PRONE POSITION: PERI-OP VISUAL LOSS (POLV), ION, CRAO, CRVO AND INDUCED HYPOTENSION
Visual loss after spinal fusion surgery is a rare but devastating complication with a reported incidence of 0.017 to 0.1 percent.
The major causes of visual loss in this patient population include
1. Ischemic Optic Neuropathy (ION)
2. Central Retinal Artery Occlusion (CRAO) and
3. Central Retinal Vein Occlusion (CRVO).
POSITIONING AND CRAO and CRVO
CRAO and CRVO are attributed to embolic load and/or direct globe compression, emphasizing the need to protect the eyes from direct pressure while in the prone position.
ISCHEMIC OPTIC NEUROPATHY (ION)*****
ION almost universally results in PERMANENT VISUAL LOSS. In 2012, the Postoperative Visual Loss (POVL) Study Group reported the findings of their multicenter case control study that used data from known ION cases registered in the American Society of Anesthesiologists POVL Registry and control cases from 17 United States academic medical centers.
INDEPENDENT RISK FACTORS FOR THE DEVELOPMENT OF ION
1. Male sex
2. Obesity
3. Use of the Wilson frame (head lower than the heart)
4. Longer anaesthetic time
5. Greater estimated blood loss and
6. Lower percent of colloid in the nonblood fluid replacement
Using these risk factors and the reported incidence of POVL from ION in the literature, the POVL Study Group created a risk prediction model that may help surgeons and anesthesia providers modify their care plans and define the risk of ION to patients .
RISK MODIFICATION STRATEGIES FOR ION AFTER PRONE SPINE SURGERY
1. Positioning with the head level with or above the heart (Tilt The Whole Table)
2. Minimizing anaesthetic duration and blood loss ( Staged Procedure if necessary)
3. Use of both colloids and crystalloids for intravascular volume replacement
4. Monitoring hemoglobin periodically during cases with significant blood loss and
5. Optimization of hemodynamics with maintenance of blood pressure within 20 percent of baseline.
Patients scheduled for historically lengthy (longer than four hours) spine surgery in the prone position should be informed of the small but increased risk of ION. When possible, staging of long procedures should be considered.
CONTROLLED/ INDUCED HYPOTENSION
The use of controlled hypotension is NOT recommended in patients undergoing spine surgery. Induced hypotension has historically been advocated as a mechanism to reduce blood loss during a variety of surgical procedures.
WHY LOW BP DOES NOT REDUCE BLOOD LOSS
Reduced wound blood flow as a result of lower arterial blood pressure has been the mechanism cited for reduction in blood loss. However, epidural venous plexus pressure and intraosseous pressure, both important determinants of blood loss in spine surgery, are independent of arterial blood pressure.
INDUCED HYPOTENSION and SPINAL CORD ISCHEMIA
The most important reason to avoid the use of induced hypotension is the potential for end-organ ischemia. In particular, patients with severe spinal stenosis are at risk for SPINAL CORD ISCHEMIA and should be maintained during anaesthesia at or near their usual blood pressure.
In addition, spinal instrumentation and distraction can reduce SPINAL CORD PERFUSION AND RESULT IN ISCHEMIA. Therefore, adequate arterial blood pressure should be maintained during instrumented spinal surgery as one measure to avoid neurologic damage.
INDUCED HYPOTENSION AND POVL
As explained above, visual loss is a rare but potentially devastating postoperative complication of prone spinal surgery. Ischemic optic neuropathy (ION) is the most common cause of postoperative visual loss. Although the causes of ischemic optic neuropathy have not been fully elucidated, tissue edema with reduced perfusion of the optic nerve is a suggested etiology.
Visual loss after spinal fusion surgery is a rare but devastating complication with a reported incidence of 0.017 to 0.1 percent.
The major causes of visual loss in this patient population include
1. Ischemic Optic Neuropathy (ION)
2. Central Retinal Artery Occlusion (CRAO) and
3. Central Retinal Vein Occlusion (CRVO).
POSITIONING AND CRAO and CRVO
CRAO and CRVO are attributed to embolic load and/or direct globe compression, emphasizing the need to protect the eyes from direct pressure while in the prone position.
ISCHEMIC OPTIC NEUROPATHY (ION)*****
ION almost universally results in PERMANENT VISUAL LOSS. In 2012, the Postoperative Visual Loss (POVL) Study Group reported the findings of their multicenter case control study that used data from known ION cases registered in the American Society of Anesthesiologists POVL Registry and control cases from 17 United States academic medical centers.
INDEPENDENT RISK FACTORS FOR THE DEVELOPMENT OF ION
1. Male sex
2. Obesity
3. Use of the Wilson frame (head lower than the heart)
4. Longer anaesthetic time
5. Greater estimated blood loss and
6. Lower percent of colloid in the nonblood fluid replacement
Using these risk factors and the reported incidence of POVL from ION in the literature, the POVL Study Group created a risk prediction model that may help surgeons and anesthesia providers modify their care plans and define the risk of ION to patients .
RISK MODIFICATION STRATEGIES FOR ION AFTER PRONE SPINE SURGERY
1. Positioning with the head level with or above the heart (Tilt The Whole Table)
2. Minimizing anaesthetic duration and blood loss ( Staged Procedure if necessary)
3. Use of both colloids and crystalloids for intravascular volume replacement
4. Monitoring hemoglobin periodically during cases with significant blood loss and
5. Optimization of hemodynamics with maintenance of blood pressure within 20 percent of baseline.
Patients scheduled for historically lengthy (longer than four hours) spine surgery in the prone position should be informed of the small but increased risk of ION. When possible, staging of long procedures should be considered.
CONTROLLED/ INDUCED HYPOTENSION
The use of controlled hypotension is NOT recommended in patients undergoing spine surgery. Induced hypotension has historically been advocated as a mechanism to reduce blood loss during a variety of surgical procedures.
WHY LOW BP DOES NOT REDUCE BLOOD LOSS
Reduced wound blood flow as a result of lower arterial blood pressure has been the mechanism cited for reduction in blood loss. However, epidural venous plexus pressure and intraosseous pressure, both important determinants of blood loss in spine surgery, are independent of arterial blood pressure.
INDUCED HYPOTENSION and SPINAL CORD ISCHEMIA
The most important reason to avoid the use of induced hypotension is the potential for end-organ ischemia. In particular, patients with severe spinal stenosis are at risk for SPINAL CORD ISCHEMIA and should be maintained during anaesthesia at or near their usual blood pressure.
In addition, spinal instrumentation and distraction can reduce SPINAL CORD PERFUSION AND RESULT IN ISCHEMIA. Therefore, adequate arterial blood pressure should be maintained during instrumented spinal surgery as one measure to avoid neurologic damage.
INDUCED HYPOTENSION AND POVL
As explained above, visual loss is a rare but potentially devastating postoperative complication of prone spinal surgery. Ischemic optic neuropathy (ION) is the most common cause of postoperative visual loss. Although the causes of ischemic optic neuropathy have not been fully elucidated, tissue edema with reduced perfusion of the optic nerve is a suggested etiology.
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