Posterior Ischemic Optic Neuropathy - Additional Notes

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) .

Therefore:

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%.