Posterior ischemic optic neuropathy
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|Posterior ischemic optic neuropath|
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Posterior Ischemic Optic Neuropathy (PION) is a medical condition characterized by damage to the retrobulbar portion of the optic nerve due to ischemia, a restriction of the blood supply to the optic nerve. Despite the term posterior this pathophysiology may be applied to cases where the ischemic damage is anterior, as the condition describes a particular mechanism of visual loss as much as the location of damage in the optic nerve. Anterior ischemic optic neuropathy (AION) is distinguished from PION by the fact that AION occurs spontaneously and unilaterally in patients with predisposing anatomy and cardiovascular risk factors.
PION is a watershed infarction of the optic nerve that may cause either unilateral or, more often, bilateral blindness. PION typically occurs in two categories of patients:
- patients who have undergone non-ocular surgery that is particularly prolonged or is associated with a significant blood loss.
- patients who have experienced significant hemorrhaging from an accident or ruptured blood vessels. In these cases, the hematocrit (percentage of blood cells in the blood) is low and often the blood pressure is also low. This combination can produce shock, and PION has sometimes been called shock-induced optic neuropathy.
The combination of low blood count and low blood pressure means that the blood is carrying less oxygen to the tissues. Many end-organs may suffer from the lack of oxygen, including the brain, kidneys or heart. What may put the optic nerve at particular disadvantage is the superimposition of a compartment syndrome. When tissues suffer from lack of oxygen, they swell. The optic nerve is, however, confined within a bony canal between the brain and eye. Not being free to swell may cause further increased pressure on the optic nerve, reducing blood flow into the nerve, particularly in combination with low blood pressure. Restricted blood flow can result in permanent damage to the optic nerve with resultant blindness, which is often bilateral, causing some patients to wake up permanently blind after long or difficult surgeries. For technical reasons this occurs more frequently with spinal surgeries .
The diagnosis of PION is often difficult since the optic nerves initially appear normal. The injury occurs posterior to that portion of the nerve visible during ophthalmoscopic examination. There may be an abnormal relative pupillary response (APD) if the injury is confined to one optic nerve, but often it is bilateral and the symmetry of pupillary responses is maintained. Furthermore, MRI scanning may not be helpful. It is not uncommon for the erroneous diagnoses of malingering or cortical blindness to be made. If possible, an urgent neuro-ophthalmology consult is most likely to lead to the correct diagnosis .
Patients with a history of high blood pressure, diabetes and smoking are most susceptible to PION as they have a compromised system of blood vessel autoregulation. Hence, extra efforts may need to be taken for them in the form of careful or staged surgery or the controlling the anemia from blood loss (by administration of blood transfusions), and the careful maintenance of their blood pressure .
Once visual loss has occurred, it becomes more problematic, but there are reports of recovered vision if blood transfusions and agents that raise blood pressure are administered within hours .
PION: In Depth 
Posterior ischemic optic neuropathy (PION) is a rare but devastating cause of vision loss. PION occurs when oxygen delivery to the posterior optic nerve is inadequate, resulting in nerve cell death. This ischemia comes from a watershed infarct. There are two types of PION, which result in optic nerve ischemia via separate mechanisms: arteritic and perioperative.
Ischemia means decreased blood flow, which decreases oxygen delivery to tissues, and may lead to cell death.
Arteritic PION (A-PION): Arteritic PION is an autoimmune disease. The risk of A-PION increases ten times per decade of life, which means that the elderly are at the highest risk. A-PION causes severe vision loss in rapid sequence from one eye to the next. Early treatment is essential to halt the progression of vision loss.
Perioperative PION: a.k.a. surgical, postoperative, or shock induced PION. Profound vision loss is discovered upon waking from surgery. Risk factors during surgery include low blood pressure, blood loss, anemia, long procedure duration, and facial swelling. Perioperative PION usually results in severe, irreversible, bilateral vision loss.
