Perioperative ischemic optic neuropathy (PO-ION) should be considered in cases where no other causes for ION are found.  Although still considered rare, its incidence is increasing along with the frequency of surgeries at risk for with prolonged surgical times, risks for high volume blood loss, perioperative anemia and hypotension.

PO-ION may occur in patients with predisposing systemic risk factors, such as a surgical complication.^1-4 The general term “perioperative vision loss” describes various subtypes of vision changes that occur during or after surgery, including ischemic optic neuropathy, cortical vision loss and/or vascular occlusions.^1-3,5-10

ION is the most common cause of postoperative vision loss. Common procedures associated with PO-ION include coronary artery bypass surgery, spine surgery and radial neck dissection surgery.^{3,5,6,8,11-15}

Risk of PO-ION is increased by a number of conditions, including perioperative anemia, hypotension and direct pressure on the globe—and pre-existing systemic vascular conditions, such as hypertension and heart disease, are factors as well.^{1,3,5-8,11,12,14-20}

A recent study showed that the incidence of symptomatic perioperative vision loss related to ION was 0.013%.⁵ A population-based study of vision loss after spine surgery showed an incidence of 0.006% cases of ION associated with spine surgery.²

PO-ION in coronary artery bypass graft surgery is fairly rare; however, it has shown an increase in frequency from 0.06% to 0.113% between the years 1976 and 2002.^15,18
PO-ION should be considered in cases when no other cause of ION can be determined. It is considered a rare condition; however, its incidence is rising due to the increased frequency of surgeries that predispose patients to PO-ION.

The Nuts and Bolts of ION
PO-ION typically involves the posterior optic nerve.^7,10,21 The posterior portion of the optic nerve extends from the lamina cribrosa to the optic foramen within the orbit. Optic atrophy develops after either anterior (AION) or posterior ischemic optic neuropathy (PION). With an acute presentation, the first can be readily distinguished from the other based upon the presence of disc swelling in the AION and absence of disc edema in PION.^{1,4-6,11,12,14} (For a breakdown of factors in cases of both types of ischemic optic neuropathy, see “Signs and Symptoms of PION vs. AION.)

Signs and Symptoms of PION vs. AION1,4,5,7,11
Posterior Ischemic Optic Neuropathy	Anterior Ischemic Optic Neuropathy
Signs
• Optic atrophy develops acutely within three to eight weeks. • Absence of optic disc edema.  • RAPD (if damage is asymmetric). • VA decrease. • Optic nerve pallor develops on temporal neuroretinal rim. • Central or paracentral VF defects. • Commonly associated with hypotension and hemodilution.	• Optic atrophy develops nearly two months later. • Presence of optic disc edema initially. • RAPD. • VA decrease. • Altitudinal or inferior nasal VF defects. • Congenitally small optic nerve cupping; “disc at risk.”
Symptoms
• Acute vision loss. • Poor prognosis for vision improvement.	• Acute vision loss upon awakening. • Visual field deficit.

Perioperative PION vs. AION
PION can present with acute vision loss post-surgically, during recovery from anesthesia. Several cases show blurred vision or loss of vision as early as day one to day five post-op.⁷ In the context of ION, it is important to observe the optic nerve appearance for edema upon acute onset of symptoms to rule out posterior vs. anterior involvement of the optic nerve.^{1,4-6,11,14,22}

It takes about two months for pallor to develop in a patient with AION, whereas pallor in PION may appear earlier, within three to eight weeks, although it typically becomes evident in the six- to eight-week range.^4-7,10,17 Pallor in PION is more likely to involve the temporal neuroretinal rim of the optic nerve, which correlates to central or paracentral visual field defects.⁴ AION, on the other hand, has classically been associated with altitudinal visual field defects or inferior nasal visual defects in about 22.4% of ION cases.^23,24

A diagnosis of PION is based on the following criteria: sudden vision loss, visual field defects related to the optic nerve, relative afferent pupillary defect in the involved eye, normal optic nerve on initial evaluation, and ruling out of any other causes of vision loss.^1,3,4,6 But, a diagnosis of AION involves disc edema upon presentation.⁶ In the setting of perioperative ischemic optic neuropathy, the acute symptoms of visual loss generally occur upon awakening after surgery and are bilateral, similar to the symptoms of AION, which can occur upon awakening as well.^4,6,17,25

Risk Factors
The exact mechanism of PO-ION is not understood, but multiple known risk factors can aid in understanding the possible pathophysiology. The risk of ischemic optic neuropathy is higher in patients with more complicated vascular disease.^{1,3,7,10,14-15,19,26} Diabetes and hypertension are microvascular conditions that involve already-compromised autoregulation; these pre-existing conditions contribute additional stress that raises the risk of ischemia in conjunction with the decreased hematocrit and hypotension of complicated surgeries. Prolonged surgical times, massive blood loss, hypotension and direct compression on the orbit are considered strong intra-operative risk factors for vision loss.^{1,4,11,15,17-19,24,27} (For more on the predisposing and perioperative risk factors for PO-ION, see “Predisposing and Perioperative Risk Factors for PO-ION.”)

