【Anesthesia Hotspot】Causes and thoughts of postoperative vision loss

Click the blue word to follow our postoperative vision loss patient, male, 57 years old, with a history of lumbar fusion, and plans to undergo another T12~L4 spinal fusion
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Past history: moderate obesity (body mass index: 30kg/m2), type 2 diabetes mellitus, and well-controlled hypertension
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The patient's physical activity was significantly limited by back pain, but a recent cardiac examination showed good left ventricular function with no evidence of myocardial ischemia
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Preoperative laboratory tests were within normal limits, including a hematocrit of 45%
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The patient's preoperative vital signs: blood pressure 150/90mmHg, heart rate 75 beats/min, oxygen saturation under air inhalation was 96%
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The operation lasted 8 hours without serious complications
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Mean arterial pressure was maintained within 20% of baseline except for hypotension (mean arterial pressure <60 mmHg) for approximately 10 minutes due to severe bleeding
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The patient is estimated to have lost approximately 3000ml of blood
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The lowest measured hematocrit was 24%
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Rehydration included 4 units of red blood cells, 6000 ml of crystalloids, and 500 ml of hydroxyethyl starch
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Shortly after extubation in the recovery room from anesthesia, the patient complained of being unable to see
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CASE DISCUSSION Postoperative visual loss after spinal surgery was previously thought to be related to cardiac surgery, and more specifically to cardiopulmonary bypass
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Fortunately, the incidence of POVL is extremely low, but appears to be increasing, especially in patients undergoing spinal surgery
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Although corneal abrasions or transient corneal edema caused by prone positioning can cause transient postoperative visual changes, true POVL occurs in 0.
1% to 0.
2% of patients with spinal cord injury
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In 2006, the ASA published the documented results of POVL in 93 patients with spinal injuries and guidelines for POVL in patients with spinal surgery
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Optic nerve and retina related anatomy The optic nerve consists of four parts: ① the intracranial segment (the part from the optic chiasm to the optic canal in the lesser sphenoid wing); ② the optic canal segment (the part in the optic canal); ③ the posterior or intraorbital segment ( The part from the optic nerve foramen to the cribriform plate); ④ the anterior segment or the intraocular segment (the part from the cribriform plate to the optic disc)
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The lamina cribrosa is a porous membrane covering the posterior scleral foramen through which the optic nerve, central retinal artery and vein enter the eyeball
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The blood supply to the retina comes from the branch of the ophthalmic artery, which is the first intracranial branch of the internal carotid artery
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Once the ophthalmic artery passes through the optic foramen, it divides into several branches: the central retinal artery and a series of posterior ciliary arteries
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These two sets of arterial systems are necessary for the maintenance of retinal function and, as peripheral vessels, have potential ischemic risk in the innervating adjacent areas
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The intraorbital segment of the optic nerve is surrounded by a sheath, and its blood supply comes mainly from branches of the central retinal artery and the retrociliary vessels
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The blood supply to the most anterior segment of the optic nerve comes mainly from branches of the short posterior ciliary artery, not from the central retinal artery
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Whether the blood supply to the optic nerve is automatically regulated when intraocular pressure increases is unclear
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Injury Mechanisms POVL has many causes, including cerebral cortical infarction, pituitary apoplexy, direct damage to the eyeball and optic tract, and ischemic damage to the optic nerve or retina
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The most common cause is ischemic damage to the optic tract, which is divided into two categories: central retinal artery occlusion (CRAO) and ischemic optic neuropathy (ION)
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ION is further divided into posterior ischemic optic neuropathy (PION: optic nerve injury in the posterior region of the cribriform plate) and anterior ischemic optic neuropathy (AION: optic nerve injury in the anterior region of the cribriform plate)
