does not to play an important part in or optic nerve Blood flow in these parts is to the arterial pressure it would seem that the maintenance of is a in the of ischemia to the optic However, postoperative optic nerve ischemia has been reported in one normal blood pressure and hematocrit the of other factors in its Postoperative ischemic optic neuropathy is the result of It is usually associated with hypotension and often with blood However, there are often other such as venous obstruction or vascular systemic vascular and are common predisposing more to be in the elderly population, postoperative loss of vision may more as the of the surgical a low hematocrit in the of other factors may more surgical patients to visual loss, it may be that the low level of at which blood is to be may not be as safe as in the of the of it be to to patients that postoperative loss of vision is a but that it has been to particularly in of hypotension and low This discussion may be most for patients to blood are to procedures associated with hypotension or hemorrhage. the postoperative an of visual function by means of and by the anesthesiologist may in of visual by ensuring and the of vision. The Ischemic Optic Neuropathy has the of a Nerve for Anterior Ischemic Optic Neuropathy and This in the surgical treatment of AION in some a of the visual