Predisposing and Perioperative Risk Factors for PO-ION7,11
Predisposing Factors	Perioperative Factors
• Diabetes • Hypertension • Anemia • Atherosclerosis • Smoking history • Age • Location of the “watershed zone” within the optic nerve head	• Prolonged surgicial procudure duration • Massive blood loss • Hemoglobin level less than 8gm/dL • Hematocrit levels between 18.5% and 39% • Direct compression of orbit  • Prone position during surgery • Head down in the “Trendelenburg position” • Spine or heart surgery • Hypotension

Pathogenesis
PO-ION can involve either anterior or posterior ischemic optic neuropathy. Perioperative mean blood pressure, intraocular pressure and anemia may contribute to a change in optic nerve head perfusion.⁶ The optic nerve head blood flow is related to several factors, including the relationship between mean arterial blood pressure, intraocular pressure and resistance to blood flow.^6,27-30

A drop in mean arterial blood pressure (MAP) of 24% to 46% from the preoperative MAP has also been associated with PO-ION.7 Arterial hypotension can be due to perioperative blood loss, and can ultimately affect optic nerve head blood flow. The optic nerve is supplied by the posterior ciliary arteries (PCA), which stem from the ophthalmic artery, which in turn gets its blood supply from the carotid artery.^5,10,31

Clinical Signs of PO-ION Ocular Signs When examining a patient, these are the signs that you should look for. Any of them can lead you to a diagnosis of PO-ION. • Acute bilateral or unilateral vision loss upon awakening after a surgical procedure. • Acute optic nerve edema (postoperative anterior ischemic optic neuropathy) • A seemingly normal optic nerve upon initial examination that develops optic atrophy three to eight weeks after initial exam (postoperative posterior ischemic optic neuropathy). • The patient has a relative afferent pupillary defect. • Visual field defect. Medical/Surgical History Make sure that you attain a full medical history. These factors can clarify a patient’s condition and help you come to a definite diagnosis. • Recent surgery, such as coronary artery bypass graft (CABG), lumbar spine surgery or neck dissection surgery. • Prolonged surgical duration. • Hypovolemic event (hemoglobin levels below 8gm/dL; Hematocrit levels between 18.5% and 39%). • Any comorbidities, such as diabetes, hypertension, anemia, etc.

The anterior optic nerve head receives its blood supply from the short posterior ciliary arteries; anastomoses of the PCAs form a ring pattern around the optic nerve, known as the Circle of Haller and Zinn.^29,31,32

The PCAs that supply the anterior optic nerve head are end arterioles and are susceptible to ischemia during decreases in perfusion pressure. These areas are defined as “watershed zones,” which have variable distribution in the ONH.³¹ It is thought that the location of the watershed zone within the optic nerve head has an influence on the type of visual field defect produced in ischemic optic neuropathy.28,31,33 The posterior optic nerve is supplied by small pial vessels and is susceptible to ischemia, because it is devoid of autoregulation.^{4,12,21,22,34} Therefore, arterial hypotension in perioperative situations can put the ONH at risk for decreased perfusion.^6,21,22,33 The optic nerve blood flow is sensitive to any changes in blood pressure that affect the perfusion pressure of the optic nerve.

Acute changes in intraocular pressure influence the mean arterial pressure and ultimately contribute to the level of optic nerve perfusion. Changes in IOP can occur because of prone positioning, external pressure (e.g., from an eye patch applied during surgery) and anesthesia. These challenges must be dealt with during complex operations.^30,35 The prone position causes an increase in IOP that becomes significant when IOP rises above 40mm Hg.^30,34,35 Maintaining the head in the prone position on the horseshoe-shaped head rest during prolonged surgery has been associated with direct pressure on the globe (termed “head rest syndrome”), which increases IOP and decreases blood flow to nerve.^1,19,35,36