PION is so rare that it is hard to calculate its incidence. PION occurs most frequently after spinal surgery. Currently, the largest retrospective review of perioperative PION estimates an incidence of 0.028% during spinal surgeries . However, PION is exceedingly rare after most other surgical procedures. Another review did not find a single case of PION out of 60,965 patients who underwent general anesthesia . Our knowledge of PION’s incidence would be improved by a large-scale prospective study .
Signs and Symptoms 
General PION signs & symptoms: PION most commonly affects the elderly. Onset of moderate to severe vision loss is abrupt and painless. One or both eyes may be affected. Color vision is impaired in most cases [4,5,6].
PION Ophthalmoscopic exam: Looking inside the patient’s eyes at the time of onset, ophthalmoscope exam reveals no visible changes to the optic nerve head. Weeks after ischemic insult, nerve atrophy originating from the damaged posterior optic nerve progresses to involve the anterior optic nerve head. Four to eight weeks after onset, atrophy of the optic nerve head is observable upon ophthalmoscope exam .
PION Pupils: If both eyes are affected by PION, the pupils may look symmetrical. However, if the eyes are asymmetrically affected, i.e. one eye's optic nerve is more damaged than the other, it will produce an important sign called an afferent pupillary defect.
Afferent Pupillary Defect (APD): Defective light perception in one eye causes an asymmetrical pupillary constriction reflex called the afferent pupillary defect (APD).
Other signs and symptoms are distinct for each type of PION:
Arteritic PION (A-PION) signs & symptoms: A-PION most commonly affects Caucasian women, with an average age of 73 [5,8]. At onset vision loss is unilateral, but without treatment it rapidly progresses to involve both eyes. Vision loss is usually severe, ranging from counting fingers to no light perception. Associated symptoms are jaw pain exacerbated by chewing, scalp tenderness, shoulder and hip pain, headache and fatigue[6,7].
Perioperative PION signs & symptoms: Vision loss is usually apparent upon waking from general anesthesia. Signs observable to a bystander include long surgery duration and facial swelling. Vision loss is usually bilateral and severe, ranging from counting fingers to no light perception [4,5,6,7,9,11].
Differential diagnosis 
In the postoperative setting, without gross eye injury, visual loss requires an assessment of the whole visual system for ischemic damage. The optic nerve is not the only tissue of the visual pathway susceptible to decreased blood flow. Decreased oxygenation of the retina or brain could also impair vision .
PION Pathogenesis: In both types of PION, ischemia, i.e. decreased blood flow, leads to the death of optic nerve cells. Ischemic injury to the optic nerve causes inflammation and swelling. Because the posterior optic nerve passes through the optic canal, a boney tunnel leading to the brain, swelling in this inelastic space causes compression. Optic nerve compression exacerbates ischemia and perpetuates the cycle of injury, and swelling, and compression .
A-PION Pathogenesis: A-PION is caused by an inflammatory disease called giant cell arteritis (GCA), a.k.a.temporal arteritis. GCA is an inflammatory disease of blood vessels. It is believed to be an autoimmune disease caused by inappropriate T-cell activity [7,13]. When T-cells damage arteries supplying the optic nerve, a thrombus forms and stops blood flow. When blood flow stops, oxygen delivery stops and optic nerve fibers die.
Perioperative PION (a.k.a. Surgical, Postsurgical, or Shock Induced PION) Pathogenesis: The exact cause of perioperative PION is unknown. Many risk factors have been identified, all of which contribute to optic nerve hypoxia. Alone, none of these risk factors is enough to cause PION. However, in susceptible patients, a combination of these risk factors produces devastating blindness. This evidence suggests that PION is a disease of multifactorial origin.
Risks of perioperative PION can be divided into two categories, intraoperative ischemic pressures, and cardiovascular risk factors.
