Posterior ischemic optic neuropathy (PION) is a medical condition characterized by damage to the retrobulbar portion of the optic nerve due to inadequate blood flow (ischemia) to the optic nerve. Despite the term posterior, this form of damage to the eye's optic nerve due to poor blood flow also includes cases where the cause of inadequate blood flow to the nerve is anterior, as the condition describes a particular mechanism of visual loss as much as the location of damage in the optic nerve. In contrast, anterior ischemic optic neuropathy (AION) is distinguished from PION by the fact that AION occurs spontaneously and on one side in affected individuals with predisposing anatomic or cardiovascular risk factors.
Signs and symptoms
PION is characterized by moderate to severe painless vision loss of abrupt onset. One or both eyes may be affected and color vision is typically impaired.
Ophthalmoscopic exam
Looking inside the person'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.
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.
Defective light perception in one eye causes an asymmetrical pupillary constriction reflex called the afferent pupillary defect (APD).
Arteritic PION
A-PION most commonly affects Caucasian women, with an average age of 73. 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.
Perioperative PION
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.
Cause
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 people: People who have undergone non-ocular surgery that is particularly prolonged or is associated with a significant blood loss. People who have experienced significant bleeding from an accident or ruptured blood vessels. In these cases, the person may develop anemia (too few oxygen-delivering red blood cells in the bloodstream) and often have low blood pressure as well. This combination can produce circulatory shock, and PION has sometimes been called shock-induced optic neuropathy.
The combination of anemia and low blood pressure means that the blood is carrying less oxygen to the tissues. The optic nerve can be at very high risk for damage from insufficient blood supply due to swelling (from lack of oxygen) in a confined bony space resulting in a compartment syndrome. Restricted blood flow can lead to permanent damage to the optic nerve and result in blindness (often in both eyes). For technical reasons this occurs more frequently with spinal surgeries.
Cardiovascular risk factors
Perioperative PION patients have a higher prevalence of cardiovascular risk factors than in the general population. Documented cardiovascular risks in people affected by perioperative PION include high blood pressure, diabetes mellitus, high levels of cholesterol in the blood, tobacco use, abnormal heart rhythms, stroke, 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. These cardiovascular risks all interfere with adequate blood flow, and also may suggest a contributory role of defective vascular autoregulation.
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.