Evidence shows that the optic nerve head’s autoregulation maintains constant blood flow. Maintaining constant blood flow to the optic nerve head is influenced by vascular tone, which in turn signals autoregulation. Blood flow is only well controlled under a critical range of perfusion pressure, and any change in perfusion pressure outside of this range puts the optic nerve tissue at risk for an ischemic event.^28,30,34,37 Such systemic vascular conditions as atherosclerosis, hypertensive vascular disease and diabetic vascular disease can also have a large impact on vascular tone and influence regulation of blood flow.^10,22,31

Extensive and complicated surgery puts patients at risk for hypotension from blood loss, decrease in hematocrit and resulting perioperative anemia. Perioperative hemoglobin levels less than 8.0gm/dL results in risk for ischemic optic neuropathy.⁷ Hemoglobin transports oxygen through the blood, so a decrease in hemoglobin levels or functionality reduces the oxygen transport to tissue, including the optic nerve.³⁸

Secondly, the sudden drop in hematocrit level from blood loss is also a common sign in patients who experience vision loss in spine surgery.^1,14,38 In a case series of perioperative PION, a drop in hematocrit of 9.5% to 19% was a contributing factor, along with drop in blood volume ranging from 2.0L to 16L.¹ Similarly, a literature case review showed that an average blood loss of 4.3L and level of hematocrit between 18.5% and 39% were common risk factors in patients with PO-ION.

Even with a wide range of values, the one commonality is that sudden drops in hematocrit and hemoglobin, as well as hypotension, are risk factors that can contribute to PO-ION.

The Relationship of IOP, Blood Pressure and Blood Flow Through the Optic Nerve Blood flow =   Perfusion pressure*  Resistance to flow *Perfusion pressure = Mean arterial blood pressure - IOP = ? So, these three factors come together to play a role in the pathogenesis of PO-ION. Mean blood pressure, IOP and anemia contribute to a change in optic nerve head perfusion.

Differential Diagnosis
Perioperative vision loss can result from retinal vascular occlusions and cortical vision loss. An ophthalmic evaluation immediately after onset of acute symptoms of vision loss postoperatively can differentiate the cause of vision loss and prevent further loss. Retinal arteriolar occlusion manifests with an edematous retina and narrowing of arterioles, which can also result in optic atrophy—but this etiology is less common than ischemic optic neuropathy.^{10,11-13,39,40} Cortical vision loss presents with normal optic nerve appearance and pupillary reactions, but distinct visual field defects. A cortical loss can be confirmed by radiological neuro-imaging—e.g., CT scan or magnetic resonance imaging.^10,13,32,39

With acute vision loss, rule out causes related to inflammation, infection and space-occupying lesions. The most important condition to rule out is GCA, which is the arteritic form of ION (whether it involves the posterior or anterior optic nerve).^32,41

If all investigative procedures (erythrocyte sedimentation rate, C-reactive protein, ESR, complete blood count, temporal artery biopsy) are negative and no other systemic medical cause is found, PO-ION can be inferred in the setting of a high-risk non-ocular surgery. Again, PO-ION is a diagnosis of exclusion after all other causes of perioperative vision loss have been ruled out.

Appropriate Management
The appropriate way to manage patients with ischemic optic neuropathy is to recognize and treat the underlying cause. In patients with PO-ION, this consists of restoration of normal blood volume and constituents as well as blood pressure. However, there is no effective treatment for ischemic optic neuropathy itself. The Ischemic Optic Neuropathy Decompression Trial (IONDT) found no evidence that optic nerve sheath fenestration improved visual acuity outcome.^42,43

When confronted with ischemic optic neuropathy, proceed with full ocular evaluation, including history, visual acuity, visual field, pupil testing and dilated fundoscopy. It is important to investigate for a primary systemic cause. Imaging, as well as laboratory tests, can help rule out space-occupying lesions or any inflammatory cause. A careful review of systemic medical and surgical history is crucial, because vascular diseases are associated risk factors for PO-ION. Additionally, patients who are contemplating spine surgery and are at risk for PO-ION should be counseled on the possibility of vision loss as a consequence of surgery.^12,16

Perioperative ischemic optic neuropathy should be included among the differential diagnoses of patients who present with unilateral or bilateral optic atrophy. It is a diagnosis of exclusion after other causes have been ruled out. Optometrists should be aware of the potential for ischemic optic neuropathy to occur in conjunction with surgery, as well as the systemic and perioperative risk factors that may predispose a patient to developing PO-ION.
Dr. Ma completed a residency in primary eye care at the VA Medical Center in West Los Angeles, California. She is currently working at a private practice optometry office and a retina clinic in southern California. Dr. Ilsen is the Assistant Chief of Optometry at the West Los Angeles VA Medical Center.

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