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Due to the increased incidence of vision loss after spinal surgery, POVL, more precisely PI-ON, has attracted the attention of attending physicians, researchers, and the public
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CRAO and ION are discussed separately below
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CRAOCRAO is mainly manifested as painless blindness in one eye after recovery from anesthesia
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Although ION is also a mechanism of POVL after cardiac surgery, CRAO may be an important mechanism for POVL after cardiac surgery due to the high risk of central retinal artery embolism (incidence as high as 4.
5%)
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ION after spinal surgery is uncommon
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The typical findings on fundus examination are pallor of the retina with the formation of a cherry erythema at the macula
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The pupil's light reflex is weakened or disappears
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According to this mechanism, central retinal artery embolism or restricted retinal perfusion pressure due to increased intraocular pressure can lead to CRAO
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Studies have shown increased intraocular pressure in prone patients
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If the patient's orbit is compressed by a head immobilizer (typically a horseshoe-shaped head immobilizer) or other object, the retinal perfusion pressure will be further reduced, a so-called head immobilization syndrome
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Due to the fluid perfusion into the entire orbit including the periocular tissues and extraocular muscles, the patient presented with marked orbital edema, limited extraocular rotation (or even complete ophthalmoplegia)
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In the ASA Vision Loss Registry, CRAO was identified in 10 of 93 patients who developed POVL after spinal surgery
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Compared with the 83 ION patients, CRAO patients were less likely to be immobilized with a Mayfield headgear (all patients had headgear), a shorter procedure, and less blood loss
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Furthermore, no bilateral lesions were seen in CRAO patients, whereas 66% of ION patients had bilateral lesions
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Although CRAO-induced visual loss appears to have a greater chance of recovery overall than ION-induced visual loss, in the ASA registry, no difference in outcomes was found between CRAO and ION patients
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ION reported 131 cases of vision loss in the ASA POVL registry between 1999 and 2004, of which 93 (72%) were associated with spinal surgery, of which 83 (89% of spinal surgeries) ) in patients with visual loss due to ION
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Clearly, this data is still increasing, but it remains unclear whether the increase is related to more complex surgical procedures, patient factors, and greater awareness of POVL
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In conclusion, ION is a devastating complication of unknown etiology, and its prevention is a challenge for surgeons and anesthesiologists
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Patients with ION present with painless binocular or monocular blindness ranging in severity from partial loss of visual field to complete loss of light perception
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Symptoms are usually detectable after waking up from anesthesia, but may not be discovered until hours after waking up
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Like CRAO, patients have diminished or absent reflexes to light and generally have no other evidence of eye damage
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The early fundus examination of AION patients showed papilledema, while the early fundus examination results of PION patients were generally normal
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Over time, patients with AION and PION developed optic nerve head degeneration on fundus examination, with a low likelihood of significant visual acuity recovery
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Although it seems easy to explain ION in terms of vascular disease and reduced oxygen supply to the optic nerve, its exact etiology is much more complex
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Intraoperative ION has been reported in patients with normal hematocrit and perfusion pressure
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There are also very few reports of ION in children, suggesting a population that is at increased risk for ION because ION development is based on the anatomy of the optic nerve blood supply or the lack of self-regulation of this blood supply
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In the ASA registry, 66% of ION patients had bilateral symptoms, suggesting that whatever the deficit was, it was systemic in nature
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Of course, patients with ION did not have evidence of ischemic damage to other vascular beds (kidney, heart), implying that the visual system may be particularly sensitive to changes in oxygen supply
.