Intraoperative ischemic pressures in perioperative PION:
Many causes of decreased blood flow during surgery are systemic, i.e. they decrease blood flow throughout the body. Studies have shown that nearly all perioperative PION patients suffered from prolonged intraoperative hypotension and postoperative anemia. The average perioperative PION patient loses 4 liters of blood during surgery, and the majority receive blood transfusions. Massive blood loss is just one cause of low blood pressure. Side effects of general anesthesia also lower blood pressure. The average surgery duration in PION cases is 7 to 9 hours, which increases the risk of prolonged low blood pressure [4,7,9,28].
Other intraoperative ischemic pressures are local, i.e. they decrease blood flow to the affected area, the optic nerve. Facial swelling, periorbital swelling, direct orbital compression, facedown position during surgery, and a tilted operating table in feet-above-head position, have all been reported to be associated with perioperative PION. All of these factors are believed to increase tissue pressure and venous pressure around the optic nerve, thereby decreasing local blood flow and oxygen delivery [4,7,9,10,14-17].
Surgeries with the highest estimated incidence of PION are surgeries with a higher risk of the aforementioned conditions. In spine surgery, patients are susceptible to significant blood loss, and they are positioned face down for long periods of time, which increases venous pressure, decreases arterial perfusion pressure, and often causes facial swelling (increased tissue pressure). Spine surgery is estimated to have the highest incidence of PION, 0.028% . Long duration of feet-above-head position in prostate surgery has also been suggested to increase risk of PION .
Cardiovascular risk factors for PION:
Perioperative PION patients have a higher prevalence of cardiovascular risk factors than in the general population. Documented cardiovascular risks in perioperative PION patients include hypertension, diabetes mellitus, hypercholesterolemia, tobacco use, cardiac arrhythmia, cerebrovascular disease and obesity. Men are also noted to be at higher risk, which is in accordance with the trend, as men are at higher risk of cardiovascular disease. [4,6,9,11,12,14,15,16,19]. These cardiovascular risks all hinder perfusion, and also may suggest a contributory role of defective vascular autoregulation [4,7,9,11].
The multifactorial origin of perioperative PION:
As illustrated by the risk factors above, perioperative hypoxia is a multifactorial problem. Amidst these risk factors it may be difficult to pinpoint the optic nerve’s threshold for cell death, and the exact contribution of each factor .
Low blood pressure and anemia are cited as perioperative complications in nearly all reports of PION, which suggests a causal relationship. However, while low blood pressure and anemia are relatively common in the perioperative setting, PION is exceedingly rare. Spine and cardiac bypass surgeries have the highest estimated incidences of PION, 0.028% and 0.018% respectively, and this is still extremely low [2,20,28]. This evidence suggests that optic nerve injury in PION patients is caused by more than just anemia and low blood pressure .
Evidence suggests that the multifactorial origin of perioperative PION involves the risks discussed above and perhaps other unknown factors. Current review articles of PION propose that vascular autoregulatory dysfunction and anatomic variation are under-investigated subjects that may contribute to patient-specific susceptibility [7,9].
There is no confirmatory test for PION. PION is a diagnosis of exclusion. To prevent impending blindness, it is urgent to rule out giant cell arteritis when a patient over 50 presents with sudden vision loss.
GCA Diagnosis: The American College of Rheumatologists have defined a combination of physical symptoms and inflammatory changes to diagnose giant cell arteritis. See: http://www.ncbi.nlm.nih.gov/pubmed/2202311
To differentiate PION from AION upon examination: At the onset of symptoms, ophthalmoscope examination can differentiate AION from PION. If optic nerve head involvement is observed, it is AION. PION does not produce optic atrophy that is observable via ophthalmoscope until four to eight weeks after onset. In addition, AION often shows a characteristic altitudinal defect on a Humphrey Visual Field test.
