Posterior ischemic optic neuropathy

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 diagnosis 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.

Causation
PION 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 causes the increased pressure on the optic nerve, further blocking 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.

Diagnosis
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.

Prevention
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.

Treatment
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.