One possible mechanism of ION is orbital compartment syndrome, in which optic nerve edema is caused by increased venous pressure (prone position) or by tissue edema caused by a large infusion of crystalloid, which can cause the optic nerve to remain within the nerve sheath or before entering the orbit.
or pressure on the eyeball
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There is insufficient evidence to link sildenafil and ION, and most IONs occur in men, but also in children
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One literature recommends discontinuing sildenafil at least 1 week before surgery
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Risk Factors for POVL Although risk factors for atherosclerotic disease, such as hypertension, diabetes, and smoking, are also thought to be risk factors for POVL (which is certainly intuitive), evidence is lacking
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As previously stated, there are certain populations at risk for POVL regardless of cause, but preoperative identification of such patients is currently not possible
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Although intraoperative hypotension and anemia put patients at risk for POVL and have been reported in multiple case reports, they are not necessarily supported in larger samples
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However, it has been reported that blood transfusion and elevated blood pressure can improve postoperative POVL in anemic patients
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Two factors, prolonged surgical procedures and massive blood loss, have long been considered risk factors for the development of POVL in patients with spinal cord injuries
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In the ASA registry, these two factors are defined as a surgical procedure exceeding 6 hours and an estimated blood loss greater than 1
L.

Of the 93 patients with POVL spinal injuries, 94% had the procedure lasting 6 hours or more
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Likewise, 82% of POVL patients had an estimated blood loss of 1 L or more
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Interestingly, although adult women had longer surgical procedures than adult men, 72% of the cases in the registry were adult men
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Prevention of POVL Because the etiology of POVL is unknown, this complication cannot be avoided
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However, there are some definite precautions that can be taken
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First, check the eyes of the prone patient frequently, at least every 15 minutes, to make sure the eyes are not pinched in any way
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Especially in patients with headrests (when opposite to the headrest), the proper initial position of the head does not guarantee that the head will not move during surgery, causing the eye sockets to come into contact with the head mount
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In addition, whenever possible to optimize retinal and optic nerve perfusion pressures, the head should be midline, at the level of the heart or slightly above
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In some patients with severe kyphosis, the fixed position of the head on the chest makes it impossible to achieve optimal positioning
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A dedicated group of anesthesiologists, spine surgeons (orthopaedic and neurosurgeons), and neuro-ophthalmologists developed an ASA guideline in which neuro-ophthalmologists assessed current data and surveyed participating anesthesiologists and spine surgeons
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Guidance is issued to aid in clinical decision making and is not intended to serve as a formal guideline or standard of practice
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Nonetheless, their review on the topic is comprehensive, with pertinent recommendations for the care of people with spinal injuries
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Their recommendations are summarized as follows: 1.
Although preoperative medical conditions, such as anemia, atherosclerosis, and obesity, may be associated with POVL, these are not currently predisposing factors
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2.
Risk factors for POVL in patients include prolonged surgery (more than 6.
5 hours) and intraoperative massive blood loss (average 45% of estimated blood volume)
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3.
Although both consulting physicians and specialists agree that controlled BP should be avoided in high-risk patients (whether or not well-controlled hypertensive patients), there is a disagreement about whether controlled BP should be used in patients without chronic hypertension
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Ultimately, there are insufficient data to argue against the use of controlled hypotension
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The guidelines recommend continuous blood pressure monitoring in high-risk patients
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4.
Likewise, there was a marked difference between the opinion of the consulting physician and the specialist regarding the minimum acceptable hemoglobin level
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These investigations indicated that the lowest acceptable mean hemoglobin level was 94 g/L
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The task force concluded that the hemoglobin concentration could not be lower, as hemoglobin has been clearly associated with the development of POVL
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5.
For patients with severe blood loss, a combination of crystalloids and colloids is recommended
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6.
Although neuroanaesthetists agree that long-term use of alpha agonists may reduce optic nerve perfusion pressure, there are insufficient data to formulate a guideline on this issue
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7.
High-risk patients should consider fractional surgery
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8.
Although all team members believe that there is no definite and effective treatment for ION, they agreed that postoperative management of POVL patients includes measures to correct anemia, increase blood pressure and increase oxygen supply
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For patients suspected of having POVL, an ophthalmologist should be consulted urgently, and an MRI examination should be performed to rule out blindness due to intracranial causes
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9.
Preoperative POVL discussion should be considered in patients with high-risk factors (prolonged surgery, unpredictable blood loss)
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POVL management Patients who develop POVL have at least two risk factors: undergoing surgery in the prone position for up to 8 hours and losing approximately 50% of their blood volume
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Whether a prior history of hypertension or diabetes increases the risk is unclear
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Intraoperative blood pressure reduction also occurred, but whether this is related to POVL has not been elucidated
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There is currently no definite and effective treatment for POVL
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However, some urgent measures should be taken for such patients, including raising blood pressure to at least its basal value, ensuring that its hemoglobin level is within a reasonable range (90g/L or above), immediately consulting an ophthalmologist, and performing fundus examination to try to assess the current type of injury, MRI was performed
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In conclusion, POVL is a catastrophic complication after spinal surgery with a generally poor prognosis
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Intraoperative attention to orbital conditions can reduce the incidence of CRAO, but the etiology of ION is more subtle and dangerous, so it is more difficult to prevent
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Due to the extremely low incidence of this complication, we have limited information and a single-center prospective study is largely impossible
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Moreover, there is currently no animal model of POVL
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Until more data are available on the mechanism of POVL, fractionated procedures, close attention to eye changes in prone patients, and frequent attention to optic nerve and retinal oxygenation are the best preventive measures at present
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Key information 1.
The most common cause of POVL is ischemic injury to the optic tract, which is divided into two main categories: CRAO and ION
.

2.
In spinal surgery patients, risk factors for developing POVL are longer surgical procedures and massive blood loss
.

3.
Although close attention to intraoperative orbital changes can reduce the incidence of CRAO, ION has a more sinister etiology and is therefore more difficult to prevent
.

Question 1.
What is the incidence of POVL in patients undergoing spinal surgery? Answer: In patients with spinal cord injury, the incidence of true POVL is 0.
1% to 0.
2%
.

2.
What is the anatomy of the optic nerve? Answer: The optic nerve consists of four parts: (a) the intracranial segment (the part from the optic chiasm to the optic canal in the lesser sphenoid wing); (b) the optic canal segment (the part in the optic canal); (c) the posterior segment or orbit Internal segment (portion of optic nerve foramen to cribriform plate); (d) anterior segment or intraocular segment (portion from cribriform plate to optic disc)
.

3.
What are the possible mechanisms by which POVL occurs? Answer: Including cerebral cortical infarction, pituitary apoplexy, direct damage to the eyeball and optic tract, and ischemic damage to the optic nerve and/or retina
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