Treatment and Prognosis 
A-PION Treatment & Prognosis: If a diagnosis of GCA is suspected, treatment with steroids should begin immediately. Temporal artery biopsy results should be obtained to confirm the diagnosis and guide future management, but should not delay initiation of treatment. Treatment does not recover lost vision, but prevents further progression and second eye involvement. High dose corticosteroids may be tapered down to low doses over approximately one year [5,6,9,16,24-27].
Treatment Links: http://ard.bmj.com/content/47/9/733, http://www.ncbi.nlm.nih.gov/pubmed/2254893, http://journals.lww.com/evidence-based-ophthalmology/Citation/2003/07000/Recovery_of_Visual_Function_in_Patients_with.5.aspx
Perioperative PION Treatment & Prognosis: Rapid blood transfusions, to correct anemia and raise blood pressure, may improve PION outcomes. In one report of a related disease, hypotension-induced AION, 3 out of 3 patients who received rapid transfusions reported partial recovery of vision . While rapid transfusions offer some hope, the prognosis for perioperative PION remains poor. Prevention remains the best way to reduce PION.
One retrospective report proposes that incidence of PION could be reduced in high-risk cases by altering surgical management. For example, for patients undergoing spine surgery, measures could be taken to minimize intraoperative hypotension, to accelerate the process of blood replacement, and to aggressively treat facial swelling .
Additional notes 
Posterior Ischemic Optic Neuropathy vs. Anterior Ischemic Optic Neuropathy (AION): PION is less common than AION . Blood supply and surrounding anatomy make the anterior and posterior portions of the optic nerve susceptible to different ischemic pressures.
The posterior optic nerve receives blood primarily from the pial branches of the ophthalmic artery. The optic canal, a boney tunnel leading to the brain, surrounds the most posterior part of this optic nerve segment.
The anterior optic nerve receives blood primarily from the posterior ciliary arteries. The anterior optic nerve, a.k.a. the optic nerve head, is surrounded by the scleral canal, and is vulnerable to crowding of nerve fibers. The portion of the optic nerve head that is visible by looking into the eye with an ophthalmoscope is called the optic disc.
This article focuses primarily on PION.
The Optic nerve perfusion equation:
Theoretically, there are three ways to hinder oxygen delivery to tissues: decrease blood pressure (BP), increase resistance to blood flow by increasing pressure in tissues (TP), or decrease the oxygen-carrying capacity of the blood (Hct) .
Optic nerve perfusion = Hct x (BP-TP)
This equation formalizes what we know about Perioperative PION, regarding both risk and prevention.
PION risk increases when optic nerve perfusion↓, caused by: Hct↓, BP↓, and/or TP↑.
PION may be prevented by maintaining optic nerve perfusion. This means: Close monitoring BP and Hct, and being ready to rapidly ↑BP and ↑Hct if necessary. Additionally, measures to prevent ↑TP, or minimize duration of ↑TP, also can prevent PION.
What factors increase/decrease Hct, BP, and TP?
Hct: 1. Increases with blood transfusions. May artificially appear increased during dehydration. 2. Decreases with blood loss, or by blood dilution with too much IV fluid.
BP: 1. Temporarily increases with blood and fluid replacement, by blood transfusion and IV fluid infusion. 2. Decreases with blood loss. Also decreases with dilation of blood vessels, which may be a physiological response, or a pharmacologic response to general anesthesia.
TP: For Perioperative PION, the most relevant cause of TP increases is the force of gravity. When the head of the operating room table is lower than the heart, fluid accumulates in tissues of the head because venous return and lymph drainage are decreased. Other causes of TP increases (e.g. heart failure, lymph obstruction and inflammation) are less relevant for PION, and beyond the scope of this discussion. TP also increases with third spacing of fluids associated with the use of crystalloids. This is particularly problematic in long surgeries and with large volumes of crystalloids in comparison to colloids that mitigate third spacing.
Note: Oxygen carrying capacity is actually affected by 2 factors, both O2 saturation and hematocrit (Hct). However, under the controlled environment of general anesthesia, O2 saturation remains near 100%.
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